International Journal of Epidemiology © O x f o r d University Press 1978
Vol 7, No 4 Printed in Great Britain
Should we Screen all Newborns for Cystic Fibrosis?
Ten Kate L P [Department of Human Genetics, State University of Groningen, Ant. Deusinglaan 4, Groningen, The Netherlands], Feenstra-de Gooyer I, Ploeg-de Groot G, Gouw W L and Anders G J P A. Should we screen all newborns for cystic fibrosis? International Journal of Epidemiology 1978, 7: 323—330. To assess the value of detecting albumin in meconium as a screening procedure for cystic fibrosis (CF) 68 000 meconium samples were examined by BM Meconium Test®, single radial immunodiffusion and benzidine reaction. The specificity and sensitivity of this combination of tests were 99.67% and 78.57% respectively. The prevalence of CF at birth was confirmed as 1:3600 in this country. This low prevalence resulted in a relatively high number of false positives. Therefore, a positive test result has a low predictive value (3.39%) and this is a serious drawback of the method. The experiences and opinions of 37 local paediatricians about the screening programme were evaluated by a simple questionnaire. Gold's decision rule was applied. The least relative cost of misclassification justifying a mass-screening programme was 3 times higher than the actual relative cost as suggested by the aggregate opinion of paediatricians in the region. These results support the view that with the methods used screening may have more disadvantages than not screening.
a programme to the aggregate opinion of paediatricians in the region.
INTRODUCTION
In a retrospective study of the prevalence of CF at birth in the Netherlands (1) we found that only one in every 3600 newborns was affected. In some parts of the country a substantially lower estimate was obtained. Since most studies have reported higher figures for Caucasian populations, these data prompted us to start a screening programme for CF in. 4 provinces of the Netherlands. The Boehringer— Mannheim (BM) Test Meconium® teststrip, supplemented with single radial immunodiffusion, was used for this purpose. In the course of the study the question came up whether or not, for the benefit of the CF patients and their families, this screening programme should be continued and extended to all newborns in the Netherlands. Although mass infant screening for cystic fibrosis has been recommended by numerous authors, others have expressed great scepticism about the desirability of such a screening programme (2). We approached this question using Gold's decision rule (3), by comparing the least relative cost of misclassification justifying such
MATERIALS AND METHODS From March 1973 to the end of 1977, meconium was obtained from 68 539 infants born either at home or hospital or maternity home in one of the 4 north-eastern provinces of the Netherlands (Groningen, Friesland, Drenthe and Overijssel). In this region home deliveries still made up about 50% of all deliveries. As to the occurrence of CF the 68 539 newborns are considered to be a representative sample of all 161 384 infants born during the same period in these 4 provinces, the only selective factor being the cooperation of the hospital or of the regional maternity aid service. 62% of the samples were from hospitals and maternity homes, 38% of the samples from home deliveries. In addition to this study group we examined 50 meconium samples from infants with either a relative with CF or a positive teststrip found in another laboratory. This series will be referred to below as the increased risk group. All samples were mailed without cooling. Senders were advised not to mail samples on Friday but to store them at 4 C until Monday. Upon arrival the
1,2,3,4,5 Department of human Genetics, State University of Groningen, Ant. Deusinglain 4, Groningen, The Netherlands.
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LP TEN KATE 1 , I FEENSTRA-DE GOOYER 2 , G PLOEG-DE GROOT 3 , W L GOUW4 and GJ PA ANDERS5
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INTERNATIONAL JOURNAL OF EPIDEMIOLOGY
prevalence of normal newborns
x
proportion misclassified by screening
prevalence of normal newborns
proportion x misclassified by not-screening
4.
Do you feel that a) initial misclassification of CF patients is as bad as initial misclassification of normal newborns? b) initial misclassification of CF patients is worse than initial misclassification of normal newborns, and, if so, how much worse (e.g. 2x, 5x, 10x, 100x)? c) initial misclassification of normal newborns is worsa than initial misclassification of CF patients, and, if so, how much worse (e.g. 2x, 5x, lOx, 10«x)? This last question was repeated in the 1977 inquiry and at that opportunity the same question was asked concerning screening for phenylketonuria (PKU). The least relative cost of misclassification justifying mass infant screening for CF was calculated by applying the decision rule proposed by Gold et al. (3). This was then compared with the answers of the paediatricians to question number 4. Gold's decision rule is based on the assumption that screening is advisable only if the cost of screening is less than the cost of not screening. In other words, if R in cost of screening R cost of not screening is less than one. Cost is#not meant here in a purely financial sense but means total burden, the total disadvantage to persons, families and society inherent in screening and in not screening. The disadvantage has 2 components: the disadvantage of misclassifying normal newborns (false positives) and the disadvantage of misclassifying CF patients (false negatives). The frequencies of these events are given by the prevalence of the disease and the sensitivity and specificity of the screening method and of not screening, respectively. Since both types of misclassification (the misclassification of normal newborns and the misclassification of CF patients) are not equally serious, they have to be weighted by a weighting factor, the relative cost of misclassification.t For the calculation of the least relative cost of misclassification we assumed a
prevalence of CF patient* prevalence + of CF patients
proportion misclassified by screening
relative cost of miclassifi cation
proportion misclassified by not-screening
relative cost of misclassification
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samples were cooled and stored till processing next day. The BM Meconium Test® was performed according to the directions provided by the manufacturer (4). Samples with a positive or dubiously positive teststrip were screened for the presence of blood (benzidine reaction). In all these samples a quantitative determination of the amount of albumin was performed by means of a single radial immunodiffusion (5, 6). After processing all samples were stored at 4°C for one year. Cases with an albumin level of 20 mg or more per gram meconium (dry weight) were referred to paediatricians for examination. In a few cases we did not receive enough meconium to perform other tests besides the BM Meconium Test®. If, in these cases, the teststrip was positive or dubiously positive, the infants (14 in total) were also referred to their paediatricians. At regular intervals we checked whether a clinical report on the infant had been received. At 4 points during the study (autumn 1974, 1975, 1976 and 1977) we asked the paediatricians in the region for identifying data on all CF patients belonging to the birth cohort under study. This was done in order to establish the number of false negatives and to compare the numbers of CF patients in the screened and unscreened population of newborns. The response rate to these inquiries was 100%. Specificity and predictive value were calculated from the results in the study group, and sensitivity from the data of both groups combined. The 1976 inquiry among the 37 regional paediatricians was accompanied by a questionnaire on their opinion of the screening programme. Four questions were asked: 1. Do you feel that at the end of the trial period the screening programme should be continued? 2. Do you think that premature and dysmature infants should be excluded from meconium testing? 3. Is the workload from this screening programme acceptable to local paediatricians?
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TABLE 1
Numbers of meconium samples tested and results of subsequent clinical evaluations of newborns with a positive test for study group and increased risk group (March 1973 — December 1977) Study group
Neonates screened
Increased risk group
Total
50
68 589
1 076
30
1 106
Meconium contaminated by blood No blood
98 978
2 28
100 1 006
Albumin < 20 mg/gr meconium (dry weight)
742
13
755
Albumin > 20 mg/gr meconium (dry weight) or unknown (11 cases)
236
15
251
8
3
11
90
6
96
138
6
144
Positive or dubiously positive teststrips
CF pre-/dysmaturity Cause unknown (may include some prc-/dysmatures) False negatives
prevalence rate of CF at birth of 1 in 3600 (see below). The proportions of normal newborns and of CF patients misclassified by screening were given by the results of this screening study. As to the proportions misclassified by not screening several assumptions were made. The proportion of CF patients misclassified by not screening was assumed to be either 100% or 80%, and the proportion of normal newborns misclassified by not screening was assumed to be either 0% or 0.033% (i.e. one tenth of the proportion misclassified by screening). RESULTS Table 1 shows the results of meconium testing and subsequent clinical evaluation of newborns with 20 or more mg albumin/gr meconium (dry weight) for both the study group and the increased risk group. In the increased risk group 3 CF patients were found and one was missed. In the study group 8 CF patients were found and 2 were missed by the test. The meconium (stored at 4 C) of the 3 false negative patients was re-analyzed after 4—9 months. It did not show any detectable amount of albumin in 2 and 17 mg albumin/gr meconium (dry weight) in one (the same amount as 7 months before).
Pulmonary symptoms dominate in these 3 patients. Table 2 summarizes the results in terms of specificity, sensitivity and predictive value. In our hands the values of specificity and predictive value of the teststrip are substantially lower than the values for the complete procedure (teststrip + test for occult blood + single radial immunodiffusion). The distribution of albumin levels in the meconia with 20 or more mg albumin/gr meconium (dry weight) is shown in Table 3. Raising the cut-off value differentiating between 'positives' and 'negatives' will result in a higher specificity and predictive value, but clearly also in a lower sensitivity. The number of CF patients in both the screened and not screened groups of the birth cohort under study are shown in Table 4 and 5. Table 4 reports the number of CF patients and the total number of TABLE 2
Specificity, sensitwuy and predictive value of a positive test
Specificity Sensitivity Predictive value of a positive test
Teststrip alone
Complete procedure
98.44% 78.57% 0.75% (1:135)
99.67% 78.57% 3.39% (1:30)
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68 539
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TABLE 3 Dsitribution of albumin levels in meconia with 20 or more mg albumin/gram dry weight (both groups combined). Range1
Normals
total
95 58 32 23 17 1 14 240
11
mg albumin/gram meconium (dry weight) see text TABLE 4
Frequency distribution of CF patients and total number of livebirtbs according to province of birth (March 1973 through August 1977).
Province
Groningen Friesland Drenthe Overijssel total
Number of CF patients*
Number of live births*
Prevalence
8(5) 8(1) 5(1) 14(3)
32 043 (22,674) 37 147 (4,776) 25 419(14,520) 66 775 (19.396)
l:4O05 1:4643 1:5084 1:4770
35(10)
161,384 (61,366)
1:4611
* Numbers in brackets refer to the study group of meconium samples. TABLE 5
Prevalence of cystic fibrosis at birth from yearly reports of local paediatricians Birth cohort
Time of report Autumn Autumn Autumn Autumn
March 3, 1973 through August 1974
Sept 1, 1974 through August 1975
1:6379 1:4104 1:3827 1:3588
1:8940 1:7152 1:5960
1974 1975 1976 1977
TABLE 6 Question
Sept 1, 1975 through August 1976
Sept 1, 1976 Total (March 3, 1973 through August through August of the 1977 year of reporting)
1:5736 1:5736
1:6379 1:5176 1:4907 l:4oll
1:4829
Opinions of 32 local paediatricians on the screening programme Answer
Frequency No
1. The screening programme should be continued at the end of the trial period
yes no don't know
23 4 5
71.9 12.5 15.6
2. Prematures and dysmatures should be excluded from meconium testing
yes no don't know
2 27 3
6.2 84.4 9.4
3. The workload from this screening programme is acceptable to local paediatricians
yes no
30 2
93.8 6.2
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20-40 40-80 80-160 160-320 320-64O >640 unknown *
CF
livebirths for each of the 4 provinces. Prevalence in these provinces is very similar. The difference in the prevalence of CF patients among the screened (10 in 61 366) and not screened newborns (25 in 100 018) is not statistically significant. Table 5 shows that at each subsequent inquiry the number of CF patients rises, reflecting the delay in diagnosis in CF patients, who were not included in the screening programme or were missed by the screening. 32 (86.5%) of the 37 paediatricians responded to the questionnaire about their opinions of the screening programme. The majority favoured continuation of the screening programme at the end of the trial period and inclusion of premature and dysmature babies in the programme (Table 6). Their
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TABLE 7
Opinion of paediatricians on tbe relative cost of initial misclassification INITIAL MISCLASSIFICATION
A inquiry
of normal newborns is as bad is of CF patients
of norm*] newborns is worse than of CF patients
of CF patients is worse than of normal newborns 17
1976 (CF)
lOOx
lOx
5x
2x
lx
2x
5x
lOx
lOOx
10
lOOx
median opinion 16
B inquiry 1977 (CF)
lOOx
lOx
5x
2x
lx
2x
5x
median opinion C inquiry
of normal newborns is worse than of PKU patients
of normal newborns is as bad as of PKU patients
of PKU patients is worse than of normal newborns
1977
21
(PKU) 15 lx
2x
5x
lOx
lOOx
median opinion
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of paediatricians answering the second time to the same question (part B of the table) was somewhat less, but the median opinion on the relative cost of misclassification was very close to the earlier one. As to the relative cost of misclassification of PKU patients (pan C of the table) 80% of the paediatricians felt that this type of misclassification was 100 times worse than misclassifying initially normal newborns. Table 8 shows the least relative cost of misclassification justifying mass infant screening for different assumptions on proportions misclassified by not screening. The values found (13—20) differ by a factor 3 from the median opinion expressed by the paediatricians. Substituting their median opinion as the relative cost of
opinion on the relative cost of initial misclassification is shown in Table 7: in the first inquiry (part A of the table) 7 paediatricians held the view that initial misclassification of CF patients was as bad as initial misclassification of normal newborns. 7 others felt that initial misclassification of normal newborns was worse than initial misclassification of CF patients, and only 17 paediatricians were of the opposite opinion. Their answers to the question how much worse they thought the worst type of misclassification was compared to the less bad type, are summarized in the table. The 'median opinion' was that initial misclassification of CF patients was 2 to 5 times worse than initial misclassification of normal newborns. The number
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TABLE 8
Least relative cost of misclassification justifying mass infant screening for CF Proportion of CF patients initially misclassified by not screening 100%
0%
20.3
15.1
0.033%
18.2
13.6
misclassification in Gold's formula results in a value ofR between 2.09 and 3.23, suggesting that the cost of screening is greater than the cost of not screening. DISCUSSION Stephan et al (4, 7) introduced the BM Test Meconium® teststrip as a screening test for cystic fibrosis. According to them an intense blue colouration of the teststrip is obtained with albumin levels of about 20 mg or more per gram of dry weight meconium, and faint blue discolourations must be regarded as negative results. In working with the teststrip we observed a whole spectrum of blue colourations, which made it difficult in individual cases to reach a decision on positiveness or negativeness of the test. We therefore decided to perform a quantitative determination of the albumin level in all cases of blue colouration, even in the faint blue ones. This explains why in our study the specificity of the teststrip alone (98.44%) was much lower and of the combined method (99.67%) higher than the value (99.5%) reported by Stephan et al (7). Kollberg and Hellsing (8) also reported a difference in the specificity of the teststrip alone (99%) and the teststrip combined with single radial immunodiffusion (99.5%). Hellsing and Kollberg (6) added known amounts of albumin to meconia of healthy newborns and studied the effect of storage temperature on the degradation of the added albumin. Finding a rapid decrease in albumin content of samples stored at room temperature and a slow decrease in samples stored at 4°C, they concluded that samples should be mailed in the deep frozen state or at 4°C. However, both Prosser et al (9) and Stephan et al (7) stored meconia with an elevated level of albumin (both true and false positives) at room temperature and did not find a decrease in the albumin content. All our samples were mailed
without cooling. If this caused any decrease of albumin in the samples, we should have found a much lower sensitivity and a much higher specificity than other studies. As this is not the case, we conclude that measuring the effect of storage temperature on the level of added or native albumin is just another way of demonstrating the lack of pancreatic enzyme activity supposed to be responsible for most cases with elevated albumin concentrations. So far the sensitivity of the screening test was 78.5%. This, of course, is an upper estimate, since by follow-up of the birth cohort some more false negatives may be found. In spite of our high values for specificity (99.67%) the predictive value of a positive test is still rather low (1 in 30). This is due to the fact that the prevalence of CF at birth in our country is only 1 in 3600. As a consequence, our predictive value cannot be applied to other populations without correction for the prevalence of CF in these populations (10). The low predictive value is a very serious drawback of the method: for every CF patient found 29 other newborns had to be evaluated. In most of the cases it took several weeks (in some cases several months) before the diagnosis CF could be ruled out with certainty. This long period of uncertainty causes much anxiety in the parents of such children, and the question arises whether or not the advantages for the few CF patients and their families counterbalance the disadvantages for the many false positive normal newborns and their families. The high proportion of premature and dysmature babies among the false positives in our study agrees with the findings of other authors (7, 8, 11, 12), although the fraction with overt or occult blood seems to be lower in our series. By excluding premature babies from meconium testing the specificity and predictive value of the test could be
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Proportion of normal newborns initially misclassifled by not screening
80%
SCREENING FOR CYSTIC FIBROSIS
theoretical point any effect would be small. Also data on the impact of initial misclassification of normal newborns are lacking. Therefore, the final decision on the introduction of a mass screening programme cannot be made until detailed quantitative data on the effects of screening for CF patients and normal newborns are available. However, as a crude and provisional approach we compared the least relative cost of misclassification justifying a mass infant screening with the aggregate opinion of paediatricians, who have experience with both types of initial misclassification. It turned out that their estimate of the relative cost was too low to justify mass infant screening. Although we do not suggest that the undesirability of a mass screening has been established by this study, our data do not support an optimistic attitude to screening for CF with present techniques in our country. Although Gold's formula does not state explicitly all elements essential for deciding on screening programmes, it is a valuable tool for a first approach in situations where detailed information for and against is lacking. In using the formula one can easily see what conditions are unfavourable for the screening programme. In our country the low prevalence of CF at birth is one of these conditions. Gold's formula also shows where improvements should be made. Even in our country screening would be worthwhile if the test was more specific. This might be achieved by adding other tests to the screening programme, such as the determination of the albumin/a!-antitrypsine ratio (17), the demonstration of a raised lactase level (18, 19), or a trypsin assay (20). Therefore, the main conclusion from this study is that screening for CF in populations with a low prevalence should preferably be done by means of methods having a higher specificity (say 99.9%) than the combination of BM Test Meconium®, a test for occult blood and single radial immunodiffusion. ACKNOWLEDGEMENTS
This study was made possible by grants from the Netherlands Praeventiefonds and by the kind support of the firm of Boehringer Mannheim, many hospitals, maternity homes, Cross organizations and regional maternity aid services. The assistance of many paediatricians, obstetricians, general practitioners, midwives, nurses and home maternity aids is gratefully acknowledged. For technical assistance we are indebted to faculty, staff and students in the. department of human genetics.
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raised. However, CF patients as well as normal infants, may be born prematurely. In fact, 2 of the CF patients with an elevated albumin level in this series weTe born after a gestation time of 34— 35 weeks. The screening programme was set up to study the prevalence of CF at birth in the Netherlands.' In a previous retrospective study (2) the following rates were found for the 4 north-eastern provinces: Groningen 1:47 072, Friesland 1:4 393, Drenthe 1: 4 958, and Overijssel 1:3 491. As Table 4 shows, these previously found regional differences in the prevalence of CF could not be confirmed. As Table 5 reveals, the estimated prevalence of CF at birth in this study each year comes closer to the previous estimate of 1 in 3600 for the country as a whole. Although there seems to be agreement on the benefit of early diagnosis for CF.patients, unbiased data giving proof to it are scarce. Some authors, eg Huang et al (13) and Schwachmann et al (14), have compared survival of patients diagnosed prior to the onset of symptoms and of patients diagnosed because of their presenting symptoms. They observed a shorter survival time in the group of patients diagnosed because of symptoms. However, this group might be a subgroup of all patients with CF, having been ascertained because of their illness, while the other group seems to be more representative for the whole population of CF patients. Other authors (e.g. Wood et al (15)) have compared survival in groups of CF patients with different clinical scores and have found that patients with a high score do better than patients with a low score. However, one cannot conclude from this that early diagnosis improves survival time, but at best that the clinical score has some predictive value. Orenstein et al (16) have compared survival in CF sibling pairs, of which the older sibling had been diagnosed at a later age than the younger one. At 7 years of age the younger sibs scored better in several pulmonary and other clinical tests. However, in 10 families the older sibling was the index case and in none of the families the younger one, suggesting a possible bias in ascertainment against families with the younger one more severely affected than the older one. Also there may be benefit for younger CF sibs from the experience of parents and physicians with older ones. In conclusion, clinical experience suggests that early diagnosis may be beneficial but this has not been proven. Data on the effect of early diagnosis on primary prevention of cystic fibrosis are lacking. From a
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INTERNATIONAL JOURNAL OF EPIDEMIOLOGY globulin content in meconium of premature and full-term infants, Abstract 5th annual meeting, European Working Group for Cystic Fibrosis. Verona. 1974. (12) Griffiths AD, Bull F E and Dykes P. Effect of gestational length on albumin content of meconium, Archives of Disease in Childhood 51. 321, 1976. (13) Huang NN, Macri C N, Girone J and Sproul A. Survival of patients with cystic fibrosis, American Journal of Diseases of Children 120. 289, 1970. (14) Schwachman H, Redmond A and Khaw K T. Studies in cystic fibrosis. Report of 130 patients diagnosed under 3 months of age over a 20-year period, Pediatrics 46. 335, 1970. (15) Wood R E, Boat T F and Doershuk C F. Cystic Fibrosis, American
Review
of Respiratory
Disease
113.
833, 1976. (16) OrensteinD M, Boat T F, Stern R C, Tucker A S, Charnock E L, Matthews L W and Doershuk C F. The effect of early diagnosis and treatment in cystic fibrosis. A 7-year study of 16 sibling pairs, American Journal of Diseases of Children 131. 973, 1977. (17)RyleyHC, Neale L M, Brogan T D and Bray P T. Screening for cystic fibrosis by analysis of meconium for albumin and protease inhibitors, Clinica Chimica Acta 64: 117, 1975. (18) Antonowicz I, Ishida S and Schwachman H. Studies in meconium: Disaccharidase activities in meconium from cystic fibrosis patients and controls, PediatricsJ(>x 782, 1975. (19) Antonowicz I, Ishida S and Schwachman H. Screening for cystic fibrosis, Lancet ii. 746, 1976. (20) Crossley J R, Berryman C C and Elliott R B. Cystic fibrosis screening in the newborn, Lancet i. 1093, 1977.
(revised version received 25 May 1978)
Alma-Ata September 1978 Primary health care was defined as 'essential health care based on scientifically sound and socially acceptable methods and technology made universally accessible to individuals and families in the community through their full participation and at a cost that the community and country can afford to
maintain at every stage of their development, in a spirit of self reliance and self-determination'. It formed an integral part of both the country's health system, of which it is the central function and of overall social and economic development.
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REFERENCES (l)Ten Kate L P. Cystic fibrosis in the Netherlands, International Journal of Epidemiology 6. 23, 1977. (2) Brimblecombe FSW and Chamberlain J. Screening for cystic fibrosis, Lancet ii. 1428, 1973. (3) Gold R J M, Maag U R, Neal J L and Scriver C R. The use of biochemical data in screening for mutant alleles and in genetic counselling, Annals of Human Genetics 37i 315, 1974. (4) Stephan U, Busch E W and Dannemann R. Ein neuer Screening-Test auf Mukoviszidose. Nachweis des erhdhten Albumingehaltes in Mekonium, Padtatriscbe Praxis 12i 487, 1973. (5)BullFE, GladwinDEO and Griffiths A D. Immunochemical method for detection of albumin in human meconium, Archives of Disease in Childhood 49. 602, 1974. (6) Hellsing K and Kollberg H. Analysis of albumin in meconium for early detection of cystic fibrosis. A methodological study, Scandinavian Journal of Clinical and Laboratory Investigation 33i 333, 1974. (7) Stephan U, Busch E W, Kollberg H and Hellsing K. Cystic fibrosis detection by means of a test-strip, Paediatrics 55. 35, 1975. (8) Kollberg H and Hellsing K. Screening for cystic fibrosis by analysis of albumin in meconium. Acta Paediatrica Scandinavian 64. 477, 1975. (9) Prosser R, Owen H, Bull F, Parry B, Smerkinich J, Goodwin H A and Dathan J. Screening for cystic fibrosis by examination of meconium. Archives of Disease in Childhood 49. 597, 1974. (10) Galen R S and Gambino R. Beyond normality. The predictive value and efficiency of medical diagnoses. John Wiley & Sons, New York. 1975. (11) Tinschmann P. Total protein, albumin and immuno-
Vol 7, No 4 Printed in Great Britain
Should we Screen all Newborns for Cystic Fibrosis?
Ten Kate L P [Department of Human Genetics, State University of Groningen, Ant. Deusinglaan 4, Groningen, The Netherlands], Feenstra-de Gooyer I, Ploeg-de Groot G, Gouw W L and Anders G J P A. Should we screen all newborns for cystic fibrosis? International Journal of Epidemiology 1978, 7: 323—330. To assess the value of detecting albumin in meconium as a screening procedure for cystic fibrosis (CF) 68 000 meconium samples were examined by BM Meconium Test®, single radial immunodiffusion and benzidine reaction. The specificity and sensitivity of this combination of tests were 99.67% and 78.57% respectively. The prevalence of CF at birth was confirmed as 1:3600 in this country. This low prevalence resulted in a relatively high number of false positives. Therefore, a positive test result has a low predictive value (3.39%) and this is a serious drawback of the method. The experiences and opinions of 37 local paediatricians about the screening programme were evaluated by a simple questionnaire. Gold's decision rule was applied. The least relative cost of misclassification justifying a mass-screening programme was 3 times higher than the actual relative cost as suggested by the aggregate opinion of paediatricians in the region. These results support the view that with the methods used screening may have more disadvantages than not screening.
a programme to the aggregate opinion of paediatricians in the region.
INTRODUCTION
In a retrospective study of the prevalence of CF at birth in the Netherlands (1) we found that only one in every 3600 newborns was affected. In some parts of the country a substantially lower estimate was obtained. Since most studies have reported higher figures for Caucasian populations, these data prompted us to start a screening programme for CF in. 4 provinces of the Netherlands. The Boehringer— Mannheim (BM) Test Meconium® teststrip, supplemented with single radial immunodiffusion, was used for this purpose. In the course of the study the question came up whether or not, for the benefit of the CF patients and their families, this screening programme should be continued and extended to all newborns in the Netherlands. Although mass infant screening for cystic fibrosis has been recommended by numerous authors, others have expressed great scepticism about the desirability of such a screening programme (2). We approached this question using Gold's decision rule (3), by comparing the least relative cost of misclassification justifying such
MATERIALS AND METHODS From March 1973 to the end of 1977, meconium was obtained from 68 539 infants born either at home or hospital or maternity home in one of the 4 north-eastern provinces of the Netherlands (Groningen, Friesland, Drenthe and Overijssel). In this region home deliveries still made up about 50% of all deliveries. As to the occurrence of CF the 68 539 newborns are considered to be a representative sample of all 161 384 infants born during the same period in these 4 provinces, the only selective factor being the cooperation of the hospital or of the regional maternity aid service. 62% of the samples were from hospitals and maternity homes, 38% of the samples from home deliveries. In addition to this study group we examined 50 meconium samples from infants with either a relative with CF or a positive teststrip found in another laboratory. This series will be referred to below as the increased risk group. All samples were mailed without cooling. Senders were advised not to mail samples on Friday but to store them at 4 C until Monday. Upon arrival the
1,2,3,4,5 Department of human Genetics, State University of Groningen, Ant. Deusinglain 4, Groningen, The Netherlands.
323
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LP TEN KATE 1 , I FEENSTRA-DE GOOYER 2 , G PLOEG-DE GROOT 3 , W L GOUW4 and GJ PA ANDERS5
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prevalence of normal newborns
x
proportion misclassified by screening
prevalence of normal newborns
proportion x misclassified by not-screening
4.
Do you feel that a) initial misclassification of CF patients is as bad as initial misclassification of normal newborns? b) initial misclassification of CF patients is worse than initial misclassification of normal newborns, and, if so, how much worse (e.g. 2x, 5x, 10x, 100x)? c) initial misclassification of normal newborns is worsa than initial misclassification of CF patients, and, if so, how much worse (e.g. 2x, 5x, lOx, 10«x)? This last question was repeated in the 1977 inquiry and at that opportunity the same question was asked concerning screening for phenylketonuria (PKU). The least relative cost of misclassification justifying mass infant screening for CF was calculated by applying the decision rule proposed by Gold et al. (3). This was then compared with the answers of the paediatricians to question number 4. Gold's decision rule is based on the assumption that screening is advisable only if the cost of screening is less than the cost of not screening. In other words, if R in cost of screening R cost of not screening is less than one. Cost is#not meant here in a purely financial sense but means total burden, the total disadvantage to persons, families and society inherent in screening and in not screening. The disadvantage has 2 components: the disadvantage of misclassifying normal newborns (false positives) and the disadvantage of misclassifying CF patients (false negatives). The frequencies of these events are given by the prevalence of the disease and the sensitivity and specificity of the screening method and of not screening, respectively. Since both types of misclassification (the misclassification of normal newborns and the misclassification of CF patients) are not equally serious, they have to be weighted by a weighting factor, the relative cost of misclassification.t For the calculation of the least relative cost of misclassification we assumed a
prevalence of CF patient* prevalence + of CF patients
proportion misclassified by screening
relative cost of miclassifi cation
proportion misclassified by not-screening
relative cost of misclassification
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samples were cooled and stored till processing next day. The BM Meconium Test® was performed according to the directions provided by the manufacturer (4). Samples with a positive or dubiously positive teststrip were screened for the presence of blood (benzidine reaction). In all these samples a quantitative determination of the amount of albumin was performed by means of a single radial immunodiffusion (5, 6). After processing all samples were stored at 4°C for one year. Cases with an albumin level of 20 mg or more per gram meconium (dry weight) were referred to paediatricians for examination. In a few cases we did not receive enough meconium to perform other tests besides the BM Meconium Test®. If, in these cases, the teststrip was positive or dubiously positive, the infants (14 in total) were also referred to their paediatricians. At regular intervals we checked whether a clinical report on the infant had been received. At 4 points during the study (autumn 1974, 1975, 1976 and 1977) we asked the paediatricians in the region for identifying data on all CF patients belonging to the birth cohort under study. This was done in order to establish the number of false negatives and to compare the numbers of CF patients in the screened and unscreened population of newborns. The response rate to these inquiries was 100%. Specificity and predictive value were calculated from the results in the study group, and sensitivity from the data of both groups combined. The 1976 inquiry among the 37 regional paediatricians was accompanied by a questionnaire on their opinion of the screening programme. Four questions were asked: 1. Do you feel that at the end of the trial period the screening programme should be continued? 2. Do you think that premature and dysmature infants should be excluded from meconium testing? 3. Is the workload from this screening programme acceptable to local paediatricians?
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SCREENING FOR CYSTIC FIBROSIS
TABLE 1
Numbers of meconium samples tested and results of subsequent clinical evaluations of newborns with a positive test for study group and increased risk group (March 1973 — December 1977) Study group
Neonates screened
Increased risk group
Total
50
68 589
1 076
30
1 106
Meconium contaminated by blood No blood
98 978
2 28
100 1 006
Albumin < 20 mg/gr meconium (dry weight)
742
13
755
Albumin > 20 mg/gr meconium (dry weight) or unknown (11 cases)
236
15
251
8
3
11
90
6
96
138
6
144
Positive or dubiously positive teststrips
CF pre-/dysmaturity Cause unknown (may include some prc-/dysmatures) False negatives
prevalence rate of CF at birth of 1 in 3600 (see below). The proportions of normal newborns and of CF patients misclassified by screening were given by the results of this screening study. As to the proportions misclassified by not screening several assumptions were made. The proportion of CF patients misclassified by not screening was assumed to be either 100% or 80%, and the proportion of normal newborns misclassified by not screening was assumed to be either 0% or 0.033% (i.e. one tenth of the proportion misclassified by screening). RESULTS Table 1 shows the results of meconium testing and subsequent clinical evaluation of newborns with 20 or more mg albumin/gr meconium (dry weight) for both the study group and the increased risk group. In the increased risk group 3 CF patients were found and one was missed. In the study group 8 CF patients were found and 2 were missed by the test. The meconium (stored at 4 C) of the 3 false negative patients was re-analyzed after 4—9 months. It did not show any detectable amount of albumin in 2 and 17 mg albumin/gr meconium (dry weight) in one (the same amount as 7 months before).
Pulmonary symptoms dominate in these 3 patients. Table 2 summarizes the results in terms of specificity, sensitivity and predictive value. In our hands the values of specificity and predictive value of the teststrip are substantially lower than the values for the complete procedure (teststrip + test for occult blood + single radial immunodiffusion). The distribution of albumin levels in the meconia with 20 or more mg albumin/gr meconium (dry weight) is shown in Table 3. Raising the cut-off value differentiating between 'positives' and 'negatives' will result in a higher specificity and predictive value, but clearly also in a lower sensitivity. The number of CF patients in both the screened and not screened groups of the birth cohort under study are shown in Table 4 and 5. Table 4 reports the number of CF patients and the total number of TABLE 2
Specificity, sensitwuy and predictive value of a positive test
Specificity Sensitivity Predictive value of a positive test
Teststrip alone
Complete procedure
98.44% 78.57% 0.75% (1:135)
99.67% 78.57% 3.39% (1:30)
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68 539
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INTERNATIONAL JOURNAL OF EPIDEMIOLOGY
TABLE 3 Dsitribution of albumin levels in meconia with 20 or more mg albumin/gram dry weight (both groups combined). Range1
Normals
total
95 58 32 23 17 1 14 240
11
mg albumin/gram meconium (dry weight) see text TABLE 4
Frequency distribution of CF patients and total number of livebirtbs according to province of birth (March 1973 through August 1977).
Province
Groningen Friesland Drenthe Overijssel total
Number of CF patients*
Number of live births*
Prevalence
8(5) 8(1) 5(1) 14(3)
32 043 (22,674) 37 147 (4,776) 25 419(14,520) 66 775 (19.396)
l:4O05 1:4643 1:5084 1:4770
35(10)
161,384 (61,366)
1:4611
* Numbers in brackets refer to the study group of meconium samples. TABLE 5
Prevalence of cystic fibrosis at birth from yearly reports of local paediatricians Birth cohort
Time of report Autumn Autumn Autumn Autumn
March 3, 1973 through August 1974
Sept 1, 1974 through August 1975
1:6379 1:4104 1:3827 1:3588
1:8940 1:7152 1:5960
1974 1975 1976 1977
TABLE 6 Question
Sept 1, 1975 through August 1976
Sept 1, 1976 Total (March 3, 1973 through August through August of the 1977 year of reporting)
1:5736 1:5736
1:6379 1:5176 1:4907 l:4oll
1:4829
Opinions of 32 local paediatricians on the screening programme Answer
Frequency No
1. The screening programme should be continued at the end of the trial period
yes no don't know
23 4 5
71.9 12.5 15.6
2. Prematures and dysmatures should be excluded from meconium testing
yes no don't know
2 27 3
6.2 84.4 9.4
3. The workload from this screening programme is acceptable to local paediatricians
yes no
30 2
93.8 6.2
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20-40 40-80 80-160 160-320 320-64O >640 unknown *
CF
livebirths for each of the 4 provinces. Prevalence in these provinces is very similar. The difference in the prevalence of CF patients among the screened (10 in 61 366) and not screened newborns (25 in 100 018) is not statistically significant. Table 5 shows that at each subsequent inquiry the number of CF patients rises, reflecting the delay in diagnosis in CF patients, who were not included in the screening programme or were missed by the screening. 32 (86.5%) of the 37 paediatricians responded to the questionnaire about their opinions of the screening programme. The majority favoured continuation of the screening programme at the end of the trial period and inclusion of premature and dysmature babies in the programme (Table 6). Their
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SCREENING FOR CYSTIC FIBROSIS
TABLE 7
Opinion of paediatricians on tbe relative cost of initial misclassification INITIAL MISCLASSIFICATION
A inquiry
of normal newborns is as bad is of CF patients
of norm*] newborns is worse than of CF patients
of CF patients is worse than of normal newborns 17
1976 (CF)
lOOx
lOx
5x
2x
lx
2x
5x
lOx
lOOx
10
lOOx
median opinion 16
B inquiry 1977 (CF)
lOOx
lOx
5x
2x
lx
2x
5x
median opinion C inquiry
of normal newborns is worse than of PKU patients
of normal newborns is as bad as of PKU patients
of PKU patients is worse than of normal newborns
1977
21
(PKU) 15 lx
2x
5x
lOx
lOOx
median opinion
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of paediatricians answering the second time to the same question (part B of the table) was somewhat less, but the median opinion on the relative cost of misclassification was very close to the earlier one. As to the relative cost of misclassification of PKU patients (pan C of the table) 80% of the paediatricians felt that this type of misclassification was 100 times worse than misclassifying initially normal newborns. Table 8 shows the least relative cost of misclassification justifying mass infant screening for different assumptions on proportions misclassified by not screening. The values found (13—20) differ by a factor 3 from the median opinion expressed by the paediatricians. Substituting their median opinion as the relative cost of
opinion on the relative cost of initial misclassification is shown in Table 7: in the first inquiry (part A of the table) 7 paediatricians held the view that initial misclassification of CF patients was as bad as initial misclassification of normal newborns. 7 others felt that initial misclassification of normal newborns was worse than initial misclassification of CF patients, and only 17 paediatricians were of the opposite opinion. Their answers to the question how much worse they thought the worst type of misclassification was compared to the less bad type, are summarized in the table. The 'median opinion' was that initial misclassification of CF patients was 2 to 5 times worse than initial misclassification of normal newborns. The number
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INTERNATIONAL JOURNAL OF EPIDEMIOLOGY
TABLE 8
Least relative cost of misclassification justifying mass infant screening for CF Proportion of CF patients initially misclassified by not screening 100%
0%
20.3
15.1
0.033%
18.2
13.6
misclassification in Gold's formula results in a value ofR between 2.09 and 3.23, suggesting that the cost of screening is greater than the cost of not screening. DISCUSSION Stephan et al (4, 7) introduced the BM Test Meconium® teststrip as a screening test for cystic fibrosis. According to them an intense blue colouration of the teststrip is obtained with albumin levels of about 20 mg or more per gram of dry weight meconium, and faint blue discolourations must be regarded as negative results. In working with the teststrip we observed a whole spectrum of blue colourations, which made it difficult in individual cases to reach a decision on positiveness or negativeness of the test. We therefore decided to perform a quantitative determination of the albumin level in all cases of blue colouration, even in the faint blue ones. This explains why in our study the specificity of the teststrip alone (98.44%) was much lower and of the combined method (99.67%) higher than the value (99.5%) reported by Stephan et al (7). Kollberg and Hellsing (8) also reported a difference in the specificity of the teststrip alone (99%) and the teststrip combined with single radial immunodiffusion (99.5%). Hellsing and Kollberg (6) added known amounts of albumin to meconia of healthy newborns and studied the effect of storage temperature on the degradation of the added albumin. Finding a rapid decrease in albumin content of samples stored at room temperature and a slow decrease in samples stored at 4°C, they concluded that samples should be mailed in the deep frozen state or at 4°C. However, both Prosser et al (9) and Stephan et al (7) stored meconia with an elevated level of albumin (both true and false positives) at room temperature and did not find a decrease in the albumin content. All our samples were mailed
without cooling. If this caused any decrease of albumin in the samples, we should have found a much lower sensitivity and a much higher specificity than other studies. As this is not the case, we conclude that measuring the effect of storage temperature on the level of added or native albumin is just another way of demonstrating the lack of pancreatic enzyme activity supposed to be responsible for most cases with elevated albumin concentrations. So far the sensitivity of the screening test was 78.5%. This, of course, is an upper estimate, since by follow-up of the birth cohort some more false negatives may be found. In spite of our high values for specificity (99.67%) the predictive value of a positive test is still rather low (1 in 30). This is due to the fact that the prevalence of CF at birth in our country is only 1 in 3600. As a consequence, our predictive value cannot be applied to other populations without correction for the prevalence of CF in these populations (10). The low predictive value is a very serious drawback of the method: for every CF patient found 29 other newborns had to be evaluated. In most of the cases it took several weeks (in some cases several months) before the diagnosis CF could be ruled out with certainty. This long period of uncertainty causes much anxiety in the parents of such children, and the question arises whether or not the advantages for the few CF patients and their families counterbalance the disadvantages for the many false positive normal newborns and their families. The high proportion of premature and dysmature babies among the false positives in our study agrees with the findings of other authors (7, 8, 11, 12), although the fraction with overt or occult blood seems to be lower in our series. By excluding premature babies from meconium testing the specificity and predictive value of the test could be
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Proportion of normal newborns initially misclassifled by not screening
80%
SCREENING FOR CYSTIC FIBROSIS
theoretical point any effect would be small. Also data on the impact of initial misclassification of normal newborns are lacking. Therefore, the final decision on the introduction of a mass screening programme cannot be made until detailed quantitative data on the effects of screening for CF patients and normal newborns are available. However, as a crude and provisional approach we compared the least relative cost of misclassification justifying a mass infant screening with the aggregate opinion of paediatricians, who have experience with both types of initial misclassification. It turned out that their estimate of the relative cost was too low to justify mass infant screening. Although we do not suggest that the undesirability of a mass screening has been established by this study, our data do not support an optimistic attitude to screening for CF with present techniques in our country. Although Gold's formula does not state explicitly all elements essential for deciding on screening programmes, it is a valuable tool for a first approach in situations where detailed information for and against is lacking. In using the formula one can easily see what conditions are unfavourable for the screening programme. In our country the low prevalence of CF at birth is one of these conditions. Gold's formula also shows where improvements should be made. Even in our country screening would be worthwhile if the test was more specific. This might be achieved by adding other tests to the screening programme, such as the determination of the albumin/a!-antitrypsine ratio (17), the demonstration of a raised lactase level (18, 19), or a trypsin assay (20). Therefore, the main conclusion from this study is that screening for CF in populations with a low prevalence should preferably be done by means of methods having a higher specificity (say 99.9%) than the combination of BM Test Meconium®, a test for occult blood and single radial immunodiffusion. ACKNOWLEDGEMENTS
This study was made possible by grants from the Netherlands Praeventiefonds and by the kind support of the firm of Boehringer Mannheim, many hospitals, maternity homes, Cross organizations and regional maternity aid services. The assistance of many paediatricians, obstetricians, general practitioners, midwives, nurses and home maternity aids is gratefully acknowledged. For technical assistance we are indebted to faculty, staff and students in the. department of human genetics.
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raised. However, CF patients as well as normal infants, may be born prematurely. In fact, 2 of the CF patients with an elevated albumin level in this series weTe born after a gestation time of 34— 35 weeks. The screening programme was set up to study the prevalence of CF at birth in the Netherlands.' In a previous retrospective study (2) the following rates were found for the 4 north-eastern provinces: Groningen 1:47 072, Friesland 1:4 393, Drenthe 1: 4 958, and Overijssel 1:3 491. As Table 4 shows, these previously found regional differences in the prevalence of CF could not be confirmed. As Table 5 reveals, the estimated prevalence of CF at birth in this study each year comes closer to the previous estimate of 1 in 3600 for the country as a whole. Although there seems to be agreement on the benefit of early diagnosis for CF.patients, unbiased data giving proof to it are scarce. Some authors, eg Huang et al (13) and Schwachmann et al (14), have compared survival of patients diagnosed prior to the onset of symptoms and of patients diagnosed because of their presenting symptoms. They observed a shorter survival time in the group of patients diagnosed because of symptoms. However, this group might be a subgroup of all patients with CF, having been ascertained because of their illness, while the other group seems to be more representative for the whole population of CF patients. Other authors (e.g. Wood et al (15)) have compared survival in groups of CF patients with different clinical scores and have found that patients with a high score do better than patients with a low score. However, one cannot conclude from this that early diagnosis improves survival time, but at best that the clinical score has some predictive value. Orenstein et al (16) have compared survival in CF sibling pairs, of which the older sibling had been diagnosed at a later age than the younger one. At 7 years of age the younger sibs scored better in several pulmonary and other clinical tests. However, in 10 families the older sibling was the index case and in none of the families the younger one, suggesting a possible bias in ascertainment against families with the younger one more severely affected than the older one. Also there may be benefit for younger CF sibs from the experience of parents and physicians with older ones. In conclusion, clinical experience suggests that early diagnosis may be beneficial but this has not been proven. Data on the effect of early diagnosis on primary prevention of cystic fibrosis are lacking. From a
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INTERNATIONAL JOURNAL OF EPIDEMIOLOGY globulin content in meconium of premature and full-term infants, Abstract 5th annual meeting, European Working Group for Cystic Fibrosis. Verona. 1974. (12) Griffiths AD, Bull F E and Dykes P. Effect of gestational length on albumin content of meconium, Archives of Disease in Childhood 51. 321, 1976. (13) Huang NN, Macri C N, Girone J and Sproul A. Survival of patients with cystic fibrosis, American Journal of Diseases of Children 120. 289, 1970. (14) Schwachman H, Redmond A and Khaw K T. Studies in cystic fibrosis. Report of 130 patients diagnosed under 3 months of age over a 20-year period, Pediatrics 46. 335, 1970. (15) Wood R E, Boat T F and Doershuk C F. Cystic Fibrosis, American
Review
of Respiratory
Disease
113.
833, 1976. (16) OrensteinD M, Boat T F, Stern R C, Tucker A S, Charnock E L, Matthews L W and Doershuk C F. The effect of early diagnosis and treatment in cystic fibrosis. A 7-year study of 16 sibling pairs, American Journal of Diseases of Children 131. 973, 1977. (17)RyleyHC, Neale L M, Brogan T D and Bray P T. Screening for cystic fibrosis by analysis of meconium for albumin and protease inhibitors, Clinica Chimica Acta 64: 117, 1975. (18) Antonowicz I, Ishida S and Schwachman H. Studies in meconium: Disaccharidase activities in meconium from cystic fibrosis patients and controls, PediatricsJ(>x 782, 1975. (19) Antonowicz I, Ishida S and Schwachman H. Screening for cystic fibrosis, Lancet ii. 746, 1976. (20) Crossley J R, Berryman C C and Elliott R B. Cystic fibrosis screening in the newborn, Lancet i. 1093, 1977.
(revised version received 25 May 1978)
Alma-Ata September 1978 Primary health care was defined as 'essential health care based on scientifically sound and socially acceptable methods and technology made universally accessible to individuals and families in the community through their full participation and at a cost that the community and country can afford to
maintain at every stage of their development, in a spirit of self reliance and self-determination'. It formed an integral part of both the country's health system, of which it is the central function and of overall social and economic development.
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REFERENCES (l)Ten Kate L P. Cystic fibrosis in the Netherlands, International Journal of Epidemiology 6. 23, 1977. (2) Brimblecombe FSW and Chamberlain J. Screening for cystic fibrosis, Lancet ii. 1428, 1973. (3) Gold R J M, Maag U R, Neal J L and Scriver C R. The use of biochemical data in screening for mutant alleles and in genetic counselling, Annals of Human Genetics 37i 315, 1974. (4) Stephan U, Busch E W and Dannemann R. Ein neuer Screening-Test auf Mukoviszidose. Nachweis des erhdhten Albumingehaltes in Mekonium, Padtatriscbe Praxis 12i 487, 1973. (5)BullFE, GladwinDEO and Griffiths A D. Immunochemical method for detection of albumin in human meconium, Archives of Disease in Childhood 49. 602, 1974. (6) Hellsing K and Kollberg H. Analysis of albumin in meconium for early detection of cystic fibrosis. A methodological study, Scandinavian Journal of Clinical and Laboratory Investigation 33i 333, 1974. (7) Stephan U, Busch E W, Kollberg H and Hellsing K. Cystic fibrosis detection by means of a test-strip, Paediatrics 55. 35, 1975. (8) Kollberg H and Hellsing K. Screening for cystic fibrosis by analysis of albumin in meconium. Acta Paediatrica Scandinavian 64. 477, 1975. (9) Prosser R, Owen H, Bull F, Parry B, Smerkinich J, Goodwin H A and Dathan J. Screening for cystic fibrosis by examination of meconium. Archives of Disease in Childhood 49. 597, 1974. (10) Galen R S and Gambino R. Beyond normality. The predictive value and efficiency of medical diagnoses. John Wiley & Sons, New York. 1975. (11) Tinschmann P. Total protein, albumin and immuno-
