R. A. Horvitz, A. yon Graevenitz
Interpretation of Blood Cultures Yielding Staphylococcus aureus Summary: FortY-eight patients with blood cultures positive for Staphylococcus aureus were classified according to clinical criteria in three groups: "definite", "possible", and "doubtful" septicemia. Using traditional blood culture sets with two bottles (thioglycollate and tryptic soy broths), we found that patients with "definite" septicemia always showed more than one positive bottle per day if more than one set was drawn, that the mean detection time was 1.7 days, and that 95% of the first positive bottles and 92% of all positive bottles grew within two clays of incubation. Patients with "doubtful" septicemia were more often (88%) positive in one bottle only, the mean detection time for all bottles was 3.7 days, and only 35°/o of the first positive bottles and 33°/0 of all positive bottles yielded growth within two days. "Possible" cases took a position between tl~Lesetwo extremes but tended more towards the "doubtful" cases. The implications of these findings for the interpretation of blood cultures with S. aureus are discussed.
Zusammen/assung: Interpretation von Blutkulturen mit Staphylococcus aureus. 48 Patienten, aus deren Blutkulturen Staphylococcus aureus geziJchtet worden war, wurden nach klinischen Kriterien in drei Gruppen eingeteilt: ,,unzweifelhafte", ,,m/Sgliche" und ,,zweifelhafte" Septik~imie. Bet Verwendung eines traditionellen Blutkultursystems mit zwei Flaschen pro Kultur (Thioglykolat- und Tryptic Soy-Bouillons) ergab sieh, dab Patienten mit ,,unzweifelhafter" Septik~imie stets mehr als eine positive Flasche pro Tag aufwiesen - sofern mehr als eine Kultur pro Tag entnommen worden war -, dab die mittlere Bebrtitungsdauer bis zur Positivit~it 1,7 Tage betrug, und dab 95o/o der ersten positiven Flaschen und 920/0 alter positiven Flaschen innerhatb von zwei Tagen Wachstum yon S. aureus zeigten. Patienten mit ,,zweifelhafter" Septik~imie zeigten hiiufiger (in 88'%) Wachstum in nur ether Flasche, die mittlere Bebrtitungsdauer betrug 3,7 Tage, und nut 350/0 der ersten positiven Flaschen und 33% aller positiven Flaschen ergaben Wachstum innerhalb yon zwei Tagen. ,,M6gliche" Septik~imien nahmen eine Zwischenstellung ein, tendierten jedoch mehr nach der ,,zweifelhaften" Kategorie. Folgerungen fiir die Interpretation yon Blutkulturen mit S. aureus werden diskutiert.
T h e presence of Staphylococcus aureus or Staphylococcus epidermidis in a blood culture may signify either staphylococcal bacteremia or contamination from areas in which staphylococci normally occur, such as the skin (5) or the inanimate environment. In our experience as well as in the experience of others (7), S. epidermidis strains found in only one bottle of a set (i. e., two bottles taken at the same time) are clinically insignificant. In this study, we tried to ascertain whether this also holds true for S. aureus, and whether there is a relationship between the time of appearance of S. aureus in a blood culture and its clinical significance.
signs of growth. If such signs were present, a gram-stained smear of both bottles of the set was made. If staphylococci were seen in one or both bottles, both were subcultured to blood agar (Tryptic Soy Agar* with 5% sheep blood) which was incubated aerobically for 24-48 hours at 37 °C. Antimicrobial susceptibility testing with Sensi Discs* was done by the Kirby-Bauer method. If no macroscopic signs of growth were found, routine subcultures were made to chocolate agar (GC Agar Base** with 8% heated sheep blood and 1% Isovitalex Enrichment**) after one and six days of incubation. The first routine subculture of the TSB was incubated in a 5-100/0 CO 2 atmosphere for 24 hours at 37 °C, while the second routine TSB and all routine Thio subcultures were incubated anaerobically (GasPak 100 Anaerobic System**) for 48 hours. S. aureus and S. epidermidis were differentiated by the coagulase test (Coagulase Plasma, Rabbit**). Laboratory records over the nine-month period from I July 1975 through 31 March 1976 were reviewed to identify hospital inpatients with blood cultures positive for S. aureus (henceforth called "positive" blood cultures, in contrast to "negative" cultures with no growth or growth of bacteria other than S. aureus). Cultures of patients under five years of age were excluded since the full volume of blood normally inoculated into a set of blood culture bottles could not always be drawn from members of that group. The hospital charts of the patients with positive blood cultures were then reviewed
Materials and Methods Five ml of venous blood were inoculated into each of two bottles of a set of culture media prepared in the Culture Media Laboratory at Yale-New Haven Hospital. The set consisted of a vented bottle containing Tryptic Soy Broth* (TSB) and an unvented bottle containing Thioglycollate Medium without Indicator 135-C (Thio)**. Both bottles were supplemented with 0.03°/0 sodium polyanethol sulfonate***. Prior to drawing the blood, the patient's skin was prepared with an iodophor and 70°/0 ethyl alcohol. Multiple blood cultures taken on the same day were spaced over a period of several hours. Cultures were incubated without shaking for eight days at 37 °C. All bottles were checked twice daily for macroscopic * Difco Laboratories, Detroit, Mich. ** BioQuest, Cockeysville, Md. *** Analabs, North Haven, Conn.
Received: 5 April 1977 Dr. R. A. Horvitz, Prof. A. von Graevenitz, Department of Laboratory Medicine, Yale University School of Medicine, New Haven, Conn., USA. Reprints: Dr. A. yon Graevenitz, 789 Howard Avenue, New Haven, Conn. 06504, USA.
Infection 5 (t977) Nr. 4
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to record subsequent blood cultures and cultures from other sites, and to determine the clinical significance of the isolate. Criteria of "definite" septicemia included: a. fever over 38.5 °C, chills, and leukocytosis, b. absence of any other (nonstaphytococcal) infectious process to which the symptoms listed in (a.) could be attributed, c. either (i), the clinical impression of gram-positive septicemia as documented in the progress notes (in which factors predisposing to staphylococcal infections such as i. v. cannulation, i.v. drug abuse, defects in neutrophilic function, renal or hepatic failure, vascular insufficiency, and diabetes mellitus figured as well); or (ii), evidence of an infection with S. aureus at another site, such as pneumonia, empyema, skin or catheter infection, abscesses, or endocarditis. Patients who showed the symptoms listed above in (a.) but who either had other concomitant (nonstaphylococcal) infections to which these symptoms could be attributed, or who could not impress the clinical staff as having outright staphylococcal septicemia were classified in the category of "possible" septicemia. Finally, patients who presented only with fever (with or without leukocytosis) or with none of the symptoms listed in (a.) but who showed other infectious nonstaphylococcal processes, did not give the impression of having staphylococcal septicemia, and did not have S. aureus cultured from any other site, were classified as cases of "doubtful" septicemia. In these cases, the finding of S. aureus could obviously not be interpreted as a sign of "definite" or even "possible" septicemia. A "positive set" was defined as a set of blood culture bottles in which either one or both bottles were positive for S. aureus. For our study, we started the count with the first positive set and included all sets-positive or negative-that were obtained within 96 hours following the last positive set. "Detection time" was defined as the number of days between, the time of incubation of the blood culture and the time of positivitymacroscopically, in the gram-stained smear, or in the routine subculture. It was rounded off to the nearest "reading" day; e. g. bottles read as positive on the day following incubation at 8:00 a.m. or at 5:00 p.m. were both said to have had a detection time of one day. Detection times for six-clay subcultures were listed as seven days. Statistical significance was determined with the X ~ test. Results
A total of 55 charts of patients with one or more positive sets (with identical sensitivities per patient) coMd be obtained for review. Two patients were excluded because their cultures had been drawn through obviously contain-
inated sites, one patient died of other causes before sufficient d a t a could be obtained to assess the category of septicemia, and four patients were eliminated due to inadequate records. The remaining 48 patients constituted the study group. Table 1 shows that 20 patients fetl into the "definite", 11 into the "possible", and 17 into the "doubtful" categories. The percentages of patients from whom muttiple sets were drawn was not significantly different (p > 0.05) between the categories. Other (nonstaphylococcai) infections were more frequently present in the "doubtful" and "possible" groups than in the "definite" one. The percentage of patients started on antibiotics (effective in vitro against the S. aureus strain) after the first blood culture was drawn was significantly lower in the "doubtful" group compared to the others-presumably a reflection of the therapeutic consequences drawn from the clinical impression. Likewise, the less frequent isolation of S. aureus from other body sites in the "doubtful" group may in part be due to the fact that patients in this group were probably less thoroughly investigated for S. aureus elsewhere than patients in the other groups. None of the patients with "definite" septicemia yielded only one positive bottle out of one or several sets taken on the same day, in contrast to 45'% of those with "possible" and 88% of those with "doubtful" septicemia. A breakdown by media shows significant (p < 0.01) differences for TSB between the "definite" and the other two categories; for Thio, a less significant (p < 0.05) difference was found between the "doubtful" and the "definite" categories. Table 2 breaks down the data for sets. The "definite" category showed more sets positive in both bottles and less sets positive in one bottle only than the other categories (denominator: all positive sets per category). The six sets from patients with "definite" septicemia which were positive in one bottle only were accompanied by other positive sets on the same day, as outlined in Table 1. Broken down by media, the differences are similar to those listed in Table 1 except for the degree of significance for some comparative figures. Table 2 also shows
Table t: Number of patients Category' of septicemia
Definite Possible Doubtful
Total patients
20 11 17
Patients with muttiple sets
20 8 16
Patients with other infections
0 7* 17", ***
* p < 0.01 compared to "definite" category. ** p < 0.01 compared to "possible" category. *** p < 0.05 compared to "possible" category.
208
InfectiOn 5 (1977) Nr. 4
Patients treated with antibiotics
20 11 6", **
Patients with S. aureus from other sites 9 6 0', **
Patients with positive bottles on same day 1 btl > 1 btl of 2 or > 2
Patients positive in one btl only (of all btls drawn)
Total 0
5* I5"
20
6 2
0
5* 15", ***
+ p < 0.05 compared to "definite" category.
TSB 0
4* 10"
Thio 0
1 5+
R. A. Horvitz, A. yon Graevenitz: Interpretation of Blood Cultures Yielding Staphylococcus aureus Table 2: Number of blood culture sets Category of septicemia
Definite Possible Doubtful Total
Total sets taken
76 27 50 153
Positive in one or both bottles
Positive in both bottles only
Positive in one bottle only Total
TSB
66 14 18 98
60 7* 1', ** 68
6 7* 17", ** 30
2 4 6* 1 12", ** 5* 20 10
Positive bottles
Thio
Total
TSB
Thio
126 21 19 166
62 13 13 88
64 8 6 78
Table 3: Detection times of pos#ive bottles Category of septicemia DefiniteTSB Thio 2 day Total 3 day Total Grand Total PossibleTSB Thio 2 day Total 3 day Total Grand Total DoubtfulTSB Thio 2 day Total 3 day Total Grand Total
1
2
26 24
30 35 115
Bottles positive on day 3 4 5 6
4 3
0 2
1 0
0 0
7
0 0
Bottles of positive sets Total Positive Negative
66 66
61 64
Mean detection time (days)
5 2
1.7 1.7
1 6
3.5* 3.4*
5 12
3.7* 3.7*
i22 125
1 1
6 4 12"
0 0
i 1
3 0
1 0
1 2
14 14
13 8
12" 21
0 0
2 4 6*
4
3
0
0
0 0
0 0
4 2
18 18
13 6
i0'
that the percentages of positive Thio (78/98) and TSB (88/98) bottles were not significantly (p > 0.05) different, i. e., neither medium grew S. aureus preferentially. Table 3 shows that the mean detection times, while not different for TSB and Thio within each category, were twice as long (3.4-3.7 days) in the "possible" and "doubtful" categories than in the "definite" category (1.7 days). Within two days of incubation 920/0, and within three days 97.50/0 of the eventually positive bottles became positive in patients with "definite" septicemia, versus 5 7 % for both dates in patients with "possible" and 33~/0 and 520/0 in patients with "doubtful" septicemia. Table 4 demonstrates that none of the patients with "definite" septicemia yielded their first positive bottle later than three days after incubation, with 19 of 20 (95'°/0) having become positive within two days. In the "possible" and "doubfful"eategories, detection times for the first positive bottle showed a much greater variation: withia two days of incubation, 6 4 ' 0 in the "possible" and 3 5 % in the "doubtful" category had become positive. Only the latter figure is significantly different from the "definite" group. Mean detection times were, again, at least twice as long as in the "definite" group.
19 Table 4: Detection times of first positive bottle Category of septicemia
1
Definite Possible Doubtful
Mean detection time (days)
Detection time (days) of first positive bottle
8 1 0
2
3
4
5
6
11 1 6 0 6* 3
0 0 3
0 2 0
0 0 0
0 2 5
1.7 3.4* 4.0*
Discussion
The blood culture system used in this study is widely employed at present (1). W i t h the same system, Washington (6) also found no differences between TSB and Thio in the ability to detect S. aureus. Using eight ml of blood divided between a p o u r plate and a bottle of trypticase soy yeast broth, MacGregor and Beaty (4) found three out of 28 isolates of S. aureus to be "contaminants", presumably due, by and large, to laboratory handling. That figure is not significantly different from our number of isolates from "doubtful" cases (19 of 166) although we would not infer that skin contamination did not play a role.
Infection 5 (1977) Nr, 4
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R. A. Horvitz, A. yon Graevenitz: Interpretation of Blood Cultures Yielding Staphylococcus aureus
The fact that the ctinical symptomatology of staphylococcal septicemia is not as clear-cut as the symptomatology of gram-negative septicemia (2) made it necessary partially to rely in our study on "clinical impressions". We felt, therefore, that introduction of the category of "possible" septicemia would improve accuracy by a sharper delineation of the "definite" and "doubtful" categories. Two of Kotin's (3) criteria of non-significance, i.e., the finding of the microorganism in one blood culture only (out of several) and its ubiquitous occurrence, could obviously not be used for our study. Neither could we use the relationship between disappearance of S. aureus from the blood and antimicrobial treatment or clinical recovery. All of the patients in the "definite" and "possible" groups and six of the 17 in the "doubtful" group had been on antibiotics. With the exception of those who had only one single blood culture and of those who died from staphylococcal septicemia (two in the "definite" group), all patients showed negative blood cultures after treatment or even (in ten of the 17 "doubtful" cases) without treatment. Since the case-fatality rate of untreated S. aureus septicemia is very high (2), we can only use the latter figure (10/17) as additional evidence that these strains were clinically insignificant. Furthermore, "recovery" in the face of concomitant nonstaphylococcal infections was most difficult to ascertain. Teichoic acid antibodies had not been determined. Thus, patients with "definite" septicemia characteristically showed (a), more than one positive bottle per day if more than one set was drawn, (b), a detection time of one to two, at most three, days for the first positive bottle, and (c), positivity in almost all eventually positive bottles within two days of incubation. These features are also observed in S. epidermidis septicemia (7). Conversely, patients with "doubtful" septicemia showed (a) growth in one bottle only in the majority of cases, (b) a variable detection time for the first bottle, and (c) positivity of all eventually positive bottles only within seven days. "Possible" cases took a position in between the two extremes but tended in most respects more towards the "doubtful" ones.
~10
Infection 5 (1977) Nr. 4
With a view towards interpretation of laboratory findings, we would say that the finding of only one positive bottle out of one or more sets drawn on the same day--provided both bottles of a set have been gram-stained--could at best be indicative of "possible" septicemia but is more likely to be clinically insignificant. Insignificance is even more likely at a time when only the Gram stain has been performed, since S. epidermidis may be present which is much less often significant in blood cultures than S. aureus (1, 2, 4). Lack of clinical significance is also more likely if a set first becomes positive on the third day of incubation or later. On the other hand, positivity within two days of incubation and/or multiple positive sets are more suggestive of "definitive" than of "possible" septicemia with S. aureus (or S. epidermidis [7]) and argue against clinical insignificance.
Literature 1. Bartlett, R. C., Ellner, P. D., Washington, J. A.: Blood cultures. Cumitech 1. American Society for Microbiology, Washington D. C. 1974. 2. Cluff, L. E., Reynolds, R. C., Page, D. L., Breckenridge, 1. L.: StaphylococcaI bacteremia and altered host resistance.
Ann. Intern. Med. 69 (1968) 859-873. 3. Kotin, P.: Techniques and interpretation of routine blood
cultures. J. Amer. reed. Ass. 149 (1952) 1273-1276. 4. MacGregor, R. R., Beaty, H. N.: Evaluation of positive
bIood cultures. Guidelines for early differentiation of contaminated from valid positive cultures. Arch. Intern. Med. 130 (1972) 84-87. 5. Rosebury, T.: Microorganisms indigenous to man, p. 12. McGraw Hilt Book Co. Inc., New York/Toronto/London
1962. 6. Washington, ]. A.: Evaluation of two commercially avail-
able media for detection of bacteremia. Appl. Microbiol. 23 (1972) 956-959. 7. Wilson, T. S., Stuart, R. D.: Staphylococcus albus in wound infection and in septicemia. Canad. Med. Ass. J. 93 (1965) 8-i6.
Interpretation of Blood Cultures Yielding Staphylococcus aureus Summary: FortY-eight patients with blood cultures positive for Staphylococcus aureus were classified according to clinical criteria in three groups: "definite", "possible", and "doubtful" septicemia. Using traditional blood culture sets with two bottles (thioglycollate and tryptic soy broths), we found that patients with "definite" septicemia always showed more than one positive bottle per day if more than one set was drawn, that the mean detection time was 1.7 days, and that 95% of the first positive bottles and 92% of all positive bottles grew within two clays of incubation. Patients with "doubtful" septicemia were more often (88%) positive in one bottle only, the mean detection time for all bottles was 3.7 days, and only 35°/o of the first positive bottles and 33°/0 of all positive bottles yielded growth within two days. "Possible" cases took a position between tl~Lesetwo extremes but tended more towards the "doubtful" cases. The implications of these findings for the interpretation of blood cultures with S. aureus are discussed.
Zusammen/assung: Interpretation von Blutkulturen mit Staphylococcus aureus. 48 Patienten, aus deren Blutkulturen Staphylococcus aureus geziJchtet worden war, wurden nach klinischen Kriterien in drei Gruppen eingeteilt: ,,unzweifelhafte", ,,m/Sgliche" und ,,zweifelhafte" Septik~imie. Bet Verwendung eines traditionellen Blutkultursystems mit zwei Flaschen pro Kultur (Thioglykolat- und Tryptic Soy-Bouillons) ergab sieh, dab Patienten mit ,,unzweifelhafter" Septik~imie stets mehr als eine positive Flasche pro Tag aufwiesen - sofern mehr als eine Kultur pro Tag entnommen worden war -, dab die mittlere Bebrtitungsdauer bis zur Positivit~it 1,7 Tage betrug, und dab 95o/o der ersten positiven Flaschen und 920/0 alter positiven Flaschen innerhatb von zwei Tagen Wachstum yon S. aureus zeigten. Patienten mit ,,zweifelhafter" Septik~imie zeigten hiiufiger (in 88'%) Wachstum in nur ether Flasche, die mittlere Bebrtitungsdauer betrug 3,7 Tage, und nut 350/0 der ersten positiven Flaschen und 33% aller positiven Flaschen ergaben Wachstum innerhalb yon zwei Tagen. ,,M6gliche" Septik~imien nahmen eine Zwischenstellung ein, tendierten jedoch mehr nach der ,,zweifelhaften" Kategorie. Folgerungen fiir die Interpretation yon Blutkulturen mit S. aureus werden diskutiert.
T h e presence of Staphylococcus aureus or Staphylococcus epidermidis in a blood culture may signify either staphylococcal bacteremia or contamination from areas in which staphylococci normally occur, such as the skin (5) or the inanimate environment. In our experience as well as in the experience of others (7), S. epidermidis strains found in only one bottle of a set (i. e., two bottles taken at the same time) are clinically insignificant. In this study, we tried to ascertain whether this also holds true for S. aureus, and whether there is a relationship between the time of appearance of S. aureus in a blood culture and its clinical significance.
signs of growth. If such signs were present, a gram-stained smear of both bottles of the set was made. If staphylococci were seen in one or both bottles, both were subcultured to blood agar (Tryptic Soy Agar* with 5% sheep blood) which was incubated aerobically for 24-48 hours at 37 °C. Antimicrobial susceptibility testing with Sensi Discs* was done by the Kirby-Bauer method. If no macroscopic signs of growth were found, routine subcultures were made to chocolate agar (GC Agar Base** with 8% heated sheep blood and 1% Isovitalex Enrichment**) after one and six days of incubation. The first routine subculture of the TSB was incubated in a 5-100/0 CO 2 atmosphere for 24 hours at 37 °C, while the second routine TSB and all routine Thio subcultures were incubated anaerobically (GasPak 100 Anaerobic System**) for 48 hours. S. aureus and S. epidermidis were differentiated by the coagulase test (Coagulase Plasma, Rabbit**). Laboratory records over the nine-month period from I July 1975 through 31 March 1976 were reviewed to identify hospital inpatients with blood cultures positive for S. aureus (henceforth called "positive" blood cultures, in contrast to "negative" cultures with no growth or growth of bacteria other than S. aureus). Cultures of patients under five years of age were excluded since the full volume of blood normally inoculated into a set of blood culture bottles could not always be drawn from members of that group. The hospital charts of the patients with positive blood cultures were then reviewed
Materials and Methods Five ml of venous blood were inoculated into each of two bottles of a set of culture media prepared in the Culture Media Laboratory at Yale-New Haven Hospital. The set consisted of a vented bottle containing Tryptic Soy Broth* (TSB) and an unvented bottle containing Thioglycollate Medium without Indicator 135-C (Thio)**. Both bottles were supplemented with 0.03°/0 sodium polyanethol sulfonate***. Prior to drawing the blood, the patient's skin was prepared with an iodophor and 70°/0 ethyl alcohol. Multiple blood cultures taken on the same day were spaced over a period of several hours. Cultures were incubated without shaking for eight days at 37 °C. All bottles were checked twice daily for macroscopic * Difco Laboratories, Detroit, Mich. ** BioQuest, Cockeysville, Md. *** Analabs, North Haven, Conn.
Received: 5 April 1977 Dr. R. A. Horvitz, Prof. A. von Graevenitz, Department of Laboratory Medicine, Yale University School of Medicine, New Haven, Conn., USA. Reprints: Dr. A. yon Graevenitz, 789 Howard Avenue, New Haven, Conn. 06504, USA.
Infection 5 (t977) Nr. 4
207
R. A. Horvitz, A. yon Graevenitz: Interpretation of Blood Cultures Yielding Staphylococcus aureus
to record subsequent blood cultures and cultures from other sites, and to determine the clinical significance of the isolate. Criteria of "definite" septicemia included: a. fever over 38.5 °C, chills, and leukocytosis, b. absence of any other (nonstaphytococcal) infectious process to which the symptoms listed in (a.) could be attributed, c. either (i), the clinical impression of gram-positive septicemia as documented in the progress notes (in which factors predisposing to staphylococcal infections such as i. v. cannulation, i.v. drug abuse, defects in neutrophilic function, renal or hepatic failure, vascular insufficiency, and diabetes mellitus figured as well); or (ii), evidence of an infection with S. aureus at another site, such as pneumonia, empyema, skin or catheter infection, abscesses, or endocarditis. Patients who showed the symptoms listed above in (a.) but who either had other concomitant (nonstaphylococcal) infections to which these symptoms could be attributed, or who could not impress the clinical staff as having outright staphylococcal septicemia were classified in the category of "possible" septicemia. Finally, patients who presented only with fever (with or without leukocytosis) or with none of the symptoms listed in (a.) but who showed other infectious nonstaphylococcal processes, did not give the impression of having staphylococcal septicemia, and did not have S. aureus cultured from any other site, were classified as cases of "doubtful" septicemia. In these cases, the finding of S. aureus could obviously not be interpreted as a sign of "definite" or even "possible" septicemia. A "positive set" was defined as a set of blood culture bottles in which either one or both bottles were positive for S. aureus. For our study, we started the count with the first positive set and included all sets-positive or negative-that were obtained within 96 hours following the last positive set. "Detection time" was defined as the number of days between, the time of incubation of the blood culture and the time of positivitymacroscopically, in the gram-stained smear, or in the routine subculture. It was rounded off to the nearest "reading" day; e. g. bottles read as positive on the day following incubation at 8:00 a.m. or at 5:00 p.m. were both said to have had a detection time of one day. Detection times for six-clay subcultures were listed as seven days. Statistical significance was determined with the X ~ test. Results
A total of 55 charts of patients with one or more positive sets (with identical sensitivities per patient) coMd be obtained for review. Two patients were excluded because their cultures had been drawn through obviously contain-
inated sites, one patient died of other causes before sufficient d a t a could be obtained to assess the category of septicemia, and four patients were eliminated due to inadequate records. The remaining 48 patients constituted the study group. Table 1 shows that 20 patients fetl into the "definite", 11 into the "possible", and 17 into the "doubtful" categories. The percentages of patients from whom muttiple sets were drawn was not significantly different (p > 0.05) between the categories. Other (nonstaphylococcai) infections were more frequently present in the "doubtful" and "possible" groups than in the "definite" one. The percentage of patients started on antibiotics (effective in vitro against the S. aureus strain) after the first blood culture was drawn was significantly lower in the "doubtful" group compared to the others-presumably a reflection of the therapeutic consequences drawn from the clinical impression. Likewise, the less frequent isolation of S. aureus from other body sites in the "doubtful" group may in part be due to the fact that patients in this group were probably less thoroughly investigated for S. aureus elsewhere than patients in the other groups. None of the patients with "definite" septicemia yielded only one positive bottle out of one or several sets taken on the same day, in contrast to 45'% of those with "possible" and 88% of those with "doubtful" septicemia. A breakdown by media shows significant (p < 0.01) differences for TSB between the "definite" and the other two categories; for Thio, a less significant (p < 0.05) difference was found between the "doubtful" and the "definite" categories. Table 2 breaks down the data for sets. The "definite" category showed more sets positive in both bottles and less sets positive in one bottle only than the other categories (denominator: all positive sets per category). The six sets from patients with "definite" septicemia which were positive in one bottle only were accompanied by other positive sets on the same day, as outlined in Table 1. Broken down by media, the differences are similar to those listed in Table 1 except for the degree of significance for some comparative figures. Table 2 also shows
Table t: Number of patients Category' of septicemia
Definite Possible Doubtful
Total patients
20 11 17
Patients with muttiple sets
20 8 16
Patients with other infections
0 7* 17", ***
* p < 0.01 compared to "definite" category. ** p < 0.01 compared to "possible" category. *** p < 0.05 compared to "possible" category.
208
InfectiOn 5 (1977) Nr. 4
Patients treated with antibiotics
20 11 6", **
Patients with S. aureus from other sites 9 6 0', **
Patients with positive bottles on same day 1 btl > 1 btl of 2 or > 2
Patients positive in one btl only (of all btls drawn)
Total 0
5* I5"
20
6 2
0
5* 15", ***
+ p < 0.05 compared to "definite" category.
TSB 0
4* 10"
Thio 0
1 5+
R. A. Horvitz, A. yon Graevenitz: Interpretation of Blood Cultures Yielding Staphylococcus aureus Table 2: Number of blood culture sets Category of septicemia
Definite Possible Doubtful Total
Total sets taken
76 27 50 153
Positive in one or both bottles
Positive in both bottles only
Positive in one bottle only Total
TSB
66 14 18 98
60 7* 1', ** 68
6 7* 17", ** 30
2 4 6* 1 12", ** 5* 20 10
Positive bottles
Thio
Total
TSB
Thio
126 21 19 166
62 13 13 88
64 8 6 78
Table 3: Detection times of pos#ive bottles Category of septicemia DefiniteTSB Thio 2 day Total 3 day Total Grand Total PossibleTSB Thio 2 day Total 3 day Total Grand Total DoubtfulTSB Thio 2 day Total 3 day Total Grand Total
1
2
26 24
30 35 115
Bottles positive on day 3 4 5 6
4 3
0 2
1 0
0 0
7
0 0
Bottles of positive sets Total Positive Negative
66 66
61 64
Mean detection time (days)
5 2
1.7 1.7
1 6
3.5* 3.4*
5 12
3.7* 3.7*
i22 125
1 1
6 4 12"
0 0
i 1
3 0
1 0
1 2
14 14
13 8
12" 21
0 0
2 4 6*
4
3
0
0
0 0
0 0
4 2
18 18
13 6
i0'
that the percentages of positive Thio (78/98) and TSB (88/98) bottles were not significantly (p > 0.05) different, i. e., neither medium grew S. aureus preferentially. Table 3 shows that the mean detection times, while not different for TSB and Thio within each category, were twice as long (3.4-3.7 days) in the "possible" and "doubtful" categories than in the "definite" category (1.7 days). Within two days of incubation 920/0, and within three days 97.50/0 of the eventually positive bottles became positive in patients with "definite" septicemia, versus 5 7 % for both dates in patients with "possible" and 33~/0 and 520/0 in patients with "doubtful" septicemia. Table 4 demonstrates that none of the patients with "definite" septicemia yielded their first positive bottle later than three days after incubation, with 19 of 20 (95'°/0) having become positive within two days. In the "possible" and "doubfful"eategories, detection times for the first positive bottle showed a much greater variation: withia two days of incubation, 6 4 ' 0 in the "possible" and 3 5 % in the "doubtful" category had become positive. Only the latter figure is significantly different from the "definite" group. Mean detection times were, again, at least twice as long as in the "definite" group.
19 Table 4: Detection times of first positive bottle Category of septicemia
1
Definite Possible Doubtful
Mean detection time (days)
Detection time (days) of first positive bottle
8 1 0
2
3
4
5
6
11 1 6 0 6* 3
0 0 3
0 2 0
0 0 0
0 2 5
1.7 3.4* 4.0*
Discussion
The blood culture system used in this study is widely employed at present (1). W i t h the same system, Washington (6) also found no differences between TSB and Thio in the ability to detect S. aureus. Using eight ml of blood divided between a p o u r plate and a bottle of trypticase soy yeast broth, MacGregor and Beaty (4) found three out of 28 isolates of S. aureus to be "contaminants", presumably due, by and large, to laboratory handling. That figure is not significantly different from our number of isolates from "doubtful" cases (19 of 166) although we would not infer that skin contamination did not play a role.
Infection 5 (1977) Nr, 4
209
R. A. Horvitz, A. yon Graevenitz: Interpretation of Blood Cultures Yielding Staphylococcus aureus
The fact that the ctinical symptomatology of staphylococcal septicemia is not as clear-cut as the symptomatology of gram-negative septicemia (2) made it necessary partially to rely in our study on "clinical impressions". We felt, therefore, that introduction of the category of "possible" septicemia would improve accuracy by a sharper delineation of the "definite" and "doubtful" categories. Two of Kotin's (3) criteria of non-significance, i.e., the finding of the microorganism in one blood culture only (out of several) and its ubiquitous occurrence, could obviously not be used for our study. Neither could we use the relationship between disappearance of S. aureus from the blood and antimicrobial treatment or clinical recovery. All of the patients in the "definite" and "possible" groups and six of the 17 in the "doubtful" group had been on antibiotics. With the exception of those who had only one single blood culture and of those who died from staphylococcal septicemia (two in the "definite" group), all patients showed negative blood cultures after treatment or even (in ten of the 17 "doubtful" cases) without treatment. Since the case-fatality rate of untreated S. aureus septicemia is very high (2), we can only use the latter figure (10/17) as additional evidence that these strains were clinically insignificant. Furthermore, "recovery" in the face of concomitant nonstaphylococcal infections was most difficult to ascertain. Teichoic acid antibodies had not been determined. Thus, patients with "definite" septicemia characteristically showed (a), more than one positive bottle per day if more than one set was drawn, (b), a detection time of one to two, at most three, days for the first positive bottle, and (c), positivity in almost all eventually positive bottles within two days of incubation. These features are also observed in S. epidermidis septicemia (7). Conversely, patients with "doubtful" septicemia showed (a) growth in one bottle only in the majority of cases, (b) a variable detection time for the first bottle, and (c) positivity of all eventually positive bottles only within seven days. "Possible" cases took a position in between the two extremes but tended in most respects more towards the "doubtful" ones.
~10
Infection 5 (1977) Nr. 4
With a view towards interpretation of laboratory findings, we would say that the finding of only one positive bottle out of one or more sets drawn on the same day--provided both bottles of a set have been gram-stained--could at best be indicative of "possible" septicemia but is more likely to be clinically insignificant. Insignificance is even more likely at a time when only the Gram stain has been performed, since S. epidermidis may be present which is much less often significant in blood cultures than S. aureus (1, 2, 4). Lack of clinical significance is also more likely if a set first becomes positive on the third day of incubation or later. On the other hand, positivity within two days of incubation and/or multiple positive sets are more suggestive of "definitive" than of "possible" septicemia with S. aureus (or S. epidermidis [7]) and argue against clinical insignificance.
Literature 1. Bartlett, R. C., Ellner, P. D., Washington, J. A.: Blood cultures. Cumitech 1. American Society for Microbiology, Washington D. C. 1974. 2. Cluff, L. E., Reynolds, R. C., Page, D. L., Breckenridge, 1. L.: StaphylococcaI bacteremia and altered host resistance.
Ann. Intern. Med. 69 (1968) 859-873. 3. Kotin, P.: Techniques and interpretation of routine blood
cultures. J. Amer. reed. Ass. 149 (1952) 1273-1276. 4. MacGregor, R. R., Beaty, H. N.: Evaluation of positive
bIood cultures. Guidelines for early differentiation of contaminated from valid positive cultures. Arch. Intern. Med. 130 (1972) 84-87. 5. Rosebury, T.: Microorganisms indigenous to man, p. 12. McGraw Hilt Book Co. Inc., New York/Toronto/London
1962. 6. Washington, ]. A.: Evaluation of two commercially avail-
able media for detection of bacteremia. Appl. Microbiol. 23 (1972) 956-959. 7. Wilson, T. S., Stuart, R. D.: Staphylococcus albus in wound infection and in septicemia. Canad. Med. Ass. J. 93 (1965) 8-i6.
