DIAGN MICROBIOLINFECTDIS 1992;15:5-11
5
The Impact of Converting to a Biphasic Blood-Culture System on the Overall Cost and the Incidence of Pseudobacteremia Judith S. Heelan, Steven M. Opal, Edward Brissette, and Michael Donahue
After converting from a conventional broth (CB) system to a biphasic (BP) agar-slide blood-culture system (Septi-Chek), our laboratory noted an increase in positive blood cultures in general, and in coagulase-negative staphylococci (CNS) in particular. To investigate these findings, we compared all blood cultures collected over a 21-month period using CB and then BP systems, totaling 28,199 blood cultures. The frequency of positive blood cultures increased from 9.2% to 12.7% (p < 0.0001), whereas CNS isolation increased from 2.6% to 5.2% (p < 0.0001). There was no significant change in the incidence of true primary or secondary bacteremia due to CNS (p = 0.9). The isolation of other pathogens, including Staphylococcus aureus, Candida albicans, Bacteroides species,
and Gram-negative bacilli increased from 6.5% to 7.1% (p < 0.05). We estimated the cost of processing 28,000 blood cultures by both CB and BP systems, using positivity rates of 9.2% and 12.7%, respectively, and standards provided by the College of American Pathologists (CAP, 1991) for workload hours of technologist time. We calculated a higher overall cost for the BP system. However, the use of this system eliminated the use of needles and syringes for subculture of bottles showing no growth, thus decreasing the risk of technologist exposure to body fluids. Despite the increased cost and more frequent occurrence of pseudobacteremia, the enhanced sensitivity and increased safety of the BP system justified its use in the prompt identification of patients with true bacteremia.
INTRODUCTION
Septi-Chek system. A properly collected blood culture represents one of the simplest a n d most useful diagnostic tests available to the clinician for conditions such as endocarditis a n d bacterial p n e u m o n i a . Although m a n y systems for blood cultures are commercially available, the Septi-Chek biphasic system offers the advantage of providing a slide that contains three types of agar a n d is attached to the aerobic bottle (Thompson et al., 1987). The system allows for frequent subculturing, which is important not only as a time-saving technique but as a safety measure, by decreasing exposure of laboratory personnel to h u m a n blood. More rapid identification of microorganisms is also possible due to preliminary identification based u p o n differential growth on the media provided. In recent years, clinical investigators have reported an increase in the isolation of coagulase-negative staphylococd (CNS) from blood cultures (Bryan, 1989). Although these organisms are frequent skin colonizers that are often present as blood-culture
Realizing the high mortality rate from septicemia a n d the importance of quickly providing the physician with accurate information on blood-culture isolates, our laboratory changed from a conventional broth (CB) blood-culture system to the biphasic (BP) From the Department of Pathology (J.S.H., E.B., M.D.), and Division of Infectious Diseases (S.M.O.), Memorial Hospital of Rhode Island, Pawtucket; and the Department of Pathology and Laboratory Medicine (J.S.H.), and Brown University Program in Medicine (S.M.O.), Brown University, Providence, Rhode Island, USA. This work was presented in part at the 90th annual Meeting of the American Society for Microbiology, 13-17 May 1990, Anaheim, California, USA [abst C286]. Address reprint requests to Dr. J.S. Heelan, Department of Pathology, Memorial Hospital of Rhode Island, 111 Brewster Street, Pawtucket, RI 02860, USA. Received 18 October, 1990; revised and accepted 14 March 1991. © 1992 Elsevier Science PublishinZ Co., Inc. 655 Avenue of the Americas, New York, NY 10010 0732-8893192/$3.50
6
contaminants, they may also be associated with significant bacteremia. Numerous recent studies have shown nosocomial CNS bacteremia to be a growing problem in the neonatal intensive care unit, the surgical patient, and the neutropenic cancer patient (Donowitz et al., 1987; Sidebottom et al., 1988; Stillman et al., 1987; Wade et al., 1982). None of these studies analyzed the changes in blood-culture procedures and their association with the observed increase in CNS isolation. In this study, we sought to determine the incidence of pseudobacteremia due to CNS and other skin contaminants using the more sensitive biphasic system as compared with the conventional broth system. We also wished to make a cost comparison between the two systems. MATERIALS AND METHODS
Study Population Memorial Hospital is a 300-bed acute care, general medical-surgical teaching hospital associated with the Brown University Program in Medicine. This study is based on results of blood cultures collected mostly from adult patients at this institution over a 42-month period, beginning in January 1984 and running through June 1987. The conventional broth system was employed from January 1984 through September 1985 (21 months) yielding a total of 13,963 cultures. The biphasic Septi-Chek System was used to collect blood from October 1985 through June 1987 (21 months) for a total of 14,236 cultures.
Collection and Processing of Specimens The conventional broth system included an aerobic bottle containing 50 ml of trypticase soy broth (TSB) and an anaerobic bottle containing 50 ml of thioglycollate broth (Difco Laboratories, Detroic, MI, USA). Blood was collected by venipuncture using aseptic technique, and 5 ml was added to each bottle. Bottles were incubated at 35°C for 1 week and subcultured on days 2 and 7. The aerobic bottle was subcultured to 5% sheep blood agar and chocolate agar, which were incubated in 5% CO2 for 48 hr. The anaerobic bottle was subcultured onto 5% sheep blood agar that was incubated anaerobically for 48 hr at 35°C. In our prospective study involving the processing of 28,000 blood cultures, the aerobic bottle would be subcultured only to chocolate agar which should allow recovery of all significant pathogens. The subculture of the anaerobic bottle would be omitted, as it has been shown that the detection of anaerobes is not significantly improved by blind subculture (Murray et al., 1978). The Septi-Chek (Roche Diagnostics, Division Hoffman-La Roche, Nutley, NJ, USA) biphasic
J.S. Heelan et al.
system included an aerobic bottle containing 70 ml TSB with 0.05% sodium polyanetholsulfonate to which a three-sided agar-slide paddle containing chocolate, MacConkey, and malt agars was attached. The assembled unit (slide and bottle) was inverted immediately, and twice daily thereafter to subculture, allowing the blood-broth mixture to cover the agar-coated paddle. The anaerobic bottle contained thioglycollate broth. After venipuncture, 7 ml of blood were added to each bottle. All bottles were incubated for 7 days at 35°C. To rule out the probability that contaminants were introduced into the system when attaching the slide, we randomly added slides to uninoculated bottles. A total of 42 slides were attached to 42 TSB bottles, the bottles inverted and incubated as described. Clinical significance of isolates of CNS and other skin contaminants was determined by the infection control coordinator using established criteria by review of patient charts. Other laboratory and clinical information included the presence of intravascular devices, signs of infection, such as fever and leukocytosis, and multiple isolates having identical susceptibility patterns. True CNS bacteremias were identified as being primary if associated with intravascular devices and as being secondary if associated with colonization from sputum, urine, and other host sites (Kirchhoff and Sheagren, 1985). All blood cultures that were not sterile, having growth in one or in both bottles, were considered to be positive. Microorganisms were isolated and identified using standard procedures (Lennett et al., 1985). Nonstaphylococcal skin contaminants included Bacillus spp. and Corynebacterium spp. and were rarely isolated (0.31% of all blood cultures). These organisms were not considered to be significant pathogens.
Cost Analysis This analysis was made based on the average technologist's salary and the College of American Pathologists (CAP, 1991) suggested workload units as well as the initial cost of blood-culture sets (two botties) and subsequent media used.
Statistical Analysis Differences in proportions were calculated by chisquare analysis of contingency tables. A p value of 50,000 needles and syringes (two subcultures per bottle) used to process the 25,424 negative cultures.
DISCUSSION Blood cultures have long been used by clinicians in the diagnosis of septicemia. However, interpretation of positive blood cultures is not always readily apparent. Contamination may occur in many ways during the processing of blood cultures. The clinician must consider several parameters when evaluating the significance of the isolate and in order to determine whether the result is a "true positive" (Bryan, 1989). The word pseudobacteremiais used to describe positive blood cultures thought not to be attributable to the presence of bacteria in the bloodstream (Reuben et al., 1989). Isolates frequently considered to be contaminants include coagulase-negative staphylococci (CNS), Bacillus species, and Corynebacterium species (diphtheroids). Contaminants may be present initially in the culture bottle, or may be introduced
Humber of Baoteremias 10 9 8
Change from oonventlonal tO 8eptl-¢hek system
Jan 1984
Apt
JuI
Oct
Jan Apt 1986
Total
~
JuI
0or
Primary
Jan 1986
[
Apr
Jul
Oct
Jan Apr 1987
] Secondary
FIGURE 3 Incidence of true coagulase-negative staphylococcal bacteremia over a 42-month period. Primary bacteremias were associated with intravascular devices, whereas secondary bacteremias were associated with colonization from sputum, urine, and other host sites. The results are expressed in 3-month increments.
Conversion to a Biphasic Blood-Culture System
TABLE 1
Study
9
Results of Blood Cultures Collected During the 42-Month Study Period
Total Number
Positives
CoagulaseNegative Staphylococcus
Other Skin Contaminants
Significant Pathogens
Ia
13,963
1281 (9.2)%
362 (2.6%)
17 (0.12%)
902 (6.5%)
2b
14,236
1812 (12.7%)
738 (5.2%)
70 (0.49%)
1004 (7.1%)
Total 1 + 2
28,199
3093 (11.0%)
1100 (3.9%)
(0.31%)
87
1906 (6.8%)
aJanuary 1984to September 1985 (21 mo) conventional broth system (CB). bOctober 1985to June 1987(21 mo) Septi-Chek biphasic system (BP).
important, but often difficult. CNS are common causes of prosthetic valve endocarditis, and ventriculoperitoneal shunts may become colonized and ultimately infected with the organism. The ability of these bacteria to cause delayed infection after the placement of orthopedic devices or vascular grafts seems to be due in part to their ability to survive in a protective biofilm (Bryan, 1989). Slime-layer production has been associated with virulence. This protective coating appears to shield the organism from phagocytosis (Patrick, 1990). Sepsis in compromised hosts is also associated with this organism (Archer, 1985). Detection of growth in
5
The Impact of Converting to a Biphasic Blood-Culture System on the Overall Cost and the Incidence of Pseudobacteremia Judith S. Heelan, Steven M. Opal, Edward Brissette, and Michael Donahue
After converting from a conventional broth (CB) system to a biphasic (BP) agar-slide blood-culture system (Septi-Chek), our laboratory noted an increase in positive blood cultures in general, and in coagulase-negative staphylococci (CNS) in particular. To investigate these findings, we compared all blood cultures collected over a 21-month period using CB and then BP systems, totaling 28,199 blood cultures. The frequency of positive blood cultures increased from 9.2% to 12.7% (p < 0.0001), whereas CNS isolation increased from 2.6% to 5.2% (p < 0.0001). There was no significant change in the incidence of true primary or secondary bacteremia due to CNS (p = 0.9). The isolation of other pathogens, including Staphylococcus aureus, Candida albicans, Bacteroides species,
and Gram-negative bacilli increased from 6.5% to 7.1% (p < 0.05). We estimated the cost of processing 28,000 blood cultures by both CB and BP systems, using positivity rates of 9.2% and 12.7%, respectively, and standards provided by the College of American Pathologists (CAP, 1991) for workload hours of technologist time. We calculated a higher overall cost for the BP system. However, the use of this system eliminated the use of needles and syringes for subculture of bottles showing no growth, thus decreasing the risk of technologist exposure to body fluids. Despite the increased cost and more frequent occurrence of pseudobacteremia, the enhanced sensitivity and increased safety of the BP system justified its use in the prompt identification of patients with true bacteremia.
INTRODUCTION
Septi-Chek system. A properly collected blood culture represents one of the simplest a n d most useful diagnostic tests available to the clinician for conditions such as endocarditis a n d bacterial p n e u m o n i a . Although m a n y systems for blood cultures are commercially available, the Septi-Chek biphasic system offers the advantage of providing a slide that contains three types of agar a n d is attached to the aerobic bottle (Thompson et al., 1987). The system allows for frequent subculturing, which is important not only as a time-saving technique but as a safety measure, by decreasing exposure of laboratory personnel to h u m a n blood. More rapid identification of microorganisms is also possible due to preliminary identification based u p o n differential growth on the media provided. In recent years, clinical investigators have reported an increase in the isolation of coagulase-negative staphylococd (CNS) from blood cultures (Bryan, 1989). Although these organisms are frequent skin colonizers that are often present as blood-culture
Realizing the high mortality rate from septicemia a n d the importance of quickly providing the physician with accurate information on blood-culture isolates, our laboratory changed from a conventional broth (CB) blood-culture system to the biphasic (BP) From the Department of Pathology (J.S.H., E.B., M.D.), and Division of Infectious Diseases (S.M.O.), Memorial Hospital of Rhode Island, Pawtucket; and the Department of Pathology and Laboratory Medicine (J.S.H.), and Brown University Program in Medicine (S.M.O.), Brown University, Providence, Rhode Island, USA. This work was presented in part at the 90th annual Meeting of the American Society for Microbiology, 13-17 May 1990, Anaheim, California, USA [abst C286]. Address reprint requests to Dr. J.S. Heelan, Department of Pathology, Memorial Hospital of Rhode Island, 111 Brewster Street, Pawtucket, RI 02860, USA. Received 18 October, 1990; revised and accepted 14 March 1991. © 1992 Elsevier Science PublishinZ Co., Inc. 655 Avenue of the Americas, New York, NY 10010 0732-8893192/$3.50
6
contaminants, they may also be associated with significant bacteremia. Numerous recent studies have shown nosocomial CNS bacteremia to be a growing problem in the neonatal intensive care unit, the surgical patient, and the neutropenic cancer patient (Donowitz et al., 1987; Sidebottom et al., 1988; Stillman et al., 1987; Wade et al., 1982). None of these studies analyzed the changes in blood-culture procedures and their association with the observed increase in CNS isolation. In this study, we sought to determine the incidence of pseudobacteremia due to CNS and other skin contaminants using the more sensitive biphasic system as compared with the conventional broth system. We also wished to make a cost comparison between the two systems. MATERIALS AND METHODS
Study Population Memorial Hospital is a 300-bed acute care, general medical-surgical teaching hospital associated with the Brown University Program in Medicine. This study is based on results of blood cultures collected mostly from adult patients at this institution over a 42-month period, beginning in January 1984 and running through June 1987. The conventional broth system was employed from January 1984 through September 1985 (21 months) yielding a total of 13,963 cultures. The biphasic Septi-Chek System was used to collect blood from October 1985 through June 1987 (21 months) for a total of 14,236 cultures.
Collection and Processing of Specimens The conventional broth system included an aerobic bottle containing 50 ml of trypticase soy broth (TSB) and an anaerobic bottle containing 50 ml of thioglycollate broth (Difco Laboratories, Detroic, MI, USA). Blood was collected by venipuncture using aseptic technique, and 5 ml was added to each bottle. Bottles were incubated at 35°C for 1 week and subcultured on days 2 and 7. The aerobic bottle was subcultured to 5% sheep blood agar and chocolate agar, which were incubated in 5% CO2 for 48 hr. The anaerobic bottle was subcultured onto 5% sheep blood agar that was incubated anaerobically for 48 hr at 35°C. In our prospective study involving the processing of 28,000 blood cultures, the aerobic bottle would be subcultured only to chocolate agar which should allow recovery of all significant pathogens. The subculture of the anaerobic bottle would be omitted, as it has been shown that the detection of anaerobes is not significantly improved by blind subculture (Murray et al., 1978). The Septi-Chek (Roche Diagnostics, Division Hoffman-La Roche, Nutley, NJ, USA) biphasic
J.S. Heelan et al.
system included an aerobic bottle containing 70 ml TSB with 0.05% sodium polyanetholsulfonate to which a three-sided agar-slide paddle containing chocolate, MacConkey, and malt agars was attached. The assembled unit (slide and bottle) was inverted immediately, and twice daily thereafter to subculture, allowing the blood-broth mixture to cover the agar-coated paddle. The anaerobic bottle contained thioglycollate broth. After venipuncture, 7 ml of blood were added to each bottle. All bottles were incubated for 7 days at 35°C. To rule out the probability that contaminants were introduced into the system when attaching the slide, we randomly added slides to uninoculated bottles. A total of 42 slides were attached to 42 TSB bottles, the bottles inverted and incubated as described. Clinical significance of isolates of CNS and other skin contaminants was determined by the infection control coordinator using established criteria by review of patient charts. Other laboratory and clinical information included the presence of intravascular devices, signs of infection, such as fever and leukocytosis, and multiple isolates having identical susceptibility patterns. True CNS bacteremias were identified as being primary if associated with intravascular devices and as being secondary if associated with colonization from sputum, urine, and other host sites (Kirchhoff and Sheagren, 1985). All blood cultures that were not sterile, having growth in one or in both bottles, were considered to be positive. Microorganisms were isolated and identified using standard procedures (Lennett et al., 1985). Nonstaphylococcal skin contaminants included Bacillus spp. and Corynebacterium spp. and were rarely isolated (0.31% of all blood cultures). These organisms were not considered to be significant pathogens.
Cost Analysis This analysis was made based on the average technologist's salary and the College of American Pathologists (CAP, 1991) suggested workload units as well as the initial cost of blood-culture sets (two botties) and subsequent media used.
Statistical Analysis Differences in proportions were calculated by chisquare analysis of contingency tables. A p value of 50,000 needles and syringes (two subcultures per bottle) used to process the 25,424 negative cultures.
DISCUSSION Blood cultures have long been used by clinicians in the diagnosis of septicemia. However, interpretation of positive blood cultures is not always readily apparent. Contamination may occur in many ways during the processing of blood cultures. The clinician must consider several parameters when evaluating the significance of the isolate and in order to determine whether the result is a "true positive" (Bryan, 1989). The word pseudobacteremiais used to describe positive blood cultures thought not to be attributable to the presence of bacteria in the bloodstream (Reuben et al., 1989). Isolates frequently considered to be contaminants include coagulase-negative staphylococci (CNS), Bacillus species, and Corynebacterium species (diphtheroids). Contaminants may be present initially in the culture bottle, or may be introduced
Humber of Baoteremias 10 9 8
Change from oonventlonal tO 8eptl-¢hek system
Jan 1984
Apt
JuI
Oct
Jan Apt 1986
Total
~
JuI
0or
Primary
Jan 1986
[
Apr
Jul
Oct
Jan Apr 1987
] Secondary
FIGURE 3 Incidence of true coagulase-negative staphylococcal bacteremia over a 42-month period. Primary bacteremias were associated with intravascular devices, whereas secondary bacteremias were associated with colonization from sputum, urine, and other host sites. The results are expressed in 3-month increments.
Conversion to a Biphasic Blood-Culture System
TABLE 1
Study
9
Results of Blood Cultures Collected During the 42-Month Study Period
Total Number
Positives
CoagulaseNegative Staphylococcus
Other Skin Contaminants
Significant Pathogens
Ia
13,963
1281 (9.2)%
362 (2.6%)
17 (0.12%)
902 (6.5%)
2b
14,236
1812 (12.7%)
738 (5.2%)
70 (0.49%)
1004 (7.1%)
Total 1 + 2
28,199
3093 (11.0%)
1100 (3.9%)
(0.31%)
87
1906 (6.8%)
aJanuary 1984to September 1985 (21 mo) conventional broth system (CB). bOctober 1985to June 1987(21 mo) Septi-Chek biphasic system (BP).
important, but often difficult. CNS are common causes of prosthetic valve endocarditis, and ventriculoperitoneal shunts may become colonized and ultimately infected with the organism. The ability of these bacteria to cause delayed infection after the placement of orthopedic devices or vascular grafts seems to be due in part to their ability to survive in a protective biofilm (Bryan, 1989). Slime-layer production has been associated with virulence. This protective coating appears to shield the organism from phagocytosis (Patrick, 1990). Sepsis in compromised hosts is also associated with this organism (Archer, 1985). Detection of growth in
