Critical assessment of blood culture techniques: analysis of recovery of obligate and facultative anaerobes, strict aerobic bacteria, and fungi in aerobic and anaerobic blood culture bottles

Routine use of anaerobic blood cultures: are they still indicated?

PURPOSE

To determine the number of patients with bacteremia and fungemia and to evaluate the utility of routine anaerobic blood cultures as part of the work-up for suspected bacteremia.

SUBJECTS AND METHODS

Retrospective review of microbiology data followed by selective chart review at a university-affiliated Veterans Affairs Medical Center. We determined the number of bacterial blood cultures drawn from January 1, 1994, to December 31, 1996, and the number of anaerobic, aerobic, and fungal isolates. Chart reviews were then performed on all patients with a positive anaerobic result.

RESULTS

There were 6,891 sets of blood cultures processed through the laboratory, yielding 1,626 patients with positive results. Anaerobic isolates were recovered from 36 patients (2.2%) in 48 bottles. Aerobic isolates were recovered from 1550 patients (95.3%), and fungal isolates were recovered from 40 patients (2.5%). Seven patients (0.4%) had true anaerobic bacteremia. All seven patients had an obvious source of anaerobic infection that was known or suspected before the cultures were drawn. Antibiotic changes were made in four of these patients after the positive anaerobic results were known. Antibiotic changes led to clinical improvement in one patient.

CONCLUSIONS

Routine use of anaerobic blood cultures rarely results in clinically important diagnostic or therapeutic benefits, based on the low incidence of anaerobic bacteremia in patients who are not at increased risk. Anaerobic blood cultures should be selectively ordered in patients at risk for anaerobic infections.

Clinical value of anaerobic blood culture: a retrospective analysis of positive patient episodes

AIM

To investigate the clinical value of anaerobic blood culture.

METHODS

Blood culture bottles (n = 25,185) submitted for culture over a two year period were reviewed.

RESULTS

The bottles yielded 1992 positive patient episodes, a positive rate of 14.4/1000 hospital admissions. Significantly more isolations were obtained from aerobic than from anaerobic bottles. Twelve of the 38 anaerobic episodes were detected in aerobic bottles. Clinical management was influenced in one of 24 patients whose cultures yielded anaerobes from anaerobic bottles only. For a further six patients it was unlikely that the result had any effect on clinical management.

CONCLUSIONS

If aerobic bottles were substituted for the anaerobic bottles, detection of positive patient episodes would increase by at least 6%. A higher yield would be achieved by using two aerobic bottles for routine culture and using anaerobic bottles only for patients where anaerobic culture may influence clinical management.

Bacteroides fragilis bacteremia and infected aortic aneurysm presenting as fever of unknown origin: diagnostic delay without routine anaerobic blood cultures

We report the case of a 71-year-old male with Bacteroides fragilis bactermia and infected aortic aneurysm that went undiagnosed, in part, because routine anaerobic blood cultures were not obtained. Bacteremia caused by anaerobes has been reported to be declining, and recommendations to discontinue routine anaerobic blood cultures have been implemented in some hospitals. To our knowledge, this is the first report of an anaerobic bacteremia and infection that had a delay in diagnosis due to this change in blood-culturing protocol. The potential impact of deleting anaerobic blood cultures from routine protocols is discussed.

Reassessment of the routine anaerobic culture and incubation time in the BacT/Alert FAN blood culture bottles

A total of 9,130 blood cultures were collected from adult patients with suspected bloodstream infections. The recommended 20 mL sample of blood was divided equally between the aerobic and anaerobic FAN bottles and monitored in the BacT/Alert Microbial Detection System for a total of 5 days. There were 757 clinically significant positive culture pairs from 291 patients. Significant differences were found with greater recovery of Pseudomonas aeruginosa (p < 0.001), Acinetobacter spp. (p = 0.002), coagulase-negative staphylococci other than Staphylococcus epidermidis (p = 0.002), and Candida spp. (p < 0.001) from the aerobic bottle and greater recovery of anaerobic bacteria (p < 0.001) from the anaerobic bottle. Significantly more episodes of P. aeruginosa bacteremia (p < 0.003) and candidemia (p < 0.001) were detected by the aerobic FAN bottle and significantly more episodes of anaerobic bacteremia (p < 0.001) were detected by the anaerobic FAN bottle (Table 2). No other significant differences between systems in their detection of bacteremias were noted. Anaerobic bacteremias were encountered in diverse and often unpredictable clinical settings. All clinically significant episodes of bloodstream infection were detected within 4 days of incubation of their cultures. We conclude routine, rather than selective, use of the anaerobic FAN bottle in the blood culture set and a 4-day incubation of blood cultures in the BacT/Alert aerobic and anaerobic FAN bottles is an appropriate routine procedure.

Reassessment of the incubation time in a controlled clinical comparison of the BacT/Alert aerobic FAN bottle and standard anaerobic bottle used aerobically for the detection of bloodstream infections

This study assessed the minimum incubation time required to detect bloodstream infections during a controlled clinical comparison of the performance characteristics of the BacT/Alert aerobic FAN bottle and the standard anaerobic bottle used aerobically except on a selective basis. Blood was collected from adults with suspected bloodstream infections and inoculated into each bottle, which was monitored in the BacT/Alert Microbial Detection System. The anaerobic bottle was vented before incubation except when cultures were obtained from patients on the colorectal and gynecologic surgical and emergency services. Statistical analysis was limited to those culture sets in which each bottle was inoculated with > or = 8 mL of blood and bacterial growth was considered to be clinically significant. A total of 682 positive cultures from 243 patients satisfied the inclusion criteria. Significantly more isolates of Staphylococcus aureus (p < 0.001), S. epidermidis (p < 0.001), other coagulase-negative staphylococci (p < 0.001), Enterococcus spp. (p = 0.04), Escherichia coli (p = 0.03), all Enterobacteriaceae (p < 0.001), Pseudomonas aeruginosa (p = 0.001), and Candida spp. (p < 0.001) were detected by the aerobic FAN bottle. Significantly more septic episodes due to S. aureus, S. epidermidis, other coagulase-negative staphylococci, Enterobacteriaceae, P. aeruginosa, and Candida spp. were detected by the aerobic FAN bottle. Significantly more bacterial isolates were detected by the aerobic FAN whether or not antibiotics were being administered at the time of blood culture, whereas there were significantly fewer positive cultures in the vented standard anaerobic bottle when patients were receiving antimicrobial therapy than when they were not. All but 5% of positive cultures were detected within three days. Only six of the cultures requiring four or five days of incubation represented true misses, and only one of these six resulted in a change in therapy which, however, did not affect the patent's outcome.

Effect on organism recovery rate from BacT/Alert blood cultures with reduced incubation period

This retrospective study evaluated 15,377 sets of BacT/Alert blood cultures to determine incubation time for blood cultures. Ninety-six per cent (1476) of total isolates signalled positive within five days and 56 isolates turned positive in five to seven days. Of the 56 organisms recovered between five and seven days, 49 were considered contaminants and seven were considered clinically significant. On assessing the medical records of the patients with the seven clinically significant isolates, it was determined that the clinical outcome would not have changed if these isolates were missed. We conclude that a five day incubation protocol reduces the recovery of skin contaminants while not significantly decreasing the recovery of clinically significant organisms. The data suggest that the incubation time can be further reduced but this policy will depend on the individual institution and their patient population mix.

Comparative study of three different BACTEC culture media for the detection of bacteremia in ambulatory and hospitalized children

To compare the yield of two aerobic and an anaerobic BACTEC blood culture media in detecting bacteremia in ambulatory and hospitalized care settings at a children's hospital, a prospective cohort study was completed. Over an 18-month period, equal blood volumes (minimum of 1 mL/bottle) were inoculated into a three-bottle culture set including aerobic BACTEC NR 6A, aerobic BACTEC PEDS Plus and anaerobic NR 7A broths. Chart reviews were completed on all children with bacteremia to determine whether the isolate was clinically significant based on predefined criteria. Among 5328 evaluable blood culture sets, 323 clinically significant organisms (110 from ambulatory and 213 from hospitalized children) were isolated. Most Streptococcus pneumoniae, Haemophilus species, and Neisseria or Moraxella species were recovered from children attending the emergency department or out-patient clinics. Important isolates in hospitalized children included most of the staphylococci and Enterobacteriaceae, and all group D enterococci, Gram-negative nonfermentative bacilli and all Candida species. Overall, significantly more isolates were detected only in the anaerobic bottle from ambulatory children (P<0.0001), including 13 of 54 (24%) patients with S pneumoniae bacteremias presenting to the emergency department. This study indicated that different BACTEC blood culture media combinations are needed in ambulatory and hospitalized pediatric care settings to ensure the optimal recovery of all types of isolates. Whereas aerobic blood culture bottles are adequate for detection of bacteremia in hospitalized children, the common occurrence of fastidious organisms mandates the need for a combined aerobic/anaerobic culture set in ambulatory pediatric care settings.

Assessment of the yield of anaerobic blood cultures

Because of the declining incidence of anaerobic bacteremia, the predictable sites of anaerobic infection and the increasing importance of aerobic isolates (eg; yeasts), the practice of routinely culturing half the volume of blood collected anaerobically has been questioned. We have assessed the yield of routine anaerobic blood cultures in our clinical setting. Blood culture isolates from November 1994 through October 1995 at Auckland (AKH) and Green Lane/National Women's Hospitals (GL/NWH) were recorded. The medical records of patients with anaerobic bacteremia were examined. For the three month period April to June 1996, all positive blood cultures were analysed with respect to which bottle (aerobic or anaerobic or both) was positive. For the period November 1994 to October 1995, 5.6% and 5.3% of blood cultures at AKH and GLH respectively were positive. At AKH and GLH anaerobes constituted 0.16% and 0.19% of all blood cultures and 3.1% and 3.5% of all positive blood cultures respectively. Twenty-one of 25 (84%) significant anaerobes were from patients in whom anaerobic infection was predictable. More isolates were recovered from aerobic than anaerobic bottles, 178 versus 71, p < 0.001. Aerobic culture also recovered more pathogens (76 versus 38, p < 0.001 more yeasts (10 versus 0) and more Pseudomonas spp. (10 versus 1) than did anaerobic culture. Only obligate anaerobes were isolated more frequently in anaerobic bottles (5 versus 0, p = 0.03). Most instances of anaerobic bacteremia occurred in patients where anaerobes could be expected. We conclude that routine use of two aerobic bottles with clinically directed use of anaerobic blood culture bottle is an appropriate and effective approach in our setting.

Advantage of combining resin with lytic BACTEC blood culture media

The BACTEC 9240 (Becton Dickinson, Sparks, Md.) automated blood culture system is based on the continuous monitoring of CO2 production by means of a fluorescent sensor attached to the bottom of a culture vial. We compared two media for this system, resin-containing Plus aerobic/F and Lytic anaerobic/F. Sets of Plus aerobic/F and Lytic anaerobic/F vials inoculated with similar volumes (9 +/- 2.5 ml) were evaluated. In the laboratory, the vials were introduced into the system in accordance with the recommendations of the manufacturer and incubated at 35 degrees C for 5 days. A total of 10,914 sets consisting of two bottles each were obtained from 3,674 patients (2.97 cultures per patient). Of these, 1,233 (11%) were culture positive, including 1,074 (10%) yielding at least one pathogen, and 178 (2%) were contaminated. A total of 1,135 isolates were considered clinically relevant in 624 septic episodes; we isolated 894 from Plus aerobic/F and 852 from Lytic anaerobic/F (P = 0.06 [not significant]). More S. aureus isolates (P = 0.05), Pseudomonas spp. (P < 0.0001), other gram-negative bacteria (P = 0.004), and yeasts (P < 0.0001) were isolated from Plus aerobic/F medium, but more streptococci (P < 0.0001), E. coli (P = 0.02) strains and anaerobes (P < 0.0001) were detected with Lytic anaerobic/F medium. Lytic anaerobic/F vials were significantly (P < 0.0001) more often positive at least 6 h before Plus aerobic/F vials (n = 112 versus 52, respectively). Significantly more (P < 0.0001) Plus aerobic/F vials (n = 210; 1.9%) than Lytic anaerobic/F vials (n = 42; 0.4%) were unconfirmed positives. Plus aerobic/F and Lytic anaerobic/F proved to be a valuable pair of blood culture media. Plus aerobic/F performs better for patients under antibiotic treatment, due to the antimicrobial-neutralizing effect of resins. For patients without antibiotic therapy, more microorganisms could be isolated from Lytic anaerobic/F due to cell lysis.

Update on detection of bacteremia and fungemia

The presence of microorganisms in a patient's blood is a critical determinant of the severity of the patient's illness. Equally important, the laboratory isolation and identification of a microorganism present in blood determine the etiologic agent of infection, especially when the site of infection is localized and difficult to access. This review addresses the pathophysiology and clinical characteristics of bacteremia, fungemia, and sepsis; diagnostic strategies and critical factors in the detection of positive blood cultures; characteristics of manual and instrument approaches to bacteremia detection; approaches for isolating specific microorganisms associated with positive blood cultures; and rapid methods for the identification of microorganisms in blood cultures.

Clinical comparison of the BACTEC 9000 Standard Anaerobic/F and Lytic/F blood culture media

An 8-month prospective, volume controlled, comparison of Standard Anaerobic/F media with a new anaerobic high blood volume lytic medium (Lytic/F) was performed. A total of 2,092 compliant sets, consisting of an aerobic resin bottle or standard aerobic bottle, Standard Anaerobic/F, and Lytic/F bottle were evaluated. A total of 220 (10.6%) positive specimens were detected from the paired anaerobic bottles. These consisted of 194 true positive and 26 false positive bottles. Of 207 total organisms isolated, 122 were considered clinically significant. A comparison of significant organism recovery revealed 79 isolates in both anaerobic bottles, 7 isolates in the standard Anaerobic/F bottle only, and 36 isolates in the Lytic/F bottle only (p < 0.001). The lytic/F bottle detected significantly more Enterobacteriaceae (p < 0.005) and Streptococci (p < 0.05). There were 24 false positive Standard Anaerobic/F bottles and 2 false positive Lytic/F bottles (p < 0.001). When both bottles were positive the Standard Anaerobic/F bottle was positive 12 hours earlier in 1 instance whereas the Lytic/F bottle was positive 12 hours earlier in 8 instances. The mean time for detection in the Standard Anaerobic/F bottle was 18.2 hours versus 13.2 hours for the Lytic/F bottle. The new Lytic/F anaerobic blood culture media was found to be superior to Standard Anaerobic/F media for both total organism recovery and time to organism detection.

Effect of number of blood cultures and volume of blood on detection of bacteremia in children

OBJECTIVE

To determine whether bacteremia can be detected more rapidly and completely by (1) obtaining two blood cultures instead of one and/or (2) collecting a larger volume of blood.

STUDY DESIGN

Prospective comparison of different strategies in 300 patients undergoing blood culture for suspected bacteremia. Each patient had two samples of blood, A (2 ml) and B (9.5 ml), obtained sequentially from separate sites. The B sample was divided into three aliquots: B1 (2 ml), B2 (6 ml), and ISO (1.5 ml, quantitative culture).

RESULTS

A pathogen was isolated from one or more blood cultures in 30 patients (10% of cases). When measured at 24 hours, the pathogen recovery rate for the B2 sample (72%) was higher than that for the individual small-volume samples (A = 37%, B1 = 33%; p < 0.01 for each comparison) and for the combination of the two small-volume samples (A + B1 = 47%; p = 0.04). At final (7-day) reading the pathogen recovery rate for the B2 sample (83%) was higher than that for B1 (60%; p = 0.02) and similar to the recovery rate observed with the combination of the two small-volume cultures (A + B1 = 73%; p = 0.55).

CONCLUSIONS

Increasing the volume of blood inoculated into blood culture bottles improves the timely detection of bacteremia in pediatric patients and spares the patients the cost and pain of an additional venipuncture.

Assessment of Difco ESP 384 blood culture system by terminal subcultures: failure to detect Cryptococcus neoformans in clinical specimens

Terminal subcultures were performed on 1,162 5-day negative blood culture sets which had been monitored by the Difco ESP 384, a continuous-monitor blood culture system. Of these, 16 (1.4%) had growth upon terminal subculture. The isolates not detected by the Difco ESP 384 were Cryptococcus neoformans (eight isolates), Candida albicans (one isolate), Staphylococcus aureus (two isolates) coagulase-negative staphylococcus (three isolates), Bacillus sp. (one isolate), and Corynebacterium sp. (one isolate). Acridine orange staining was performed on 200 randomly selected negative blood culture sets from the study group. Of these, two sets were positive and grew out C. neoformans, as did the terminal subculture. A review of patient's medical records indicated that many of these false-negative isolates were clinically insignificant. The Difco ESP 384 failed to detect 1.4% of the isolates in this study, 50% of which were C. neoformans, indicating a deficiency in the detection mechanism of the system. Further studies demonstrated that while these isolates (C. neoformans) grew in the Difco media, the system did not detect this growth when the standard 5-day protocol was used.

Detection kinetics for positive blood culture bottles by using the VITAL automated system

The VITAL system principle is based on homogeneous fluorescence technology. During an 11-month period, a total of 19,706 blood cultures from adult patients hospitalized in various establishments of the Montpellier Teaching Hospital were collected in VITAL bottles, of which 1,939 were declared positive. Only 204 bottles (1.04%) were false positives. The 1,735 true-positive bottles were collected from 130 patients. The final visual control permitted the detection of 10 falsely negative bottles (0.05%), of which 5 contained clinically significant microorganisms from four patients. The kinetics of detection for all microorganisms showed that 66.6% were detected within 24 h, 83.1% within 48 h, 95.5% within 120 h, and 100% within 150 h. No clinical episode would have been missed had a 5-day protocol been used instead of a 7-day protocol. Among the positive bottles, 65.7% were detected by the SLOPE algorithm, 20.1% by the DELTA algorithm, and 14.2% by the THRESHOLD algorithm. This retrospective study of our results shows that a 5-day protocol is sufficient for the detection of septic episodes using the VITAL system.

Value of routine anaerobic blood cultures for pediatric patients

OBJECTIVE

Anaerobic bacteremia rarely occurs in children. Therefore we assessed the usefulness of routinely obtaining anaerobic blood cultures in our pediatric patients.

STUDY DESIGN

Records of 9360 paired aerobic anaerobic blood culture bottles (Bactec NR660 System) containing blood specimens from pediatric inpatients and outpatients at Duke University Medical Center, Durham, N.C., were reviewed retrospectively. Yield and speed of detection were calculated for each bottle and compared for statistical significance by the McNemar test.

RESULTS

A total of 723 clinically important microorganisms were isolated; only 15 (2.1%) were strict anaerobes. Significantly more microorganisms (p < 0.001), especially staphylococci, nonfermenting gram-negative rods, enteric gram-negative rods, and yeasts, were detected by use of the aerobic bottle. The anaerobic bottle was important in identifying an anaerobic microorganism as the cause of sepsis in only five patients, all of whom were at increased risk of having anaerobic infection.

CONCLUSIONS

Anaerobic blood cultures are rarely helpful in the majority of pediatric patients and usually show positive results only in clinical settings associated with anaerobic infection. Microorganisms that prefer an aerobic environment, such as Pseudomonas aeruginosa and yeasts, are now far more common than anaerobes in children; aerobic culturing of the entire volume of blood collected might increase the yield from pediatric blood cultures.

Value of terminal subcultures for blood cultures monitored by BACTEC 9240

Blood cultures collected in BACTEC Plus Aerobic/F bottles and BACTEC Plus Anaerobic/F bottles were monitored for 5 days by BACTEC 9240 and subsequent terminal subcultures. Of the 13,471 bottles subcultured, 11.0% (1,477 of 13,471) were culture positive. Of these, 94.0% (1,388 of 1,477) were detected by BACTEC 9240; the additional 6.0% (89 of 1,477) were considered to be false negatives by BACTEC 9240 since they were detected by terminal subculture only. The false-negative bottles consisted of 17 BACTEC Plus Aerobic/F and 72 BACTEC Plus Anaerobic/F bottles, accounting for 2.2 (17 of 786) and 10.4% (72 of 691) of the total positive aerobic and anaerobic bottles, respectively. The positive blood culture bottles most frequently not detected by BACTEC 9240 grew Pseudomonas spp. (24), Staphylococcus spp. (21), and yeasts (24). Of the 86 blood cultures represented by the 89 false-negative bottles, 41 would not have been identified as positive since the other bottle in the blood culture set was either a false negative or a true negative. In general, terminal subcultures of false-negative BACTEC bottles had heavy growth, indicating that BACTEC Plus media were able to support the growth of microorganisms, but the BACTEC 9240 instrument was unable to detect this growth.

Continuous quality improvement for introduction of automated blood culture instrument

Despite the critical nature and high cost of blood cultures, hospitals rely on manufacturers' test site data. As a result, in-hospital testing and compliance evaluation of newly acquired instruments are seldom done. The goal of this study was to apply a continuous quality improvement approach and to develop assessment criteria for all stages from the purchase order, through the on-site instrument evaluation, to the compliance evaluation. Despite the introduction of an automated high-blood-volume instrument (BacT/Alert) in our hospital, 56% of adult patients had only one venipuncture and 89.5% had < or = 20 ml of total blood volume sampled. False positives were associated with overfilling of bottles. These problems occurred because the phlebotomists did not like to perform multiple venipunctures on ill patients; therefore, they were drawing 20 ml of blood from one venipuncture and splitting it between two bottles. Unknown to the staff, the vacuum in the bottles draws significantly more than 10 ml of blood; therefore, the first bottle in the set was frequently overfilled and the second bottle was frequently underfilled. A diagrammatic guideline for a new blood culture protocol based on two venipunctures, taken one immediately after the other, to inoculate three bottles was developed. Compliance evaluation demonstrated that within 1 month of starting the new protocol, 74% of patients had at least two or more venipunctures and 60% had > or = 30 ml of blood drawn per patient episode. This study demonstrates the need for continuous quality improvement, including compliance evaluation, to ensure that the potential benefits of newer blood culture technology are actually realized.

Effects of volume and periodicity on blood cultures

Blood specimens collected for culture by using the high-volume resin-based BACTEC system over an 18-month period at the Seattle Veterans Administration Center were examined in this study. Of 7,783 cultures obtained, 624 were classified as true positives. Patients in this group had between 20 and 60 ml of blood drawn per culture and separated into 10-ml aliquots for incubation. Analysis of the results stratified by cultured volume and time interval between specimen collection accorded yield advantage to culture volume at the maximal amounts tested. No advantage was observed with any particular interval of collection. Increasing cultured volume from 20 to 40 ml increased yield by 19%. Increasing cultured volume from 40 to 60 ml increased yield by an additional 10%. The same effect was seen whether cultures were drawn simultaneously or serially within 24 h. These observations support other reports demonstrating increased yield with increased cultured blood volume. However, they demonstrate increases in yield at volumes much higher than previously considered. In conclusion, this study demonstrates that high-volume blood cultures drawn serially or simultaneously return the best yields.

Clinical comparison of isolator and thiol broth with ESP aerobic and anaerobic bottles for recovery of pathogens from blood

The recovery of pathogens and the speed of their detection were determined for our conventional blood culture system (an Isolator [Wampole] and a 100-ml Thiol bottle [Difco]) compared with automated ESP aerobic and anaerobic bottles (80 ml each; Difco). Each of the four culture devices was inoculated with approximately 10 ml of blood from symptomatic patients weighing more than 80 lb (ca. 36 kg). From 7,070 sets of cultures for 2,841 patients, 607 clinically significant isolates were recovered: 456 (75.1%) from the Isolator, 353 (58.2%) from Thiol, 377 (62.1%) from ESP aerobic bottles, and 346 (57.0%) from ESP anaerobic bottles. Of the 607 isolates, 149 (24.5%) were detected only with the conventional system (Isolator and/or Thiol), and 65 (10.7%) were detected only with the ESP two-bottle system (P < 0.001). Our conventional system allowed for detection of significantly more isolates of members of the family Enterobacteriaceae (P < 0.001), Staphylococcus aureus (P < 0.01), Staphylococcus spp. (coagulase-negative) (P < 0.01), and Enterococcus spp. (P < 0.05), and ESP facilitated detection of significantly more isolates of S. pneumoniae (P < 0.01). When all four devices in a culture set were positive for the same isolate, no microbial species or group was detected significantly earlier ( > or = 24 h) by either blood culture system. The Isolator contamination rate (4.8%) was > or = 6 times the rate for any of the bottles. Of pathogens detected by the Isolator, 50% were recovered in counts of < or = 1.0 CFU/ml and 18% were recovered only as a single colony. The ESP system offered an automated, less labor-intensive blood culture system for which routine subcultures were not required, but the important considerations of culturing large volumes of blood and of obtaining at least two sets from each patient in our population were reemphasized.

An assessment of the anaerobic vial of paired NR 6A and NR 7N Bactec blood cultures

OBJECTIVE

To determine the number of obligate anaerobes recovered in anaerobic blood culture vials and to determine if their recovery had a significant impact on patient care.

DESIGN

Retrospective review.

SETTING

Tertiary care teaching hospital.

MAIN RESULTS

Six thousand nine hundred and five pairs of Bactec blood cultures were submitted (each set consisted of one 6A and one 7N vial). Of these, 690 sets were culture-positive in at least one of the vials (10% of pairs). Both vials were positive in 406 (58.8%). The aerobic vial alone was positive in 176 (25.5%) and the anaerobic vial alone was positive in 107 (15.5%). Of these, most were facultative anaerobes; however, 20 blood culture sets from 18 patients were positive for obligate anaerobes. In five of the 18, the isolate was judged to be a contaminant. In 11 of 13 patients, the clinically significant obligate anaerobic bacteremia might have been predicted on clinical grounds, and in eight patients, empirical antianaerobic antibiotics had been started before the results of blood cultures were known.

CONCLUSIONS

Clinical laboratories should carefully examine the use of the routine anaerobic blood culture and consider its replacement with larger volume aerobic blood culture vials.

Comparison of two BACTEC anaerobic culture media for recovery of anaerobic bacteria

A selection of 22 anaerobic isolates, originally recovered from sterile body sites or wounds, was cultured in parallel in BACTEC anaerobic NR7A and lytic 37A bottles supplemented with volunteer human blood. The overall detection rate was 95% for the 37A medium and 75% for the NR7A medium (P = 0.003). The growth indices were consistently higher in the 37A bottles, and a positive result occurred sooner in the 37A medium in 11 of the 32 bottles positive in both media. In an analysis of patient specimens, the NR7A bottle had a 14% false-positive rate compared with 3% for the 37A bottle. The increase in recovery, coupled with the decrease in false-positive readings, make the lytic 37A bottle more efficacious than the NR7A bottle as an anaerobic culture medium.

Rationale for selective use of anaerobic blood cultures

Because of the declining frequency of anaerobic bacteremia, routinely using half the collected blood volume for anaerobic culture has been challenged. There is no data indicating whether more clinically relevant isolates would be recovered if all or most of the given blood sample were cultured aerobically. In this two-part study, we reviewed cases of anaerobic bacteremia to determine what proportion occurred in situations when anaerobes would be expected and then estimated the yield of different culture approaches by reanalyzing the data from a large prospective clinical blood culture study. The records of 61 patients who had an anaerobic isolate (excluding Propionibacterium species) recovered only from an anaerobic bottle were examined to define clinical settings in which such isolates occur. Fifty-six (92%) patients had clinically important isolates, and the source of infection was obvious at the time of culture in 47 of the 56 (84%). Of 56 patients, 36 (64%) had abdominal signs and symptoms, including 12 with recent abdominal surgery. Of nine patients without an obvious source of infection, six were on high-dose steroids. Relative yields were compared for (i) one aerobic bottle and one anaerobic bottle (5 ml to each) for all blood cultures, (ii) two aerobic bottles (5 ml to each), or (iii) two aerobic bottles plus an extra anaerobic bottle (only for clinically suspected anaerobic sepsis) (5 ml to each). The third approach had the highest yield (475 isolates), because the routine use of two aerobic bottles recovered more Candida spp., members of the family Enterobacteriaceae, and nonfermenters than did the first approach (448 isolates) (P < 0.02), and clinically directed culturing for anaerobes would recover anaerobes missed with the second approach (458 isolates). Our data suggest that the use of two aerobic bottles with selective culturing for anaerobes could increase the number of clinically relevant isolates by at least 6% compared with the current practice of inoculating an aerobic bottle and an anaerobic bottle with equal volumes of blood.