Rapid detection of positive blood cultures with the BACTEC NR-660 does not require first-day subculturing

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.

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.

Osteomyelitis and intervertebral discitis caused by Blastoschizomyces capitatus in a patient with acute leukemia

We describe the first known case of vertebral osteomyelitis and discitis caused by Blastoschizomyces capitatus in a leukemic patient and the results of therapy. We also reconfirm the microbiological characteristics which differentiate this species from other yeastlike pathogens.

Recovery of clinically important microorganisms from the BacT/Alert blood culture system does not require testing for seven days

Recently, we published a comparison of the BacT/Alert blood culture system with the BACTEC 660/730 nonradiometric blood culture system using blood inocula of 5 ml per bottle. By reanalyzing data collected during that study, we found that, for true-positive isolates causing bacteremia or fungemia, 363 (97.6%) of 376 and 341 (97.7%) of 349 isolates were recovered by the end of day 5 of testing, and 364 (97.9%) of 376 and 343 (98.3%) of 349 isolates were recovered by the end of day 6 of testing for aerobic and anaerobic bottles, respectively. Most isolates recovered on days 6 (24 of 27) and 7 (20 of 25) of testing were either contaminants or indeterminate as a cause of sepsis. When used as recommended by the manufacturer, only six (1.3%) of 464 clinically important isolates recovered on test days 6-7 would have gone undetected had testing been limited to 5 days and four (0.9%) of 464 had testing been limited to 6 days. We conclude that BacT/Alert bottles can be tested for as few as 5 days and then discarded with minimal loss of true-positive isolates and maximal reduction of contaminants.

Faster isolation of Brucella spp. from blood by isolator compared with BACTEC NR

Brucella spp. was isolated from blood cultures in a mean time of 3.1 days (range, 2-5) in seven cases of human brucellosis processed with the Isolator lysis-centrifugation system. Simultaneous BACTEC NR blood cultures in the same patients were positive in six, with a mean detection time of 20.6 days (range, 17-29). These data suggest a higher speed of recovery of Brucella spp. from blood specimens with the lysis-centrifugation procedure compared with the BACTEC NR system.

Occurrence of bacteremia in hematologic patients

In the present study we reviewed eighty-six episodes of bacteremia occurred in 60 neutropenic patients and thirty-one episodes occurred in 30 non-neutropenic patients. Twenty-four out of 60 neutropenic patients suffered from multiple episodes of bacteremia, while only one out of 30 non-neutropenic patients presented multiple episodes. In neutropenic patients, 29 episodes of bacteremia were polymicrobial, whereas only one non-neutropenic patient had polymicrobial bacteremia. Intravascular catheters were the most common source of bacteremia (23.2%) in neutropenic patients, as compared with infections of the genito-urinary tract (45.1%) among non-neutropenic patients. In both groups, aerobic gram-positive cocci were the microorganisms most frequently isolated (71.6%). Anaerobic microorganisms showed an higher incidence in polymicrobial episodes than in monomicrobial episodes x 2 = 5.39 p = 0.02 OR = 2.97 95% CI (1.2-7.7).

Determination of growth value thresholds for BACTEC PLUS aerobic blood culture vials

Growth value thresholds used to identify positive blood culture vials can be defined by users for each BACTEC NR-660 bacteremia detection instrument. Growth values were compared with the recovery of organisms from vials flagged as positive during the testing of 3.056 high-volume vials containing aerobic (BACTEC PLUS 26) medium over a 2-month period. Results showed that optimal threshold values for our use of these vials varied from those recommended by the manufacturer; if the thresholds defined from these data had been used during the study period, total vials flagged as positive from which no organisms were recovered (false alarms) would have been reduced from 181 (5.9/100 vials tested) to 71 (2.3/100 vials tested), with a minimal decrease in the identification of vials containing usual or occasional pathogens (hits). Adjustments of growth value thresholds by the individual user can make the use of BACTEC instruments more efficient by decreasing further processing of vials from which no organisms are recovered.

Controlled comparison of the BacT/Alert and BACTEC 660/730 nonradiometric blood culture systems

In a collaborative study at three university hospitals, the recovery of microorganisms and the speed of detection of microbial growth by the BacT/Alert (Organon Teknika Corporation, Durham, N.C.) and BACTEC 660/730 (Becton-Dickinson Diagnostic Instrument Systems, Sparks, Md.) nonradiometric blood culture systems were compared. A total of 5,918 comparisons were made between BacT/Alert aerobic and BACTEC NR 6A bottles and 5,992 comparisons were made between BacT/Alert anaerobic and BACTEC NR 7A bottles. Each bottle was inoculated with 5 ml of blood. The overall recoveries of microorganisms from the two aerobic bottles were comparable; members of the family Enterobacteriaceae were recovered more often from BacT/Alert aerobic bottles alone (P less than 0.001). The overall recoveries of microorganisms from the anaerobic bottles were not significantly different. Growth of Staphylococcus aureus (P less than 0.001), coagulase-negative staphylococci (P less than 0.01), streptococci (P less than 0.001), Escherichia coli (P less than 0.01), other members of the family Enterobacteriaceae (P less than 0.02), and Pseudomonas aeruginosa (P less than 0.05) was detected earlier in BacT/Alert aerobic bottles. Growth of S. aureus (P less than 0.001), coagulase-negative staphylococci (P less than 0.05), enterococci (P less than 0.01), Streptococcus pneumoniae (P less than 0.02), viridans group streptococci (P less than 0.05), E. coli (P less than 0.001), Klebsiella pneumoniae (P less than 0.01), and other members of the family Enterobacteriaceae (P less than 0.001) was detected earlier in BacT/Alert anaerobic bottles. In a system-versus-system comparison, more gram-positive cocci were recovered from the BACTEC system alone (P < 0.05), and more members or the family Enterobacteriaceae were recovered from the BacT/Alert system alone (P < 0.001). As a system, the BacT/Alert system detected growth of S. aureus (P < 0.001), coagulase-negative staphylococci (P < 0.01), streptococci (P < 0.001), E. coli (P < 0.001), other members of the familyEnterobacteriaceae (P < 0.001), and P. aeruginosa (P < 0.05) earlier than the BACTEC system did. Significantly fewer (40 versus 1,183) false-positive results occurred with the BacT/Alert system. We conclude that the BacT/Alert and BACTEC 660/730 nonradiometric systems are comparable for recovering clinically significant microorganisms form adult patients with bacteremia or fungemia, but that the BacT/Alert system detects microbial growth earlier than the BACTEC system does, with significantly fewer false-positive results.

Colorbact, a visually read blood culture system using darkening of haemoglobin in aerobic blood cultures as an early growth indicator, compared with Bactec 6A and 7A

Colorbact-A (aerobic), a blood culture system using darkening due to oxygen reduction and acidification of haemoglobin as an initial growth indicator during the first day of incubation of continuously agitated, aerobic blood cultures, was compared with the 24 h agitated aerobic Bactec 6A in 1767 consecutive paired cultures. Both systems were examined simultaneously twice daily during the first two days and once daily the next five days of incubation. In addition, during the agitation period Colorbact-A was observed for darkening four to five times during day duty between 8 a.m. and 4 p.m. The final seven days detection rate found by Colorbact-A using observation for darkening during initial agitation was similar to the rate found by Bactec 6A for both clinically significant strains and "contaminants", but Colorbact-A was significantly faster (p less than 0.001) in finding clinically significant strains. Colorbact-AN (anaerobic) was more rapid (p less than 0.01) and yielded more positive cultures than the anaerobic Bactec 7A. The Colorbact system was flexible, inexpensive, reliable and especially promising for laboratories which receive blood cultures from other hospitals once daily, as many positive cultures can be found immediately on arrival undisturbed by agitation during transport.

Comparison of the automated Bactec NR-660 with a conventional blood culture system

The Bactec NR-660 blood culturing system was compared with a conventional system using 347 consecutively obtained clinical blood samples tested simultaneously. Of 46 clinically relevant isolates, 12 were detected by the conventional system only, 5 by Bactec only and 29 by both methods (0.05 less than p less than 0.10). The difference could not be explained by the results of additional in vitro tests. Of 12 isolates considered contaminants, 7 were isolated in the conventional system only, 2 in the Bactec system only and 3 in both systems (0.05 less than p less than 0.10). The two systems were approximately equal in speed of detection. A high rate of false indications of growth in the Bactec system could be reduced by applying different CO2 cut-off values.