Minimizing the workup of blood culture contaminants: implementation and evaluation of a laboratory-based algorithm.

An algorithm was implemented in the clinical microbiology laboratory to assess the clinical significance of organisms that are often considered contaminants (coagulase-negative staphylococci, aerobic and anaerobic diphtheroids, Micrococcus spp., Bacillus spp., and viridans group streptococci) when isolated from blood cultures. From 25 August 1999 through 30 April 2000, 12,374 blood cultures were submitted to the University of Iowa Clinical Microbiology Laboratory. Potential contaminants were recovered from 495 of 1,040 positive blood cultures. If one or more additional blood cultures were obtained within +/-48 h and all were negative, the isolate was considered a contaminant. Antimicrobial susceptibility testing (AST) of these probable contaminants was not performed unless requested. If no additional blood cultures were submitted or there were additional positive blood cultures (within +/-48 h), a pathology resident gathered patient clinical information and made a judgment regarding the isolate's significance. To evaluate the accuracy of these algorithm-based assignments, a nurse epidemiologist in approximately 60% of the cases performed a retrospective chart review. Agreement between the findings of the retrospective chart review and the automatic classification of the isolates with additional negative blood cultures as probable contaminants occurred among 85.8% of 225 isolates. In response to physician requests, AST had been performed on 15 of the 32 isolates with additional negative cultures considered significant by retrospective chart review. Agreement of pathology resident assignment with the retrospective chart review occurred among 74.6% of 71 isolates. The laboratory-based algorithm provided an acceptably accurate means for assessing the clinical significance of potential contaminants recovered from blood cultures.

Critical appraisal of E test for the detection of fluoroquinolone resistance.

The ability of E test to accurately detect fluoroquinolone resistance was compared with an agar dilution reference method. Nearly 300 isolates belonging to 26 different species (62.5% with documented ciprofloxacin resistance) were tested with ciprofloxacin, fleroxacin, levofloxacin, norfloxacin, ofloxacin, and sparfloxacin. In contrast to earlier reports, E test MIC values for pneumococci and all rapid growing aerobes were routinely higher (approximately 0.5 log2 dilution step) than agar dilution results. The E test stable-gradient method also efficiently identified fluoroquinolone-resistant subpopulations which were not detected by the reference procedure with the standard inoculum. Categorical agreement for 1710 test comparisons was approximately 90% with no very major, false-susceptible errors. We found the E test to be a valid, reproducible method for fluoroquinolone susceptibility testing that provided quantitative results and produced a conservative (1.6% false-resistant results) assessment of susceptibility of bacteria including isolates of Streptococcus pneumoniae and Pseudomonas aeruginosa to compounds in this antimicrobial class.

Evaluation of the Etest for antimicrobial spectrum and potency determinations of anaerobes associated with bacterial vaginosis and peritonitis.

One hundred ninety-seven anaerobic organisms (24 Gardnerella vaginalis, 16 Mobiluncus spp., 19 Peptostreptococcus spp., 20 Lactobacillus spp., 20 Prevotella bivia/disiens, 81 Bacteroides fragilis group, 12 Clostridium spp., and five Fusobacterium spp.) were processed by the Etest (AB Biodisk, Solna, Sweden) and a reference (Brucella blood agar) method against 10 antimicrobial agents. For the bacterial vaginosis-associated pathogens, the Etest was more reproducible and correlated acceptably with the reference agar test: within +/- 1 log2 dilution for 74.4% of Mobiluncus spp. to 96.0% for Peptostreptococcus spp. (all organisms, 83.4%). The quantitative correlation +/- 2 log2 dilution steps between test results was 94.3%. Results with B. fragilis group strains demonstrated 97.3% correlation (+/- 2 log2 dilution) with a trend toward slightly lower Etest minimum inhibitory concentrations for ampicillin-sulbactam, cefotaxime, imipenem, and clindamycin. The absolute qualitative interpretive agreement between Etest and the reference agar dilution method results was 94.4%, with only a 0.4% false-susceptible error rate. The Etest appears to be a very practical, quantitatively accurate, alternative procedure for clinical microbiology laboratories routinely testing the susceptibilities of anaerobes and, by these presented data, organisms associated with female tract infections.

Antimicrobial spectrum of cefpirome combined with tazobactam against the Bacteroides fragilis group.

Cefpirome, a so-called fourth-generation cephalosporin, was tested alone and in combination with the sulfone beta-lactamase inhibitor, tazobactam, against 63 members of the Bacteroides fragilis group. The cefpirome MIC50 was only 64 micrograms/ml, but the MIC was reduced eightfold with tazobactam (2:1 ratio). The addition of tazobactam to cefpirome or the use of metronidazole as a codrug appear to be alternative choices to enhance the antianaerobic spectrum. Over 98% of strains had cefpirome-tazobactam MICs of less than or equal to 32 micrograms/ml.