Performance of a rapid direct-from-blood antibiotic susceptibility test based on MIC test strips on Enterobacterales blood cultures.

Abstract

Objectives: To assess the technical performance of a rapid direct-from-blood antibiotic susceptibility test (AST) using MIC test strips (MTS-AST) directly on enterobacteria-positive blood culture broth.

Methods: Fifty contrived blood culture bottles (BACT/ALERT FA plus) were prepared using 10 mL of sheep blood and a diluted bacterial suspension. A collection of 50 enterobacterial isolates was used, including 25 strains previously recovered from blood cultures and 25 additional strains exhibiting various mechanisms of resistance. Blood culture bottles were incubated in the BACT/ALERT VIRTUO system until they flagged positive. Then they were pulled and two drops of blood culture broth (approximately 100 µL) were directly poured and streaked onto a Mueller-Hinton agar plate. MIC test strips were immediately laid on the plate, before incubation at 35°C ± 1°C in an aerobic atmosphere for 5 h. Seven β-lactams were tested including amoxicillin, amoxicillin/clavulanate, piperacillin/tazobactam, cefotaxime, ceftazidime, cefepime and meropenem. Comparator MICs were obtained using MIC test strips according to the manufacturer's instructions with a standardized inoculum and 24 h of incubation. Essential agreement (EA), categorical agreement (CA) and bias were determined.

Results: EA and CA between the MTS-AST and the comparator method were ≥94% and ≥90%, respectively, with no very major error. Positive bias was apparent for all antibiotics.

Conclusions: MTS-AST provided rapid results (in 5 h) that correlated well with the comparator method on blood culture broth containing Enterobacterales. Given the shorter time to results, the reduced time cost, the limited handwork and the reduced cost per test, the MTS-AST constitutes a fast and reliable method.