Evaluation of the Cobas-Bact® System for Direct and Rapid Identification and Antimicrobial Susceptibility Testing of Enterobacteriaceae from Positive Urine Specimens

Abstract

A direct antimicrobial susceptibility test and a direct identification of Gram-stained urine specimens positive for Enterobacteriaceae using a new instrument (Cobas-Bact®) were compared by means of the conventional Kirby-Bauer agar diffusion disk method and the spot indole test, an in-house set of identification tests or API 20E. Bacteria from 191 cases of monomicrobial bacteriuria due to members of the family Enterobacteriaceae were tested. Direct susceptibility testing was performed in 180 cases (94%), providing 1649 antibiotic-microorganism combinations. A complete agreement was reached in 82% and essential agreement in 92% of cases. Minor discrepancies were found in 163 (9.9%) major ones in 125 (7.6%) and very major ones in 10 (0.6%) of examinations. 72% of all minor and 71 of all major discrepancies were caused by two antibiotics: cephalothin and nitrofurantoin. Of the very major discrepancies, 50% were due to amoxicillin. Of 171 direct identifications obtained, 130 (76%) were “2high-confidence” correct identifications (percentage of likelihood p ⩾ 80%), 25 (14.6%) “low-confidence” identifications (percentage of likelihood p ⩾ 80%) and 16 (9.4%) misidentifications. On the whole, this direct and rapid Cobas-Bact® identification and susceptibility testing procedure provided satisfying information within 5–6 h after collection of urine specimens positive for members of the Enterobacteriaceae family.