Optimization of the β LACTA test for the detection of extended-spectrum-β-lactamase-producing bacteria directly in urine samples.

The β LACTA™ test (BLT) is a chromogenic test detecting resistance to third-generation cephalosporins on bacterial colonies. Some studies have shown that this test can be used directly in urine samples. The aim of this study was to determine the optimal conditions of use of this test in order to detect the ESBL-producing bacteria directly in urine samples. During a 4-months period, a total of 365 consecutive urine samples were tested with the BLT using the recommendation of the manufacturer. We isolated 56 ESBL-producing bacteria and 17 AmpC-overproducing bacteria. ESBL- and/or AmpC β-lactamase-producing bacteria isolates were systematically characterized by disc diffusion antibiotic susceptibility testing interpreted according to the guidelines of EUCAST. The sensitivity and the specificity for 3GC-resistance detection, regardless the mechanism of resistance, were, respectively, 60.3% and 100%, whereas for ESBL detection, it was, respectively, 75.4% and 99.7%. We applied then modification of the initial protocol considering urines with a bacteriuria >1000/μL, a reading time at 30 min and considering any change of the initial colour as positive. The overall sensitivity was 81% and the sensitivity for ESBL-detection raised to 95.7%.