Evaluation of βLACTA™ test, a rapid test detecting resistance to third-generation cephalosporins in clinical strains of Enterobacteriaceae.

Since decades, third-generation cephalosporins (3GC) are major drugs to treat infections due to Enterobacteriaceae; growing resistance to these antibiotics makes important to rapidly detect such resistance. βLACTA™ test is a chromogenic test developed for detecting within 15 minutes 3GC-resistant isolates from cultures on solid media. A multi-center prospective study conducted in 5 French and Belgian hospitals evaluated the performances of this test on clinical isolates. Based on antibiotic susceptibility testing, strains resistant or intermediate to cefotaxime or ceftazidime were classified as 3GC-resistant and molecular characterization of this resistance was performed. The rate of 3GC-resistance was globally 13.9% (332/2387), 9.4% in Escherichia coli (132/1403), 25.6% in Klebsiella pneumoniae (84/328), 30.3% in species naturally producing inducible AmpC beta-lactamases (109/360), and 5.6% in Klebsiella oxytoca and Citrobacter koseri (7/124). The overall sensitivity and specificity of βLACTA™ test were 87.7% and 99.6% respectively, were 96 and 100% for E. coli and K. pneumoniae and were 67.4% and 99.6% for species naturally producing inducible AmpC beta-lactamase. False negative results were mainly related to 3GC-resistant strains producing AmpC beta-lactamase. Interestingly, the test was positive for all 3GC-resistant extended-spectrum beta-lactamase producing isolates (n=241). Positive predictive value was 97% and would remain ≥ 96% for prevalence of 3GC-resistance ranging between 10 and 30%. Negative predictive value was 99% for E. coli and K. pneumoniae and 89% for the species producing inducible AmpC beta-lactamase.In conclusion βLACTA™ test was found to be easy to use and efficient to predict resistance to third-generation cephalosporins, particularly in extended-spectrum beta-lactamase producing strains.