Detection of carbapenemases in gram-negative bacilli by chromogenic, MALDI-TOF and molecular tests

Early detection of carbapenemase-producing organisms is important to prevent their dissemination in health care settings. This study aimed to determine the distribution of carbapenemase genes in Enterobacterales and Pseudomonas isolates in our hospital and to evalute the sensitivity and specificity of 3 different phenotypic tests (Carba NP, MALDI-TOF and DKT) in detecting carbapenemases in these isolates. 85 Enterobacterales and 32 Pseudomonas isolates from various clinical samples in the Medical Microbiology Laboratory of Eskişehir Osmangazi University Health Practice and Research Hospital between 10.09.2022 and 02.08.2024 were included in our study. Among Enterobacterales isolates, 30.6% (n=26) were found to be KPC, 36.5% (n=31) were found to be OXA-48, 10.6% (n=9) were found to be NDM, 9.4% (n=8) were found to be NDM+OXA-48, 2.3% (n=2) were found to be KPC+OXA-48, 1.2% (n=1) were found to be OXA-51 positive, and 9.4% (n=8) were found to be carbapenemase negative. Of the 32 Pseudomonas spp. isolates, 40.7% (n=13) were found to be VIM, 3.1% (n=1) were found to be NDM, 3.1% (n=1) were found to be VIM+OXA-48 positive, and 53.1% (n=17) were found to be carbapenemase negative. IMP gene was not detected in any Enterobacterales or P. aeruginosa isolates. Of the carbapenemase-producing Enterobacterales members, 92.2% (71/77) gave positive results with Carba NP, 85.7% (66/77) with MALDI-TOF, and 63.6% (49/75) with DKT. All KPC, NDM+OXA-48 and KPC+OXA-48 producing strains gave positive results with all three tests. Of the OXA-48 producing Enterobacterales members, 87.1% (27/31) gave positive results with Carba NP, 71% (22/31) with MALDI-TOF, and 16.1% (5/31) with DKT tests. All three tests gave negative results with OXA-51 positive K. aerogenes. The Carba NP test gave positive results with all carbapenemase-positive Pseudomonas isolates. DCT test gave positive results in 73.3% (11/15) of carbapenemase positive Pseudomonas isolates. Pseudomonas isolates were not tested for carbapenemase production by the MALDI-TOF method.