A Pilot Study on Carbapenemase Detection: Do We See the Same Level of Agreement as with the CLSI Observations

Rapid identification of carbapenemase producing organisms is of great importance for timely detection, treatment and implementation of control measures to prevent the spread. The Modified Hodge Test (MHT) and Carba NP test is recommended by CLSI for the detection of carbapenemases in Enterobacteriaceae. However, MHT may give false positive results or fail to detect metallo β-lactamases (MBLs). In the US, MHT is the most widely used test for detection of carbapenemases and has been found to have a sensitivity and specificity of >90% for bla KPC producers. However, in India, the prevalence of bla NDM is higher than bla KPC producers.

To evaluate the usefulness of CarbaNP in an Indian setting.

A total of 260 isolates of carbapenem resistant E.coli (n=57), Klebsiella spp. (n=85), Pseudomonas aeruginosa (n=60), and Acinetobacter baumannii (58) isolated from clinical specimens between 2012-2014 at the Christian Medical College, Vellore were included in the study. All the carbapenem resistant isolates were subjected to CarbaNP, MHT and multiplex PCR for detection of carbapenemase genes.

CarbaNP was found to be positive in 88% (n=50/57), 81% (n=69/51), 38% (n=23/60) and 81% (n=47/58) for E.coli, Klebsiella spp., P. aeruginosa, and A. baumannii respectively. While in MHT it showed, 89% (n=51/57) and 81 % (n=69/85) for E.coli and Klebsiella spp. respectively. In P.aeruginosa, synergy testing of imipenem plus cloxacillin showed that, 65% of CarbaNP negatives were ampC producers. Overall, the sensitivity and specificity of CarbaNP was found to be 94% and 100 for bla NDM; 77% and 100 % for bla OXA-48 like producers and 81% and 100% for CarbAcinetoNP respectively.

This observation was more than what was reported in CLSI guidelines. Therefore, it is advisable to evaluate an assay for better laboratory diagnosis at respective regions.