Aim:

Rapid and accurate detection of carbapenemase producing Enterobacteriaceae isolates plays an important role in preven-ting the spread and defining treatment strategies.The modified Hodge test (MHT) is an easy and inexpensive tool that can be used to detect carbapenemase activity. Another method used for the same purpose are inhibitor-based tests (IBTs). The aim of this study is to compare the performances of these two phenotypic methods in the detection of carbapenemases in Enterobac-teriaceae isolates.

Materials and Methods:

A total of 112 non-repeat Enterobacteriaceae strains isolated from various clinical samples at Ibni Sina Hospital Central Microbiology Laboratory between 2010-2014 that showed decreased sensitivity and resistance to ertapenem by Kirby-Bauer disk diffusion method were included in the study.

Results:

When 1:10 dilution [0,05 McFarland (McF)] was performed, out of 94 isolates positive for carbapenemase-encoding genes (CEGs), false negativity was detected in seven isolates, whereas out of 18 isolates negative for CEGs, false positivity was detected in 16 isolates; so the sensitivity of MHT for carbapenemase detection was found to be 92,6%. Without dilution (0.5 McF) the number of false negative isolates decreased to three and the sensitivity of MHT increased to 96,8%, (p<0,05). When the performance of IBTs for detection of carbapenemases was evaluated, all isolates which were positive for CEGs were also found positive with IBTs for carbapenemase production. Sensitivity of IBTs was found 100% and significantly higher than MHT (for both McF 0,05 and 0,5) (p<0,05). Six of 18 strains negative for CEGs were also found negative with IBT; so false positivity was detected in 12 isolates, which were defined incorrectly as OXA-48 producers. With IBTs, carbapenemase types of 89 of 94 isolates which were positive for CEGs were found in accordance with polymerase chain reaction (PCR) results. The sensitivity of IBTs for defining carbapenemase types was calculated as 94,6%.

Conclusion:

IBTs had higher sensitivity and specifity than MHT. Although IBTs were more than 90% compatible with PCR for detection of carbapenemases, also false positivity can ocur occasionally. MHT has a sensitivity more than 90% for OXA-48 pro-ducers in endemic regions such as our country. Although false positivity ratio is high, according to our results it’s considered that MHT can be used in laboratories with poor facilities. Performing MHT with McF 0,5 turbidity without dilution which leads increased sensitivity and reduced laboratory workload is recommended.

Keywords: Enterobacteriaceae, Carbapenemase, Modified Hodge Test, İnhibitor-Based Tests