Performance of Quantification of Modified Hodge Test : An Evaluation with Klebsiella pneumoniae Carbapenemase-Producing Enterobacteriaceae Isolates

Modified Hodge Test (MHT) has been suggested as screening tests for carbapenemases, but concerns regarding its difficult interpretation and common false-positive results obtained in the presence of other β-lactamases have been noted. This study aimed to quantify the enhanced growth formed by the indicator strain and thus evaluate the performance of a quantitative interpretation of MHT for KPC screening. MHT was performed in 50 KPC-producing isolates and 334 non-carbapenemase-producing isolates, using ertapenem (ETP) and meropenem (MEM) as substrates. The size of enhanced growth of indicator strain was measured for each isolate tested and for the positive control used, and a ratio was calculated. Our results revealed 17 different ETP and MEM ratios, with distinct sensitivity (SN) and specificity (SP). Higher SN combined to higher SP was achieved when ETP and MEM ratios were 0.45, with a SN value of 96% for both substrates and SP values of 99.4% and 100% for ETP and MEM, respectively. The quantification with both substrates increased SP of the test for KPC detection. Considering that MHT is the unique phenotypic test that is referred to by CLSI, a more accurate approach for its interpretation could be applied to make it a more useful tool.