Combined Disc Test and Modified Hodge Test for Detection of Carbapenemase-Producing Gram-Negative Bacilli

Abstract

Introduction: Identification of carbapenem resistance and its mechanism in clinical isolates is important for patient management, epidemiological studies and control of their spread. The objective of this study was to evaluate phenotypic methods; the Modified Hodge Test (MHT) and Combined Disc Test (CDT) in the detection and typing of carbapenemase-producing isolates.

Methods: A total of 396 clinical isolates (117 Pseudomonas spp, 126 Acinetobacter spp, 86 E. coli and 67 Klebsiella pneumoniae) were screened for carbapenem resistance by modified Kirby-Bauer Disc diffusion method using Imipenem. Carbapenem-resistant isolates were further tested for carbapenemase production by MHT and CDT.

Results: Seventy-four isolates were carbapenem-resistant isolates which included 24 Pseudomonas spp, 27 Acinetobacter spp, 14 Klebsiella pneumoniae and 9 E. coli. MHT detected 59(79.72%) while CDT detected 62(83.43%) as carbapenemase producer (p-value >0.05). CDT further distinguished carbapenemases as; 20 molecular class A, 33 molecular class B, and 9 co-producing class A and B while 12 were negative in CDT for any carbapenemase. As high as 85.71% of Klebsiella pneumoniae isolates, 85.15% Acinetobacter spp, 77.77% E. coli and 70.83% Pseudomonas spp were found to be carbapenemase producers by MHT. On the other hand, carbapenemase detection by CDT was noticed among 95.83% Pseudomonas spp, 92.85% Klebsiella pneumoniae, 88.88% E. coli, and only 66.66% Acinetobacter spp.

Conclusion: MHT and CDT are acceptable phenotypic methods for the detection of carbapenemase production and differentiation. These methods are easier, cheaper and can be performed along with routine AST.