Modified Hodge Test (MHT)- Principle, Procedure, Results, Uses


Carbapenems are a class of broad-spectrum β-lactam antibiotics that are effective against many Gram-negative and Gram-positive bacteria. They are often used as the last resort for treating infections caused by multidrug-resistant organisms. However, some bacteria have developed resistance to carbapenems by producing enzymes called carbapenemases, which can hydrolyze and inactivate these antibiotics. Carbapenemase-producing organisms (CPOs) pose a serious threat to public health as they can spread rapidly and cause outbreaks of infections that are difficult to treat.

Therefore, it is important to detect CPOs in clinical specimens and monitor their prevalence and epidemiology. There are different methods for detecting carbapenemase production, such as molecular methods, biochemical methods, and phenotypic methods. Molecular methods involve detecting the genes encoding carbapenemases by PCR or sequencing. Biochemical methods involve detecting the enzymatic activity of carbapenemases by using specific substrates or inhibitors. Phenotypic methods involve detecting the phenotypic expression of carbapenemase production by using antibiotic susceptibility testing or other indicators.

Modified Hodge Test (MHT) is one of the phenotypic methods for detecting carbapenemase production. It was first described by Lee et al. in 2001 and later modified by the Clinical and Laboratory Standards Institute (CLSI) in 2009. It is based on the principle that the carbapenemase enzyme produced by the test organism hydrolyzes the carbapenem antibiotic resulting in the enhanced growth of carbapenem-susceptible E. coli. The test involves inoculating a Mueller Hinton Agar (MHA) plate with E. coli and placing a carbapenem disc at the center. Then, the test organism is streaked from the edge of the disc to the edge of the plate. After incubation, if the test organism produces carbapenemase, it will enhance the growth of E. coli within the zone of inhibition of the disc, forming a clover leaf-like pattern.

MHT is a simple and inexpensive method that can be performed in most laboratories without requiring specialized equipment or reagents. However, it also has some limitations, such as false positive results due to other β-lactamase enzymes, false negative results due to low-level carbapenemase production, and interference from some metabolites produced by the test organism. Therefore, MHT should be used in conjunction with other methods for confirming carbapenemase production.

In this article, we will discuss the objectives, principle, requirements, procedure, observation, result interpretation, advantages and limitations of MHT in detail.