Evaluation of the GenoType Mycobacterium CM/AS Assay for Species Identification of Mycobacteria

Background: The early differentiation of Mycobacterium tuberculosis (MTB) from nontuberculous mycobacteria (NTM) and the identification of species among NTM are crucial for the immediate implementation of the appropriate therapy because susceptibility to drugs varies widely among different species. Identification to the species level by classical biochemical methods is time-consuming, requires a large battery of tests to be run, and results are obtained in four to six weeks of obtaining the isolate. The introduction of molecular biological methods has greatly improved the speed and accuracy of the process. Recently, DNA strip assays for the identification of Mycobacterium to the species level have been developed. These assays are based on reverse hybridization of a polymerase chain reaction (PCR) product to a nitrocellulose strip with immobilized probes for different mycobacterial species. One such assay is GenoType Mycobacterium CM/AS assay (Hain Lifescience GmbH, Nehren, Germany). Aim: To evaluate the diagnostic accuracy of the GenoType Mycobacterium CM/AS assay for the species identification of mycobacteria in the culture isolates in comparison with conventional phenotypic and biochemical methods. Material and methods: A total of 160 mycobacterial isolates on solid Lowenstein-Jensen media or liquid MGIT 960 were subjected to species identification by GenoType Mycobacterium CM/AS assay and biochemical methods. Results: Sensitivity, specificity, positive predictive value (PPV) and negative predictive Value (NPV) were found to be 100% for all the mycobacterial isolates except M. terrae, where sensitivity and NPV were 0% but specificity was 100% and PPV was 98.13%. Hence the overall sensitivity of the GenoType Mycobacterium CM/AS assay was 98.13% and the specificity was 100%. Conclusion: The GenoType assay is a simple, rapid and reliable method for the identification of clinically important mycobacteria, and it is well suited for use in a routine laboratory.