A comparative study of the genotype MTBDRplus® assay against a standard culture method for diagnosis and drug susceptibility testing of tuberculosis
Date of Award
Master of Medicine (MMed)
Dr. G. Revathi
Dr. J. Chakaya
Dr. G. Tesfaledet
Pathology (East Africa)
Background: Nearly one-third of the global population, i.e. two billion people, is infected with Mycobacterium tuberculosis and is at risk of developing the disease. More than eight million people develop active tuberculosis (TB) every year, with about two million dying from the disease annually. Diagnosis of tuberculosis inKenya remains dependent on smear microscopy. New methods of TB diagnosis are needed which have better accuracy and are still cost effective. Molecular methods of TB diagnosis have come under investigation in a lot of studies recently. However, they have their own limitations including the inability to differentiate between active organisms and dead bacilli in specimens.
Aim: To determine the sensitivity and specificity of the Genotype® MTBDRplus assay in the detection of resistance to Isoniazid and Rifampicin in mycobacterium tuberculosis isolates.
Materials and methods: This is a descriptive and comparative study in which the performance of a new laboratory assay will be compared to the existing (reference) method. All sputum specimens submitted for TB culture and sensitivity to the Aga Khan University Hospital Laboratory microbiology section during the period from August 2008 – April 2009 were included in the study. All sputum specimens submitted underwent smear microscopy, culture and sensitivity testing by the MGIT 960 system and DNA extraction and Genotype MTBDRplus assay for both the direct specimen as well the positive culture tube.
Results: 202 sputum specimens were included in the study. Valid phenotypic DST results were obtained for 135 cultures (68%). The number of strains resistant toRIF and INH was 4.5%, 14% respectively. Valid GenoType® MTBDRplus assay results were obtained for 116 DNA extracts (86%).
Sensitivity and specificity of the test for the detection of Isoniazid resistance were 0.68 (95% CI: 0.43 - 0.87) and 0.99 (95% CI: 0.95 - 0.99) respectively. Sensitivity and specificity of the test for the detection Rifampicin resistance was 0.78 (95% CI: 0.40 - 0.97) and 0.99 (95% CI: 0.96 - 1.00) respectively.
Comparative analysis demonstrated scientifically acceptable overall agreement between molecular (sputum specimens) and phenotypic DST results as shown below (Cohen’s Kappa = 0.76 (95% CI: 0.59 - 0.93) for Isoniazid resistance and Cohen’s Kappa = 0.81 (95% CI: 0.60 - 1.02) for Rifampicin resistance
Conclusions: The Genotype MTBDRplus assay demonstrated acceptable sensitivity and specificity for use in laboratory diagnosis of Tuberculosis and the detection of drug resistance.
Thamaini, P. W. (2009). A comparative study of the genotype MTBDRplus® assay against a standard culture method for diagnosis and drug susceptibility testing of tuberculosis (Unpublished master's dissertation). Aga Khan University, East Africa.