Evaluation of mycobacterium growth indicator tube and gen-probe amplified mycobacterium tuberculosis direct test for rapid diagnosis of mycobacterium tuberculosis in sputum specimens

ABSTRACT

The Mycobacterium Growth Indicator Tube [MGIT] and Gen-Probe Amplified Mycobacterium tuberculosis Direct [AMTD] test were evaluated using 52 respiratory clinical specimens collected from suspected pulmonary tuberculosis patients. Microbiological culture on Lowenstein-Jensen medium was used as the reference method. The 52 sputum specimens, 29 [55.8%] acid fast smear positive and 23 [44.2%] acid fast smear negative, were cultivated in liquid MGIT and on solid U media. The recovary rate of MTB by MGlT was [80.8%]. The Kappa coefficient calculation showed a very good agreement [Kappa=0.83] between the 2 tests but there was no statistical difference in the sensitivity of detection of MTB between MGIT and Li methods [P>0.05]. There was a statistically significant difference [P<0.05] between the 2 methods as regards the mean time of growth detection. The mean [range] time of detection of MTB was 12.5 [5 to 30] and 19.5 [14-30] days with MGIT and Li, respectively. The recovery rate of MTB 1mm both media in combination [MGIT plus LJ] was higher [84.6%] than the recovery rate on each media separately and the difference between the 2 methods was not statistically significant [P>0.05]. Regarding the detection of MTB from smear positive and smear negative sputum, it was found that MOLT and U detected 93% and 86.2% respectively of the 29 smear positive specimens whereas they detected 65.2% and 60.8% respectively of the 23 smear negative specimens and there was no statistical difference between the 2 methods in relation to the smear type. The recovery rate of MTB by AMTD was 71%. The assay had sensitivities of 100 and 85.7% for acid fast smear positive and negative specimens, respectively. The specificity of the assay was 100% for both types of smears. Our overall results for all specimens, regardless of smear status, showed a sensitivity of 94.8%, a specificity of 100%, a positive predictive value of 100% and a negative predictive value of 86.6%. In conclusion, Both MGIT and AMTD are reliable methods for rapid diagnosis of pulmonary tuberculosis but they should not be used instead of a solid medium; rather, they should be used in addition to it