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

Abstract EN

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 LJ media.

The recovery rate of MTB by MGIT 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 LJ 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 LJ, respectively.

The recovery rate of MTB from 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 MGIT and LJ 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.