Evaluation of an immunochromatographic test kit for detecting Mycobacterium tuberculosis complex in sputum samples and on solid and in liquid cultures.

A rapid, cheap and effective method for diagnosing tuberculosis (TB) is essential for TB control. We evaluated the performance of an immunochromatographic assay (ICA) kit (SD Bioline TB Ag MPT64 rapid test) designed for detecting Mycobacterium tuberculosis complex (MTC) in liquid and solid cultures to determine its ability to detect and differentiate MTC directly in sputum samples. We attempted to optimize antigen extraction using several sputum solvents under various conditions. Adding the sputum solvent prior to using the ICA kit gave a sensitivity of 71.7% (43/60) for all acid-fast bacillus (AFB) stain positive specimens and a 100% specificity (20/20) among AFB negative specimens. Without sputum solvent, the ICA kit had 0% sensitivity for detecting MTC in sputum. We also evaluated the ICA test kit for its designed purpose of detecting MTC in 80 solid and liquid culture specimens positive for MTC using the niacin accumulation test or polymerase chain reaction (PCR) (16s-23s ITS). The ICA kit gave 100% sensitivity and specificity. We also evaluated the ICA test kit on 3 reference specimens of MTC, 15 nontuberculous mycobacteria (NTM) species, 7 bacterial species and 5 Candida albicans specimens. The tested ICA kit gave 100% specificity. The tested ICA kit was useful for detecting and differentiating MTC in solid and liquid cultures, but not useful for detecting MTC in sputum samples even treated with sputum solvent. The tested ICA kit should be used only for liquid and solid culture specimens. Therefore, the tested kit is inappropriate for use in evaluating sputum samples.