Performance of a simple flow cytometric assay in diagnosing active tuberculosis.

A flow cytometric assay measuring Mycobacterium tuberculosis-specific CD4 T-cell responses using co-expression of CD25/CD134 (OX40 assay) was explored as a diagnostic tool for active tuberculosis (TB) in a Thai population with and without HIV infection. Peripheral blood mononuclear cells (PBMC) obtained from 133 participants at TB diagnosis were cryopreserved. Seventy-six participants had a clinical diagnosis of TB which were confirmed by a positive culture. CD4 T-cell responses were measured after stimulation with a pool of overlapping peptides covering RD-1 antigens: CFP-10 + ESAT-6. The performance of the assay was also compared to the Xpert MTB/RIF assay. The overall sensitivity of the OX40 assay was 94.7% (95%CI 87.1-98.5); its specificity was 71.9% (95%CI, 58.5-83). The sensitivity of the OX40 assay among HIV-infected participants was 100% (95%CI, 88.8-100) with a specificity of 92.9% (95%CI, 66.1-99.8). OX40 assay performed particularly well in those with active TB and HIV infection.