Diagnostic usefulness of a T-cell based assay for extrapulmonary tuberculosis

Diagnosing extrapulmonary tuberculosis (E-TB) remains a challenge. A recently developed Mycobacterium tuberculosis-specific region of difference 1 gene-based assay for diagnosing tuberculosis infection showed promising results. However, the diagnostic usefulness of this assay remains to be determined compared with tuberculin skin test (TST) in patients with suspected E-TB in clinical practice.

All patients with suspected E-TB were prospectively enrolled in a tertiary care hospital during a 9-month period. In addition to the conventional tests for diagnosing E-TB, the interferon gamma-producing T-cell responses to early secreted antigenic target 6 and culture filtrate protein 10 by enzyme-linked immunospot (ELISPOT) assay were performed. Final diagnosis in patients having suspected E-TB was classified by clinical category.

Seventy-two patients with suspected E-TB were enrolled; 34 (47%) had immunosuppressive conditions. Of 72 patients, 32 (44%) were classified as having E-TB, including 22 with confirmed tuberculosis and 10 with probable tuberculosis, and 35 (49%) were classified as not having tuberculosis. The remaining 5 (7%) had possible tuberculosis and were excluded from the final analysis. Chronic caseating granulomas, acid-fast bacilli stain, M tuberculosis polymerase chain reaction, and cultures for M tuberculosis were positive in 22 (69%), 5 (16%), 15 (47%), and 18 (56%), respectively, of 32 patients with E-TB. The sensitivity and specificity of the TST (induration size, > or =10 mm) were 47% (95% confidence interval [CI], 29%-65%) and 86% (95% CI, 70%-95%), respectively. By comparison, the sensitivity and specificity of the ELISPOT assay were 94% (95% CI, 79%-99%; P < .001 between TST and ELISPOT) and 88% (95% CI, 72%-97%; P =.99 between TST and ELISPOT), respectively.

The ELISPOT assay is a useful adjunct test for diagnosing E-TB.