T-SPOT. TB assay and tuberculin skin test for diagnosis and screening of tuberculosis: First report in a Sri lankan population

Abstract

Objective: Guidelines encourage evaluation of an Interferon Gamma Release Assay (IGRA) in the local setting, particularly in low/middle income, Bacillus Calmette Guerin (BCG) vaccinated populations where the assays have shown variable utility. The T-SPOT.TB assay and the Tuberculin Skin Test (TST) were compared in diagnosis of active tuberculosis (TB) and in contact screening in an adult Sri Lankan population. Materials and Methods: A prospective study including confirmed TB cases (n=75), controls (n=-74) and close contacts of smear positive cases (n=27) was carried out at the regional Chest Clinic in Kandy district. Clinical history and investigation findings, including TST results were recorded and the T-SPOT. TB(Oxford Immunotec) performed. Results: The presence of diabetes and cavitation were significant predictors of T-SPOT.TB positivity, while TST had no significant clinical predictors. In the diagnosis of active TB, T-SPOT.TB sensitivity was 73.3% (95% CI, 58.6-84.6%) and a specificity was 72% (95% CI, 62.0-85.5%) while the TST sensitivity was 70.7% (95% CI, 54.2-83.3%) and specificity was 64.1% (95% CI, 49.7-76.5%). In contact investigation neither test showed an association with exposure level. Cost estimate was LKR 9400.00 per T-SPOT.TB test compared to LKR 200.00 per TST. A high (21%) indeterminate result rate was seen with the T-SPOT.TB assay. Conclusions: This study did not show any advantage in using T-SPOT.TB over TST in this setting.