Enzyme-linked immunospot and tuberculin skin testing to detect latent tuberculosis infection

Rationale: Diagnosis of latent tuberculosis infection (LTBI) is currently based on the tuberculin skin test. The enzyme-linked immunospot assay (ELISPOT) is a new blood test to diagnose LTBI. Objective: To compare the ELISPOT and the tuberculin skin test for detecting LTBI in contacts of patients with tuberculosis. Methods: Prospective study of 413 contacts of patients with tuberculosis. Measurements and Main Results: Because there is no gold standard for LTBI, the sensitivity and specificity of the ELISPOT and tuberculin skin test cannot be directly measured. For each contact, we therefore estimated the likelihood of having LTBI by calculating a contact score that quantified exposure to and infectiousness of the index case. We analyzed the relationship of contact score to ELISPOT and tuberculin skin test results. The likelihood of a positive ELISPOT (p = 0.0005) and a tuberculin skin test (p = 0.01) increased significantly with rising contact scores. The contact score was more strongly related to the ELISPOT than to the tuberculin skin test results, although this difference was not statistically significant. Among U.S.-born persons and those who were not vaccinated with bacille Calmette-Guerin, approximately 30% had positive ELISPOT or tuberculin skin test results. Foreign-born, bacille Calmette-Guerin-vaccinated persons were significantly more likely to have a positive tuberculin skin test than a positive ELISPOT result (p < 0.0001). Conclusions: Compared with the tuberculin skin test, the ELISPOT appears to be at least as sensitive for diagnosis of LTBI in contacts of patients with tuberculosis.