Clinical

Elispot assay: a promising substitute for tuberculin skin test for latent TB

Last Updated: 2008-08-26 8:30:44 -0400 (Reuters Health)

NEW YORK (Reuters Health) - Elispot, an enzyme-linked immunospot assay that detects Mycobacterium tuberculosis antigens, is a useful screening test to rule out latent tuberculosis infection in rheumatic disease patients starting treatment with a tumor necrosis factor antagonist, according to physicians in Greece.

Because it is not affected by previous bacillus Calmette-Guerin vaccination or immunosuppression, Elispot may be preferable to the tuberculin skin test in such patients, Dr. Dimitrios Vassilopoulos and his associates at the Athens University School of Medicine report in the July issue of the Journal of Rheumatology.

The research team compared tuberculin skin test and Elispot results in 70 patients with various rheumatic diseases. Discordant results were obtained for 15 patients (21%) who were Elispot-negative/tuberculin skin test-positive and 4 (6%) who were Elispot-positive/tuberculin skin test-negative.

In multivariate analysis, a history of bacillus Calmette-Guerin vaccination was associated with Elispot-negative/tuberculin skin test-positive discordant results, and steroid use was associated with Elispot-positive/tuberculin skin test-negative results.

The frequency of Elispot-negative/tuberculin skin test-positive discordancy suggests a high rate of potentially unnecessary exposure to isoniazid prophylaxis, the authors maintain.

"This is particularly important," they explain, "because isoniazid-related liver toxicity may be even higher in rheumatic patients who, especially with rheumatoid arthritis, are being treated with other potentially hepatotoxic medications such as methotrexate, nonsteroidal antiinflammatory drugs, leflunomide, sulfasalazine, etc."

On the other hand, spotting steroid-treated patients with negative tuberculin skin test results and positive Elispot results could lead to appropriate initiation of prophylactic treatment.

Still, the Greek research team does not at this time advocate replacing the tuberculin skin test with the Elispot assay.

They conclude: "More data examining the test's cost, feasibility, and reproducibility, as well as the outcome of anti-TNF-treated rheumatic patients with discordant tuberculin skin test/Elispot results, are needed before evidence-based recommendations can be made."

J Rheumatol 2008;35:1271-1276.