Use of an Interferon-γ Release Assay in the Detection of Latent Tuberculosis in Dermatology: A Descriptive Analysis

With the increasing use of biologic therapy in psoriasis, it is becoming more important to identify and treat latent tuberculosis (TB) infection (LTBI). Tuberculin skin test (TST) has been traditionally used to detect LTBI, but interferon-γ release assays (IGRAs), such as the T-SPOT.TB test (T-Spot), are increasingly being used in its place. The indications and results of 51 T-Spot tests performed at the National Skin Centre in Singapore between 2008 and 2010 were analyzed and compared with TST results, decision on LTBI treatment, and previous use of immunosuppressants. T-Spot was most commonly performed as part of a prebiologic workup in patients with psoriasis. A total of 14 (27.5%) results were positive, and no patients had features of active TB. Ten of these patients also underwent TST, five of whom had negative TST results. Six patients (11.8%) had equivocal results with T-Spot test. This study shows poor concordance between T-Spot test and TST. A high incidence of equivocal results in IGRA may limit the utility of the T-Spot test.