Evaluation of an IFN-gamma Assay in the Diagnosis of Latent Tuberculosis in Patients with Psoriasis in a Highly Endemic Setting

Screening for latent tuberculosis infection is mandatory before starting anti-tumour necrosis factor treatments, but its diagnosis still poses a challenge. While studies performed in developed countries have demonstrated superior performance of T-cell based interferon-gamma release assay (IGRA) compared with the tuberculin skin test, there is a debate about whether this holds true in tuberculosis endemic areas. The performance of an IGRA kit (T-SPOT.TB) was evaluated in 33 moderate-to-severe untreated psoriasis patients and, as controls, 30 patients with common dermatological diseases at a tuberculosis highly endemic setting. The frequency of positive tuberculin skin test responses and induration size in controls were higher than in psoriasis patients (53% vs. 18% and 9.3 +/- 1.4 vs. 2.6 +/- 0.7 mm, respectively, p<0.001). In contrast, the frequency of positive response and mean number of spots elicited with the T-SPOT.TB test were not significantly different between patients and controls (47% vs. 43% and 14.7 +/- 3.2 vs. 20.5 +/- 3.1 spots/well, respectively). The two tests presented good agreement in the control, but not the psoriasis group (kappa values of 0.625 and 0.375, respectively). Thus, in a highly tuberculosis-endemic setting the T-SPOT.TB test was superior to the tuberculin skin test in diagnosing latent tuberculosis infection in psoriasis, probably because the immune dysregulation of psoriasis shows a lower interference in the in vitro test.