Tuberculin skin test overestimates tuberculosis hypersensitivity in adult patients with psoriasis.

Background: Tuberculin skin testing (TST) is still the reference method for the diagnosis of latent tuberculosis infection (LTBI).

Objective: To evaluate the effect of psoriasis on TST.

Methods: Comparison of TST results of consecutive dermatology (n = 91) and internal medicine (n = 615) inpatients. TST was uniformly ordered, performed and evaluated according to the Mantoux method using purified protein derivative.

Results: (a) Significantly larger TST indurations were measured in dermatology (median: 7 mm) compared to internal medicine inpatients (median: <1 mm; p < 0.0001). More dermatology inpatients showed 'positive' (>5 mm) and 'strongly positive' (>or=15 mm) TST results (53 vs. 29% and 22 vs. 13%, respectively). (b) Among dermatology inpatients, the TST reactions were significantly larger (p < 0.01) in psoriatics (n = 28) compared to the remaining patients (n = 63). (c) In psoriatics, the TST correlated positively with the psoriasis area and severity index score (p = 0.015).

Conclusion: Overt psoriasis is associated with increased TST measurements. This observation is suggestive of a possible overtreatment of these patients for LTBI.