Sensitivity and Specificity of QuantiFeron -TB Gold Test (QFT-G)in Comparison With Tuberculin Skin Test in Patients With Psoriasis and Psoriatic Arthritis

Patients with psoriasis and psoriatic arthritis are candidates to receive anti-TNF alpha therapies which require prior screening for latent tuberculosis (LTB). Currently, screening for LTB is based on tuberculin skin test (TST) , chest X rays, and a questionaire on predisposing factors for TB. The main drawbacks of TST are the lack of specificity due to cross reactivity with Bacille Calmette- Guerin (BCG) and other nontuberculosis mycobacteria and the risk of anergy in immunosuppressed patients. Furthermore, it has been suggested that the skin of psoriatic patients may be more sensitive resulting in increased TST which does not obligatory reflect the status of LTB.

Recently, a new assay for LTBI has been developed, which evaluates interferon (IFN) -γ release by memory effector T-cells stimulated in vitro with specific mycobacterial antigens, ESAT-6 (early secretory antigen target-6) and CFP-10 (culture filtrate protein-10) [9-10]. The QuantiFeron -TB Gold test (QFT-G) uses ELISA to measure IFN-γ concentrations in supernatants in plate format and "In tube" format (QFT-GIT) while the enzyme-linked immunospot (ELISPOT) detects individual IFN-γ producing T-cells (TS-TB, Oxford Immunotech, Abingdon, UK).

The whole blood IFN-γ assays was approved by The Centers for Disease Control as an alternative screening strategy to TST in immunocompetent individuals [11], but its clinical utility as a single test for detection LTBI in immunocompromised patients is controversial.Furthermore, its utility in patients with psoriasis and psoriatic arthritis has not been yet established One hundred patients with psoriasis and psoriatic arthritis and 50 healthy control will participate in this study.

Enrolled subjects will be requested to complete a detailed sociodemographic and TB screening questionnaire including gender, age, place of birth and work, prior BCG vaccination, close contact with TB patients or TB prophylaxis in the past. Screening workup will includ assessment of clinical disease activity using the Disease Activity Score 28 (DAS-28)and Psoriasis Area Severity Index (PASI),documentation of past or current treatment with systemic corticosteroids and immunosuppressive drugs, and imaging (chest X-ray).

All the subjects will undergo a TST and QFT-G test A 2-TU dose of PPD will be administered by a certified technician using the Mantoux method and induration measured after 72 h. TST will be deemed positive if bove or equally to 5 mm for RA patients and 10 mm for controls The absence of induration of <2 mm in diameter will be recorded as anergic and negative TST results was defined as having more than 2 but less than 5 mm reactions for RA patients.