Su1101 Indeterminate and Inconclusive Results Are Common When Using Interferon Gamma Release Assay As Screening for TB in Patients With IBD

Abstract

Introduction Anti-TNF treatment is widely used in inflammatory bowel disease (IBD) but has been linked with reactivation of tuberculosis (TB). Screening for active and latent TB prior to initiation of anti-TNF therapy is therefore mandated. ECCO recommends interferon gamma release assays (IGRAs) as, unlike tuberculin skin test, positive tests are not caused by previous Bacillus Calmette–Guerin (BCG) vaccine. However, immunosuppressive agents can result in indeterminate or unreportable results[i] and there is no clear guidance on managing them. We quantified the prevalence of indeterminate or unreportable TB IGRA Elispot results in a large tertiary centre cohort of patients with IBD. Methods A single centre retrospective study of IGRA tests performed on IBD patients prior to commencement of anti-TNF therapy between Oct 2010 and Oct 2013. Results We included 140 patients (median age 34, range 24–86, 50% males). 92% had Crohn’s disease, 4% ulcerative colitis, and 4% IBD-unclassified. At the time of IGRA testing, 115 patients were on immunomodulators and 6 on prednisolone. 3 were positive for latent TB and were referred to infectious disease (ID) department prior to anti-TNF therapy. 3 had indeterminate results; all were on immunosuppressants (2=azathioprine, 1=methotrexate). 2 had a lymphocyte count 10 had unreportable results, 9 of whom were on azathioprine. On repeat testing, 4 were negative, and the remainder were still unreportable, one of whom had risk factors for TB and was treated with isoniazid chemoprophylaxis on the advice of the ID team. The remaining 5 patients started anti-TNF based on the absence of risk factors for TB. No patient had reactivation of latent TB at follow up (range 1–18 months). Lymphopaenia was found to be associated with non-reportable cases as compared to the reported cases (median lymphocyte count unreportable = 0.4, reportable = 1.2; p = 0.015). Conclusion Our results demonstrate TB IGRA is a useful test to screen for latent infection before initiating anti-TNF therapy. However, a minority of results are indeterminate or unreportable. In such cases repeat testing can produce definitive results. Low lymphocyte counts in association with immunosuppression may contribute to unreportable and indeterminate results; clinical risk stratification appears to be a safe way of managing such cases in this small cohort. Reference Papay P et al . Predictors of indeterminate IFN-γ release assay in screening for latent TB in inflammatory bowel diseases. Eur J Clin Invest. 2011 Disclosure of Interest None Declared.