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P207. Indeterminate and inconclusive results are common when using interferon gamma release assay as screening for TB in patients with IBD

I. Nasr, R. Goel, M. Ward, S. Fong, K. Patel, M. Sastrillo, S. Anderson, J. Sanderson, P. Irving, Guy's and St Thomas' NHS Foundation Trust, Gastroenterology, London, United Kingdom

Background

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 the use of interferon gamma release assays (IGRAs) to screen for latent TB as, unlike tuberculin skin test, positive tests are not caused by previous Bacillus Calmette-Guérin (BCG) vaccine. However, immunosuppressive agents can result in indeterminate or unreportable results and there is no clear guidance on managing them.

Aim: To quantify the prevalence of indeterminate or unreportable TB IGRA Elispot results in a large tertiary centre cohort of patients with IBD.

Methods

We performed 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

140 patients were included (median age 34, range 24–86, 50% males). 92% had Crohn's disease (CD), 4% ulcerative colitis (UC), and 4% IBD-unclassified. At the time of IGRA testing, 115 patients were on immunomodulators (81 azathioprine, 11 mercaptopurine, 2 tioguanine, 11 methotrexate) or prednisolone (6).

Three patients were positive for latent TB infection at screening and were referred to the infectious disease (ID) department prior to anti-TNF therapy.

Three patients had indeterminate results; all were on immunosuppressants (2 azathioprine, 1 methotrexate). 2 had a lymphocyte count <1. In 2 cases the IGRA was repeated, one was negative and the second was unreportable on 2 occasions. None had TB risk factors and all were started on anti-TNF. To date, none have developed TB (follow up range 6–18 months).

Ten patients had unreportable results, 9 of whom were taking 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). Lymphopenia 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.

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