Introduction Two commercial Interferon Gamma release assays (IGRA) are approved in the UK by NICE to detect M tuberculosis (Mtb) infection. They use different test platforms. Both may provide borderline (neither clear positive or negative) or indeterminate (failure of control samples) results, especially in people taking immunomodulatory therapy. Since 2008, we have applied a standard IGRA-based assessment for latent Mtb infection in inflammatory bowel disease (IBD) patients being considered for anti-TNFα therapy. Initially this involved T-Spot.TB (TSTB) but in December 2010, our service switched to Quantiferon Gold In-tube (QFGIT). Here we review the performance and cost-effectiveness of these assays within our protocol.
Method Adult IBD patients were assessed using symptom review, chest radiograph (CXR) and IGRA Indeterminate/borderline IGRA were repeated and patients with persistently indeterminate/borderline results plus TB risk factors, or a positive result were referred to TB services. Cost per patient assessment used the average of assay costs and repeated assays plus onward referrals for those with indeterminate/borderline tests. Appointment costs were taken from standard NHS tariffs. Immunomodulators were defined as thiopurines, methotrexate or prednisolone >20mg/day.
Results Between October 2008 and November 2010, 90 patients were tested with TSTB, of which 2 had a positive TSTB result and 5 borderline/indeterminate. From December 2010 until July 2012, 82 patients were tested with QFGIT, of which 3 had a positive result and 12 indeterminate/borderline (Table). 170 (99%) had normal CXR and a negative clinical assessment. 4 of 13 patients had two sequential indeterminate IGRA and also required assessment in TB clinic. The average price per patient was £60.66 for TSTB and £52.41 for QFGIT. 88% (152/172) have subsequently received treatment with either infliximab or adalimumab. No subjects have gone on to develop active tuberculosis.
Conclusion Using either platform, we find a comparable, low rate of LTBI in our IBD population. There appears to be a higher frequency of indeterminate results using QFGIT. This raises the average cost per patient, but overall, QFGIT remains more cost-effective than TSTB. Despite differing length of follow-up, the average time was sufficient in both, otherwise comparable, cohorts to detect likely development of active TB disease.