Introduction Interferon-gamma (IFN-γ) release assay is used to identify patients with latent tuberculosis prior to biological treatment initiation. A lack of stimulation capacity of lymphocytes by the test control results in an indeterminate result. This is not uncommon in inflammatory bowel disease (IBD) patients receiving immunosuppressive agents.1,2,3 The aim of this study is to compare the clinical and laboratory characteristics of patients with indeterminate results to those with determinate results.
Methods The national database of IBD patients receiving biological agents was accessed. IFN-γ release assay (QuantiFERON-TB Gold) results were noted. Parameters at the time of testing were collected, namely age, diagnosis, disease activity, blood results and drug treatment. Comparisons between the indeterminate group and determinate group were made using IBM® SPSS® Statistics Version 20. The study was approved by the University and Research Ethics Committee of Malta.
Results Data from 136 subjects was analysed. 8.8% (n = 12) had an indeterminate IFN-γ release assay result, 2.2% (n = 3) had a positive result, while 89% (n = 121) had a negative result. The indeterminate group had a significantly lower lymphocyte count (p = 0.041), globulin level (p = 0.042), immunoglobulin A (IgA) (p = 0.031) and haemoglobin (p = 0.016). C-reactive protein was higher in the indeterminate group (p = 0.024). No significant differences were recorded for age, gender, diagnosis, disease activity, white cell count, neutrophils, platelets, renal/liver function, albumin levels, IgM, IgG, or drug treatment. Critically, only 25% of indeterminate result subjects had a repeat test, all reported as negative.
Conclusion Indeterminate results are common in IBD patients, affecting nearly 1 in 10 patients. Patients with low lymphocyte counts, low globulin, low haemoglobin, low IgA and high CRP levels are more likely to have an indeterminate result when tested. The potential of a false negative result is likely to be higher in these patients. One must exert caution when requesting IFN-γ release assays in such IBD patients since the probability of a determinate result may be low. We advise test repetition in all patients with indeterminate results and screening with chest x-ray.
References 1 Helwig U, Müller M, Hedderich J, et al, Corticosteroids and immunosuppressive therapy influence the result of QuantiFERON TB Gold testing in inflammatory bowel disease patients. J Crohns and Colitis 2012;6:419–424.
2 Jeong SJ, Han SH, Kim CO, et al, Predictive factors for indeterminate result on the QuantiFERON test in an intermediate tuberculosis-burden country. J Infect. 2011;62:347–354.
3 Kobashi Y, Sugiu T, Mouri Y, et al, Indeterminate results of QuantiFERON TB-2G test performed in routine clinical practice. Eur Respir J 2009;33:812–815.
Disclosure of Interest None Declared