Comparison of in vitro-specific blood tests with tuberculin skin test for diagnosis of latent tuberculosis before anti-TNF therapy

Introduction

Latent tuberculosis infection (LTBI) is detected with the tuberculin skin test (TST) before anti‐TNF therapy. We aimed to investigate in vitro blood assays with TB‐specific antigens (CFP‐10, ESAT‐6), in immune‐mediated inflammatory diseases (IMID) for LTBI screening.

Patients and methods

Sixty‐eight IMID patients with (n?=?35) or without (n?=?33) LTBI according to clinico‐radiographic findings or TST results (10?mm cutoff value) underwent cell proliferation assessed by thymidine incorporation and PKH‐26 dilution assays, and IFNγ‐release enzyme‐linked immunosorbent spot (ELISPOT) assays with TB‐specific antigens.

Results

In vitro blood assays gave higher positive results in patients with LTBI than without (p<0.05), with some variations between tests. Among the 13 patients with LTBI diagnosed independently of TST results, 5 had a negative TST (38.5%) and only 2 a negative blood assays result (15.4%). The 5 LTBI patients with negative TST results all had positive blood assays results. Ten patients without LTBI but with intermediate TST results (6–10?mm) had no different result than patients with TST result ?5?mm (p>0.3) and lower results than those with LTBI (p<0.05) on CFP‐10+ESAT‐6 ELISPOT and CFP‐10 proliferation assays.

Conclusion

Anti‐TB blood assays are beneficial for LTBI diagnosis in IMID. Compared with TST, they show a better sensitivity, as seen by positive results in 5 patients with certain LTBI and negative TST, and better specificity, as seen by negative results in most patients with intermediate TST as the only criteria of LTBI. In the absence of clinico‐radiographic findings for LTBI, blood assays could replace TST for antibiotherapy decision before anti‐TNF.TNFα blocker agents are approved for the treatment of immune‐mediated inflammatory diseases (IMID) and provide marked clinical benefit. However, they can reactivate tuberculosis (TB) infection in patients previously exposed to TB bacilli.^1,2 The presence of quiescent mycobacteria defines latent TB infection (LTBI).^3,4 Thus, screening for LTBI is necessary before initiating therapy with TNF blockers.⁵ However, to date, no perfect gold standard exists for detecting LTBI, and tuberculin skin test (TST) remains largely used. The recommendations for detecting LTBI differ worldwide.^3,6,7 In France, recommendations were established in 2002 by the RATIO (Research Axed on Tolerance of Biotherapies) study group for the Agence Française de Sécurité Sanitaire des Produits de Santé.^8,9 Patients are considered to have LTBI requiring treatment with prophylactic antibiotics before starting anti‐TNFα therapy if they had previous TB with no adequate treatment, tuberculosis primo‐infection, residual nodular tuberculous lesions larger than 1?cm³ or old lesions suggesting TB diagnosis (parenchymatous abnormalities or pleural thickening) as seen on chest radiography or weals larger than 10?mm in diameter in response to the TST. Adequate anti‐TB treatment was defined as treatment initiated after 1970, lasting at least 6?months and including at least 2?months with the combination rifampicin–pyrazinamide. The choice of the threshold of 10?mm for the TST result was established in 2002 in France since the programme of vaccination with bacille Calmette–Guérin (BCG) was mandated in France, and nearly 100% of the population has been vaccinated. Nevertheless, after July 2005, the threshold was decreased to 5?mm as in most of all other countries.¹⁰The TST is the current method to detect LTBI but has numerous drawbacks. Indeed, the TST requires a return visit for reading the test result. It has a poor specificity, since previous BCG vaccination and environmental mycobacterial exposure can result in false‐positive results in all subjects.^6,11,12 This poor specificity can lead to unnecessary treatment with antibiotics, with a significant risk of drug toxicity.^13,14,15 On the other hand, TST in IMID may often give a more negative reaction than in the general population, mainly because of the disease or immunosuppressive drug use.^16,17 This poor sensitivity can lead to false‐negative results, with a subsequent risk of TB reactivation with anti‐TNF therapy.The identification of genes in the mycobacterium TB genome that are absent in BCG and most environmental mycobacteria offers an opportunity to develop more specific tests to investigate Mycobacterium tuberculosis (M. tuberculosis) infection, particularly LTBI.¹⁸ Culture fibrate protein‐10 (CFP‐10) and early secretory antigen target‐6 (ESAT‐6) are two such gene products that are strong targets of the cellular immune response in TB patients. In vivo‐specific T‐cell based assay investigating interferon gamma (IFNγ) release or T‐cell proliferation in the presence of these specific mycobacterial antigens could be useful in screening for LTBI before anti‐TNF therapy. New IFNγ‐based ex vivo assays involving CFP‐10 and ESAT‐6 (T‐SPOT TB, Oxford Immunotec, Abingdon, UK) and QuantiFERON TB Gold (QFT‐G; Cellestis, Carnegie, Australia) allow for diagnosis of active TB, recent primo‐infection or LTBI.¹² These tests seem to be more accurate than the TST for this purpose in the general population.¹² To date, the performance of the commercial assays in detecting LTBI in patients with IMID receiving immunosuppressive drugs has not been demonstrated, and the frequency of indeterminate results is still debated.^19,20,21We aimed to investigate the performance of homemade anti‐CFP‐10 and anti‐ESAT‐6 proliferative and enzyme‐linked immunosorbent spot (ELISPOT) assays in detecting LTBI in patients with IMID before anti‐TNFα therapy. We analysed two subgroups of patients: those with confirmed LTBI independent of TST result, and those with LTBI based exclusively on a positive TST result between 6 and 10?mm.