Tumour necrosis factor α (TNFα) has a key role in chronic arthritis,¹ but it also participates in the defence against infection. TNFα, together with interferon γ, contributes to the immune response against mycobacteria.² This can explain the higher incidence of tuberculosis seen in patients receiving anti-TNFα treatment.³ Therefore, a tuberculin purified protein derivative (PPD) test has been recommended before starting anti-TNFα treatment to better identify patients at risk for tuberculosis reactivation requiring isoniazid treatment.⁴ However, patients with chronic arthritis can be anergic to delayed-type hypersensitivity (DTH) tests, owing to either active disease or to immunosuppressive treatment.⁵ Some authors have previously shown that the DTH response in patients given second line treatment improves, but the effect of anti-TNFα treatment is not known.⁶

We have studied, …