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Correspondence Dapsone in rheumatoid arthritisSir, We read with interest the paper by Swinson et al.1 on a double-blind trial of dapsone against placebo in the treatment of rheumatoid arthritis. We recently undertook a similar double-blind study using dapsone in an identical dose regimen to that already described (50 mg daily for one week and thereafter 100 mg daily) in 14 white patients with seropositive rheumatoid arthritis and comparing it against placebo (vitamin C) in 14 matched controls.While we observed a similar trend towards improvement in both clinical and serological parameters in the dapsone-treated group, the incidence of anaemia secondary to haemolysis in our dapsone-treated patients was greater than that previously recorded.2 Four of the 14 patients receiving dapsone had falls in haemoglobin of greater than 3 g/dl, and a further 3 patients had falls of between 1 and 3 g/dl. In all cases the fall in haemoglobin occurred within 6 weeks of commencing the therapy. Only one patient in the placebo group had a fall in haemoglobin of greater than 1 g/dl.It has been demonstrated that the haemolytic activity of dapsone is dependent upon its being metabolised to an acetylated form.3 Therefore, in an attempt to elucidate further why some of our patients should develop severe haemolysis, we assessed the acetylator status (using isoniazid) in 13 of the patients receiving dapsone, 3 of whom had had falls in haemoglobin of greater than 3 g/dl and 3 of whom had had falls between 1 and 3 g/dl. The acetylator studies were performed in the University Department of Pharmacology, Newcastle upon Tyne. Eight of the patients were fast acetylators, but we failed to observe any association between the acetylator status and the severity of haemolysis.This pilot study, therefore, while supporting the view that dapsone is an effective therapy in rheumatoid arthritis, also reveals a higher incidence of significant haemolysis than has previously been described. It does not appear that the identification of the acetylator status of rheumatoid patients is of any benefit in identifying patients who may develop significant haemolysis.