Sulfasalazine associated agranulocytosis in Sweden 1972-1989. Clinical features, and estimation of its incidence.

During the 18 year period 1972-1989 a total of 62 cases of agranulocytosis associated with the use of sulphasalazine were reported to the Swedish Adverse Drug Reactions Advisory Committee (SADRAC). The median age of the patients was 52 y and median duration of sulphasalazine treatment was 43 days. The fatality rate was 6.5%, and among patients who recovered the median recovery time was 10 days. Twelve patients were treated concomitantly with other drugs generally suspected to induce agranulocytosis. From sales and prescription data the average incidence of agranulocytosis during sulphasalazine therapy was estimated to be 1/1750 patient years of exposure. From an ongoing Prescription Monitoring Project in a Swedish county it was possible to calculate the proportion of patients receiving sulphasalazine for different periods of time. The incidence of agranulocytosis during the first 30 days was estimated to be 1/2400 patients, while it was 1/700 between Days 31-90 and 1/11200 during Days 91-365. The risk of developing agranulocytosis during sulphasalazine treatment is considerable during the first three months of treatment, and the traditional way of expressing the risk (1/1750 patient years) underestimates the risk for the individual patient.