Comparing risk estimates of sulphonamide-induced agranulocytosis from the Swedish Drug Monitoring System and a case-control study.

A comparison has been made of risk estimates for agranulocytosis connected with sulphasalazine and trimethoprim-sulphamethoxazole (T-SM) therapy calculated from data in the Swedish Drug Monitoring System ("spontaneous" reports, sales and prescription information) and a population based case-control study (the IAAAS). The relative risk for agranulocytosis during sulphasalazine treatment was calculated to be 107 and 123 by the spontaneous reporting system and the case-control study, respectively. The corresponding excess risk in both systems was 1.8. For T-SM the relative risk was 17 in the spontaneous reporting system and 12 in the case-control study, while the excess risk calculated for 3 days of treatment was 0.9 in the spontaneous reporting system, and 1.6 for 3 or more days of treatment in the case-control study. It is concluded that the Swedish Drug Monitoring System gives an appropriate estimate of the risk of developing agranulocytosis in association with the two drugs studied.