Allopurinol is the most commonly used urate-lowering therapy in gout. Allopurinol hypersensitivity syndrome (AHS) is a rare but potentially fatal adverse event. Dosing guidelines based on creatinine clearance have been proposed based on the recognition that dosages of ≥300 mg/day may be associated with AHS, particularly in patients with renal impairment. However, the relationship between the allopurinol starting dose and AHS is unknown. This study was undertaken to determine the relationship between allopurinol dosing and AHS.

A retrospective case-control study of patients with gout who developed AHS between January 1998 and September 2010 was undertaken. For each case, 3 controls with gout who were receiving allopurinol but did not develop AHS were identified. Controls were matched with cases for sex, diuretic use at the time of initiating allopurinol, age (±10 years), and estimated glomerular filtration rate (estimated GFR). Starting dose and dose at the time of the reaction in cases were compared between cases and controls.

Fifty-four AHS cases and 157 controls were identified. There was an increase in the risk of AHS as the starting dose of allopurinol corrected for the estimated GFR increased. For the highest quintile of starting dose per estimated GFR, the odds ratio was 23.2 (P < 0.01). Receiver operating characteristic analysis indicated that 91% of AHS cases and 36% of controls received a starting dose of allopurinol of ≥1.5 mg per unit of estimated GFR (mg/ml/minute).

Our findings indicate that starting allopurinol at a dose of 1.5 mg per unit of estimated GFR may be associated with a reduced risk of AHS. In patients who tolerate allopurinol, the dose can be gradually increased to achieve the target serum urate level.