Gout is a common and challenging problem in South Auckland, New Zealand. Allopurinol is widely used but urate reduction remains unsatisfactory. Allopurinol dosing guidelines and a therapeutic range for plasma oxypurinol levels have been published.
We aimed to determine the appropriateness of allopurinol dosing according to current guidelines and to assess the relationship between plasma creatinine, oxypurinol and urate. In addition, we assessed the clinical usefulness of the oxypurinol level.
Thirty-one patients, on a stable dose of allopurinol for at least three weeks, had plasma creatinine, urate and oxypurinol measured as part of routine clinical assessment. Relationships between the various methods were examined using regression analysis. Fisher's exact test was used to test associations with categorical variables.
Fifty-five per cent of patients were on higher than recommended doses of allopurinol. There was a statistically significant relationship between calculated creatinine clearance and plasma oxypurinol level. Only 50% of patients with a plasma oxypurinol within the therapeutic range (30-100 micromol/L) had a plasma urate < 0.42 mmol/L and this did not increase significantly in the patients with an oxypurinol level > 100 micromol/L.
There is poor adherence to the current recommended dosing guidelines for allopurinol. Creatinine clearance rather than plasma creatinine needs to be used to predict the dose of allopurinol. The current role of the oxypurinol level is to identify non-compliers with allopurinol therapy. We need further research to clarify whether increasing the dose of allopurinol outside the recommended dose range to reach an oxypurinol level of close to 100 micromol/L may be of benefit in those who have not had sufficient urate reduction.