Oxypurinol, the active metabolite of allopurinol, is the major determinant of the hypouricemic effect of allopurinol. Monitoring oxypurinol concentrations is undertaken to determine adherence to therapy, to investigate reasons for continuing attacks of acute gout and/or insufficiently low plasma urate concentrations despite allopurinol treatment, and to assess the risk of allopurinol hypersensitivity, an adverse effect that has been putatively associated with elevated plasma oxypurinol concentrations.
An audit of request forms requesting plasma oxypurinol concentration measurements received by the pathology service (SydPath) at St Vincent's Hospital, Darlinghurst, Sydney was undertaken for the 7-year period January 2005-December 2011. Patient demographics, biochemical data, including plasma creatinine and uric acid concentrations, comorbidities, and concomitant medications were recorded.
There were 412 requests for determination of an oxypurinol concentration. On 48% of occasions, the time of allopurinol dosing was recorded, while just 79 (19%) blood samples were collected 6-9 hours postdosing, the time window used to establish the therapeutic range for oxypurinol. For these optimally interpretable concentrations, 32 (8%) were within the putative therapeutic range (5-15 mg/L), while 5 (1%) were below and 41 (10%) above this range. The daily dose of allopurinol was documented on only one-third of the request forms. Individually, plasma urate and creatinine concentrations were requested concomitantly with plasma oxypurinol concentrations in 66% and 58% of the cases, respectively; while plasma oxypurinol, urate, and creatinine concentrations were requested concomitantly in 49% of the cases.
Requesting clinicians and blood specimen collectors often fail to provide relevant information (dose, times of last dose, and blood sample collection) to allow the most useful interpretation of oxypurinol concentrations. Concomitant plasma urate and creatinine concentrations should be requested to allow more complete interpretation of the data.