What laboratory tests should be done while on allopurinol (xanthine oxidase inhibitor)?

Laboratory Monitoring for Allopurinol Therapy

Monitor serum uric acid levels regularly using a treat-to-target strategy, aiming for levels <6 mg/dL, with dose titration guided by serial measurements. 1

Essential Laboratory Tests

Serum Uric Acid Monitoring

• Measure serum uric acid regularly to guide dosing adjustments and ensure therapeutic targets are achieved 1, 2
• The therapeutic goal is to maintain serum uric acid below 6 mg/dL (360 μmol/L), which is below the saturation point for monosodium urate crystal formation 1
• Serial measurements should guide dose titration in increments of 100 mg every 2-4 weeks until target is reached 1

Renal Function Monitoring

• Monitor renal function parameters regularly, particularly BUN, serum creatinine, and creatinine clearance 2
• This is critical because allopurinol and its active metabolite oxypurinol are eliminated by the kidneys, and changes in renal function profoundly affect dosing requirements 2
• Renal function monitoring is especially important in patients with concurrent conditions affecting kidney function such as hypertension and diabetes mellitus 2
• Patients on thiazide diuretics require particularly close renal function monitoring, even in the absence of baseline renal failure, as combined therapy may enhance allopurinol toxicity 2

Liver Function Tests

• Perform periodic liver function tests during early stages of therapy in patients with pre-existing liver disease 2

Additional Monitoring Considerations

For Patients on Anticoagulants

• Reassess prothrombin time periodically in patients receiving dicumarol who are given allopurinol, as allopurinol prolongs the half-life of this anticoagulant 2

For High-Risk Populations

• Consider HLA-B*5801 testing before initiating allopurinol in patients of Southeast Asian descent (Han Chinese, Korean, Thai) and African American patients, as this allele is associated with markedly elevated risk for allopurinol hypersensitivity syndrome 1
• Testing is conditionally recommended against in other ethnic/racial backgrounds due to lower prevalence 1

Monitoring During Dose Adjustments

Renal Impairment Considerations

• Patients with decreased renal function require lower initial doses and closer observation during early administration 2
• In severely impaired renal function, the half-life of oxipurinol is greatly prolonged, potentially requiring doses as low as 100 mg per day or 300 mg twice weekly 2
• Dose reduction of 50% or more is recommended in renal failure 1

Drug Interaction Monitoring

• For patients on 6-mercaptopurine or azathioprine, monitor for toxicity as these drugs require dose reduction of 65-75% when used concomitantly with allopurinol 1
• Monitor cyclosporine levels when co-administered, as allopurinol may increase cyclosporine concentrations 2

Common Pitfalls to Avoid

• Do not use fixed-dose allopurinol (e.g., 300 mg) without titration to target serum uric acid levels, as this approach is inferior to treat-to-target strategies 1
• Do not discontinue monitoring once symptoms are controlled, as this leads to recurrence of gout flares 3
• Do not assume adequate dosing based solely on creatinine clearance calculations without measuring actual serum uric acid response, as approximately 89% of patients can safely achieve target levels with doses above traditional creatinine-based recommendations 4
• Patients should be instructed to discontinue allopurinol immediately and contact their physician at the first sign of skin rash, painful urination, blood in urine, eye irritation, or swelling of lips/mouth 2