Macrobid and Allopurinol: No Significant Drug Interaction
There are no clinically significant drug interactions between nitrofurantoin (Macrobid) and allopurinol, and with a GFR of 90 mL/min, nitrofurantoin is safe and appropriate for treating this UTI.
Safety of Nitrofurantoin at GFR 90
Nitrofurantoin is safe and effective at this level of renal function. The patient's GFR of 90 mL/min represents normal kidney function (CKD stage 1 if any kidney damage is present, or normal if no damage), well above the threshold where nitrofurantoin becomes problematic 1
Nitrofurantoin should be avoided when GFR falls below 30-50 mL/min/1.73 m² due to increased risk of pulmonary adverse events and potential ineffectiveness, but this patient is far from that threshold 1
At GFR 90, there is no increased risk of treatment failure or serious adverse events compared to patients with normal renal function 1
Drug Interaction Assessment
No documented pharmacokinetic or pharmacodynamic interactions exist between nitrofurantoin and allopurinol. These medications work through completely different mechanisms and do not affect each other's metabolism or efficacy
The allopurinol dose of 100 mg daily is appropriate as a starting dose for gout management, consistent with guideline recommendations to start low and titrate 2
Both medications can be safely continued concurrently during the 5-day nitrofurantoin course
Allopurinol Dosing Context
The patient's allopurinol 100 mg daily dose is appropriate as an initial or maintenance dose, particularly given the normal renal function 2, 3
With GFR 90, this patient does not require dose reduction of allopurinol and may eventually need dose escalation above 300 mg/day to achieve target serum uric acid <6 mg/dL 2, 3
Starting allopurinol at ≤100 mg/day reduces the risk of allopurinol hypersensitivity syndrome, which is more common with higher starting doses 4
Clinical Recommendation
Proceed with Macrobid 100 mg twice daily for 5 days as planned, continuing allopurinol 100 mg daily without interruption
No dose adjustments, additional monitoring, or medication holds are necessary
The only caveat is ensuring the patient is not experiencing an acute gout flare during the UTI treatment, as this would require separate anti-inflammatory management with corticosteroids (NSAIDs and colchicine have their own considerations in various clinical contexts) 5