Background Nitrofurantoin is probably the agent of choice for urinary tract infections (UTIs), but its use is limited by its lack of efficacy in impaired renal function.

Purpose The British National Formulary says to avoid in patients with renal impairment (estimated glomerular filtration rate [eGFR] <60 ml/min), but the Renal Drug Handbook recommends use if >20 ml/min. This pilot study was to look at which guidance provided the best outcome.

Materials and Methods Patients over 18 years from a single city centre medical practise were reviewed if they had received nitrofurantoin prescriptions and an eGFR had been recorded. Where there was low eGFR, a Cockcroft & Gault Creatinine Clearance (C&G-IBW-ClCr) based on the ideal body weight (IBW) was performed. Outcomes were reviewed. Success was assumed if there were no further antibiotics, no admission to hospital for a related episode or not recorded as still symptomatic on their medical records.

Results Of 164 patients, 37 were reviewed. Average age: 72 (range 21–100); median 80 years. Average eGFR/1.73 m² = 73.8 ml/min (range 33–130) and C&G-IBW-ClCr = 55 ml/min (24–127). Of 15 patients with C&G-IBW-ClCr >60 ml/min, none needed further antibiotics or were recorded as still symptomatic.

22 patients with C&G-IBW-ClCr <60 ml/min (average eGFR 61.7 ml/min and CrCl 38.7 ml/min), eighteen (81.8%) had further antibiotics or were recorded as still symptomatic. Only seven patients (31.8%) had an eGFR/1.73 m² <60 ml/min. Twelve had further antibiotics, 4 were still symptomatic, 1 went into hospital (unrelated) and 1 went back onto prophylactic antibiotics. No sample stated resistance but 6 samples stated sensitivity. The successfully treated patients had an eGFR of 75, 57, 55, & 53 ml/min /1.73 m² & a CrCl of 36, 39, 50 & 53 ml/min.

Conclusions Nitrofurantoin should not be recommended where renal function is impaired. This pilot study shows that eGFR is not a good indicator of renal function, and that CrCl should be used. Over 80% with a CrCl < 60 ml/min needed further treatment. This will progress to a larger study.

No conflict of interest.