The frusemide test: simple screening test for renal acidification defect in urolithiasis.

Urinary pH was first labelled as a risk factor for patients with recurrent calcium stone disease of the renal tract in 1978. The standard diagnostic test for patients with a history of stones, who appear to have urinary acidification defects is the ammonium chloride test. However, the performance of this test results in significant morbidity, particularly nausea and vomiting, and as a result there is little enthusiasm for screening for such defects. We describe a screening test that may be used to determine which patients require more definitive testing. It involves an oral dose of frusemide (40 mg) followed by half-hourly urine sampling for pH. For the detection of a renal tubular acidification defect, the frusemide test had sensitivity = 100%, specificity = 82%, predictive value of a positive result = 40%, predictive value of a negative result = 100%, screening efficiency = 84%, and there was no morbidity. During the ammonium chloride test 80% of the patients vomited or felt very nauseated.