Selective screening for distal renal tubular acidosis in recurrent kidney stone formers: initial experience and comparison of the simultaneous furosemide and fludrocortisone test with the short ammonium chloride test.

Background: Distal renal tubular acidosis (dRTA) is associated with renal stone disease, and it often needs to be considered and excluded in some recurrent calcium kidney stone formers (KSFs). However, a diagnosis of dRTA, especially when 'incomplete', can be missed and needs to be confirmed by a urinary acidification (UA) test. The gold standard reference test is still the short ammonium chloride (NHCl) test, but it is limited by gastrointestinal side effects and occasionally failure to ingest sufficient NHCl. For this reason, the furosemide plus fludrocortisone (F+F) test has been proposed as an easier and better-tolerated screening test. The aim of the present study was to assess the usefulness of the F+F test as a clinical screening tool for dRTA in a renal stone clinic.

Methods: We studied 124 patients retrospectively in whom incomplete dRTA was suspected: 71 had kidney stones only, 9 had nephrocalcinosis only and 44 had both. A total of 158 UA tests were performed: 124 F+F and 34 NHCl; both tests were completed in 34 patients.

Results: The mean age was 45.4 ± 15 years, and 49% of patients were male. The prevalence of complete and incomplete dRTAs was 7 and 13.7%, respectively. Of the 34 patients tested using both tests, 17 (50%) were abnormal and 4 (12%) were normal. Thirteen (39%) patients were abnormal by F+F, but normal by NHCl [sensitivity 100% (95% CI 80-100), specificity 24% (95% CI 7-50), positive predictive value 57% (95% CI 37-75), negative predictive value 100% (95% CI 40-100)].

Conclusions: The F+F test is characterized by an excellent sensitivity and negative predictive value, and the diagnosis of incomplete dRTA can be excluded reliably in a patient who acidifies their urine normally with this test. However, its lack of specificity is a drawback, and if there is any doubt, an abnormal F+F test may need to be confirmed by a follow-up NHCl test. Ideally, a prospective blinded study in unselected KSFs is needed to accurately assess the reliability of the F+F test in diagnosing, rather than excluding, dRTA.