Ammonium and sodium urates precipitating from synthetic urine and fine structure of urate renal calculi

A. study of ammonium and sodium urate precipitation in vitro and the fine structure of several urate renal calculi was carried out to contribute to an understanding of the participation of ammonium and sodium urates in urolithiasis. Ammonium urate precipitated in vitro in two different morphologies: a typical spherulite morphology formed at high supersaturation and disorganized needle-like crystals formed at low supersaturation. In all cases sodium urate precipitated in vitro as bundles of curved fibrils, its crystallization being inhibited by calcium in concentrations between 20 and 60 mg/l depending on the sodium urate supersaturation. From a collection of 1300 renal calculi, only three had ammonium urate as their main component (0.2%), three were mixed calculi (0.2%) consisting of ammonium urate and calcium oxalate (two) or uric acid (one), and in one calculus ammonium urate was present as a minor component. Only in a mixed calculus of uric acid and calcium oxalate was sodium urate detected in a very low quantity. The study of the fine structure of the renal calculi constituted mainly by ammonium urate demonstrated similar patterns in which spherulites, needle-like individual crystals and an amorphous mass of ammonium urate with abundant organic matter in non-organized structures coexist. As minor components, struvite or calcium oxalate crystals were found. A general mechanism of the formation of such calculi is proposed.