Formation of urate uroliths (eg, ammonium urate [AU], uric acid, calcium urate, sodium urate) occurs from urine supersaturation with purine metabolites. Uric acid derives from the metabolic degradation of endogenous purine ribonucleosides and dietary nucleic acids (Figure 1) and is typically converted by uricase within hepatocytes to allantoin. Compared with uric acid and xanthine, allantoin is relatively soluble in urine.
Figure 1. Metabolism of AU uroliths. Image courtesy Mal Rooks, CMI