A risk factor model of stone-formation.

A simple method for assessing the biochemical risk of forming uric acid and/or calcium-containing stones would be extremely useful for screening patients with urinary stone disease before and during their clinical management and follow-up. This paper describes a simplified procedure for calculating the overall biochemical risk of forming stones consisting of uric acid, calcium oxalate, calcium phosphate or various combinations of these common stone constituents making use of seven analyses normally carried out on 24-h urine samples in most Stone Clinics. The contribution of each risk factor towards the overall biochemical risk of forming stones (P(SF)) is calculated from a set of "risk curves" derived from frequency distributions of the seven risk factors measured in the 24-h urine samples from a large number of idiopathic stone-formers and their controls. PSF discriminates well between stone-formers and normal subjects and predicts the likely severity of the disorder in a given individual as defined by the number of stone episodes per year experienced by the patient concerned.