Saturation-Inhibition Index as a Measure of the Risk of Calcium Oxalate Stone Formation in the Urinary Tract

Abstract

Studies were carried out on multiple urine-samples from eight patients with recurrent idiopathic-calcium oxalate stone formation and eight normal-persons to define an index of the risk of forming-calcium oxalate stones. Under the same conditions of dietary and fluid intake the urine samples of the patients with stone formation were more supersaturated with calcium oxalate (P < 0.001) and had lower concentrations of protective inhibitors of crystallization (P < 0.001) than those of the controls. However, the best separation between the groups was defined by a discriminant line relating inhibitory activity and urine-saturation. A measure of the risk of forming large crystals, the saturation-inhibition index, was defined as the distance of each urine from the discriminant line. The patients with stone formation had a significantly higher-mean saturation-inhibition index than the controls (P << 0.001). Both the percentage of large calcium oxalate crystals excreted (P < 0.001) and the stone episode rate (P <0.005) were significantly correlated with the saturation-inhibition index. (N Engl J Med 294:-249–252, 1976)