Divergence between stone composition and urine supersaturation: Clinical and laboratory implications

Purpose: In general high urine supersaturation with respect to calcium oxal ate, calcium phosphate or uric acid is associated with that phase in stones . We explore the exceptions when supersaturation is high and a correspondin g solid phase is absent (type 1), and when the solid phase is present but s upersaturation is absent or low (type 2), Materials and Methods: Urine supersaturation values for calcium oxalate, ca lcium phosphate and uric acid, and other accepted stone risk factors were m easured in 538 patients at a research clinic and 178 at stone prevention si tes in a network served by a single laboratory. Results: Of the patients 14% lacked high supersaturation for the main stone constituent (type 2 structural divergence) because of high urine volume an d low calcium excretion, perhaps from changes in diet and fluid intake prom pted by stones. Higher calcium excretion and low urine volume caused type 1 divergences, which posed no clinical concern. Conclusions: Type 1 divergence appears to represent a condition of low urin e volume which raises supersaturation in general. Almost all of these patie nts are calcium oxalate stone formers with the expected high supersaturatio n with calcium oxalate as well as high uric acid and calcium phosphate supe rsaturations without either phase in stones, Type 2 divergence appears to r epresent an increase in urine volume and decrease in urine calcium excretio n between stone formation and urine testing.