A method for studying inhibitory activity in whole urine.

A method has been developed for inducing and quantifying calcium oxalate crystallisation in whole human urine. The propensity of a given urine to induce crystal formation was described in two ways: its ability to resist spontaneous nucleation of calcium oxalate crystals was assessed by titrating 20 mls of the urine with increasing quantities of sodium oxalate (0-150 mumol) to determine its practical metastable limit. This limit was inversely related to the endogenous calcium concentration; its capacity to inhibit crystal growth was quantified by determining the rate of growth of calcium oxalate crystals precipitated in response to a fixed oxalate load (30 mumol) above its metastable limit. The crystals produced were predominantly calcium oxalate dihydrate and were morphologically identical to those occurring naturally in urine. Citrate had no effect on the metastable limits of 3 urines examined, but markedly inhibited crystal growth. Pyrophosphate had a similar effect on crystal growth, and in addition, raised the metastable limit of one of the urine samples.