We devised a simple and economical technique for measuring crystallization inhibitory activities in calcium oxalate urolithiasis. Calcium ions and oxalate ions diffuse towards one another from two adjacent wells in a layer of gel on a microscopic slide. Where they meet, a streak of calcium oxalate crystals forms. Substances which influence crystallization are caused to diffuse into this middle zone. Depending on the inhibitory potency of the substance, the calcium oxalate streak varies in density and extent. These differences are recorded photometrically. By comparison with a sample containing distilled water an inhibition index for each probe is calculated. Some of the best-known inhibitors were tested and showed a definite effect in this model. Magnesium and citrate in physiological concentrations are of special significance. The expected range of variation of inhibitory activities in human urine is covered by this method.
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