A simple and easy-to-handle method for measuring calcium oxalate crystal formation in whole urine samples using a gel system is described. To prove reliability of the method, the crystallization-inhibiting effect of a number of well-known, so-called inhibitors of crystallization is demonstrated. In testing urine samples from recurrent stone formers and healthy controls, we found a significant difference in the inhibitor index, the stone formers producing more crystals than the controls. Experiments in test persons with oral loading of magnesium compounds and alkalinizing substances suggest that this effect is probably caused by alteration of the urine composition, e.g., a lower excretion of inhibitors by the stone formers.
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