Calcium oxalate dihydrate formation in urine.

Factors that promote the formation of calcium oxalate dihydrate (COD) in urine were investigated. Crystals resulting from the incubation of 25-ml aliquots of the solution to be tested and 1 ml of 0.05 M ammonium oxalate were examined by infrared spectrophotometry. With reference spectra of known mixtures, the fractional content of COD could be estimated. At pH 6.5, only COD formed in human urine. In a supersaturated inorganic solution of calcium oxalate, the percentage of COD was 7.5 +/- 1.4. Pyrophosphate (1 to 8 X 10(-5) M), citrate (10(-4) to 2 X 10(-3) M), RNA from yeast (5 X 10(-9) to 0.5 X 10(-7) M), or heparin (2 X 10(-9) to 2 X 10(-7) M) added to a supersaturated solution of calcium oxalate increased the percentage of COD proportional to the concentration of the additives. Chondroitin sulfate and magnesium had no effect. An increase in pH increased the formation of COD in inorganic solutions containing citrate, pyrophosphate, and heparin and in undiluted urine. RNA-citrates and citrate-pyrophosphate mixtures showed additivity. Urine showed an effect that was inversely proportional to its dilution. Substances that promote COD formation in this system had their phase-stabilizing effects at concentrations normally found in urine. Further, these same substances are known inhibitors of calcium oxalate crystal formation. With both inhibition and phase stabilization, there is additivity of effects, and changes in pH alter the response.