A CYSTINE calculus represents a chemical precipitation of a definite crystalloid, cystine, (S.CH2.CH[NH2].COOH)2, when its concentration in solution passes a certain critical level. The solubility of cystine has been determined by previous chemical studies.1 Figure 1 depicts the solubility of cystine at various pH levels, according to Sano. Figure 2 indicates the solubility change with temperature.
Reference to the solubility curves shows that at pH 7 cystine will be held in solution up to a concentration of 16.2 mg. per 100 at body temperature, but at pH 8 35.3 mg. per 100 cc. can be held in solution.
The urinary cystine excretion in normal persons varies from 0.8 to 84 mg. a day.2 A patient with cystinuria excretes 400 to 1,000 mg. per day. The urine of such persons may or may not be supersaturated with cystine, depending upon the urinary volume and pH. This explains why not all persons