Clinical use of cystine supersaturation measurements

Purpose: We measured the concentration and solubility of cystine in urine f rom patients with cystinuria or calcium stones and from normal subjects to determine whether urine cystine supersaturation can be calculated from a st andard nomogram of solubility versus pH or needs to be measured directly. W e also evaluated whether increasing pH of the 24-hour collection recovered enough crystallized cystine to increase cystine supersaturation. Materials and Methods: Cystine concentration, pH and usual stone risk facto rs were measured on 50 ml. aliquots of 24-hour collections from 24 patients with cystinuria, 22 calcium stone formers and 15 normal subjects. After 48 hours of incubation with sodium bicarbonate, a second aliquot was taken fr om the 24-hour collection for cystine concentration. The original urine at its ambient pH was incubated with an excess of cystine crystals for 24, 48, 72 or 96 hours at 37C to determine solubility and kinetics of equilibratio n. Results: Cystine solubility varied so widely at any pH range that no predic tive nomogram could be relied on for calculating supersaturation. Addition of sodium bicarbonate to the 24-hour urine significantly increased cystine concentration. Urine from stone formers had higher cystine solubility than urine from normal subjects. Conclusions: Clinical management of cystinuria can be improved by direct me asurement of cystine solubility because it varies widely at any given pH. I ncreasing 24-hour collection pH with sodium bicarbonate additionally improv es accuracy of supersaturation measurement by recovering crystallized cysti ne.