MEASUREMENT OF DIURNAL-VARIATIONS IN URINARY CYSTINE SATURATION

In an attempt to improve the diagnostic value of urine analysis in pat ients with homozygous cystinuria, we studied the diurnal variation in urine composition. A simplified estimate of the ion-activity product o f cystine was used to increase the probability of identifying patients with a particular risk of stone formation. Eight 6-h urine samples we re collected during two 24-h periods. The highest urinary excretion of cystine was recorded between 1200 and 1800 hours and the lowest betwe en 0000 and 0600 hours, whereas the urinary cystine concentration was highest between 0000 and 0006 hours and lowest between 1200 and 1800 h ours. The approximate ion-activity product of cystine had a maximal le vel between 0000 and 0600 hours but a minimal level between 0600 and 1 200 hours. The differences between different periods were numerically more pronounced in terms of the ion-activity product of cystine than i n terms of concentration. The peak concentrations of cystine in 6-h sa mples were about 90% higher than the corresponding concentrations in 2 4-h urine samples. It is concluded that the analysis of cystine in 6-h urine samples reveals transient episodes of cystine supersaturation t hat otherwise will remain undetected. Further studies are, however, ne eded to establish its usefulness in clinical practice.