Variability of 24-hour urinary creatinine excretion by normal subjects.

Creatinine excretion was studied in eight healthy males who collected 54 to 97 24-hour urine specimens. Significant differences among subjects in mean creatinine excretion were only partly explained by differences in body weight and surface area. Considerable daily within-subject variation in creatinine excretion during normal activity was found. Standard deviations were from 10.5 to 14.4 per cent of the mean, and ranges varied from 50 to 79 per cent of the mean. Day-to-day variation appeared to be time dependent rather than entirely random, and could not be explained by unreliability of the assay technique or by incomplete collections. Creatinine excretion values were normally distributed in seven of eight subjects. Individual variation from day to day limits the value of urinary creatinine excretion as an index of the completeness of 24-hour urine collections.