Evidence for systematic temporal variation in 24-hour urinary creatinine excretion.

Five hundred fifty-seven 24-hour urinary creatinine specimens from eight healthy males collected in six-day sequences separated by two- to ten-week intervals over six to ten months were analyzed for factors contributing to the large observed standard deviation and range (14% and 50% of the total mean, respectively). Subject means correlated significantly with weight and surface area. Between-sequence differences were significantly greater than within-sequence differences in all subjects. Statistically significant linear and/or curvilinear time dependences were found in each subject. Elimination of subject variation and pooling yielded one cycle of a biphasic function, approximately 12 months in duration, oscillating about the mean with an amplitude approximately 6% of the mean; the maximum and minimum exhibited a seasonal correspondence. Upper limits to sources of eror are estimated as measurement +/- 3%, biological +/- 6%, mixed biologic-temporal +/- 6%, and random +/- 5%.