Endogenous Overnight Creatinine Clearance Compared with 51 Cr-EDTA Clearance during the Menstrual Cycle

ABSTRACT Fourteen healthy women aged 21–41 years, who did not take oral contraceptives and who all had ovdatory cycles, were examined once in the follicular phase and once in the luteal phase of a single menstrual cycle. The glomerular filtration rate (⁵¹Cr-EDTA clearance) increased from the follicular to the luteal phase by a median of 7.0% (95% confidence interval: 0.7–10.3%). Endogenous overnight (22.00–08.00 hours) creatinine clearance increased by a median of 7.3% (95% confidence interval: 1.0–14.6%). The urinary creatinine excretion rate also increased with a median of 7.3% (95% confidence interval: 1.511.9%) whereas the serum concentrations of creatinine and β₂-microglobulin, urine flow and urinary excretion rate of urea did not change. The results confirm previous observations of an increase in creatinine clearance in the luteal phase of the menstrual cycle and indicate that the increase in overnight creatinine clearance reflects a true change in glomerular filtration rate.