MID-MENSTRUAL CYCLE DECLINE IN CREATININE AND UREA CLEARANCES

In 6 pre-menopausal and 6 post-menopausal women we compared urine volu me and excretion of creatinine and urea daily from day 2 of the 1st pe riod of menstruation to day 1 of the 2nd, along with clearances of cre atinine (CCr) and urea (C-urea). Clearances were based on 7 serum samp les which were taken on day 2 of the 1st menstruation and on days S, 1 4, 15, 16, 21, and day 1 of the next menstruation, when 4 cycling horm ones were also assayed. Mean group ages were 35 and 59 years for pre- and post-menopausal women. CCr cycled only in the pre-menopausal group , with a nadir around the time of ovulation. The mean CCr combining da ys 14 and 15 was lower than that for the average of the 5 other days b y 22% (p < 0.003) in the pre-menopausal but not in the post-menopausal group (p = 0.99). Average CCr values fell from the peak mean of 129 m l/min on day 8 in the follicular phase to the mean nadir of 93 ml/min for days 14 and 15 and recovered to 117 ml/min by day 1 of the 2nd men struation; the percent fall in CCr ranged from 9 to 39%. A mid-cycle n adir was also present for C-urea (p = 0.03), also found only in the pr e-menopausal group. We conclude that, in pre-menopausal women, there i s generally a fall in CCr and C-urea from a peak in the follicular pha se to a nadir around ovulation.