Monitoring the menstrual cycle using urinary oestrone glucuronide: the relationship between excretion rate and concentration

Oestrone glucuronide (E1G) is monitored during the menstrual cycle because it reflects the follicular production of oestradiol (E2), the rate of which increases as the follicles grow. Despite several recent assertions to the contrary, the urinary E1G concentration is an inadequate indicator of E2 because it is confounded by variations in the rate of urine production. We show that (a) the serum E2 concentration is proportional to the E1G excretion rate, which is the product of the urinary concentration of E1G and the urine production rate, (b) that it is almost as important to measure the urine production rate as it is to measure the urinary E1G concentration itself, and (c) that the distribution of urine production rate is positively skewed, from which it is inferred that extreme values are more likely than would be expected were the distribution normal. If urinary E1G is to be used to monitor the menstrual cycle it is essential that the concentration is corrected for fluctuations in urine production rate: the E1G excretion rate is reliable, urinary E1G concentration is not