Optimal time in the menstrual cycle for serum progesterone measurement to diagnose luteal phase defects.

Inadequate production of progesterone by the corpus luteum results in luteal phase deficiency, which is a frequent cause of recurrent spontaneous abortion. The diagnosis is made by assessment of endometrial biopsy specimens. Measurement of serum progesterone offers a less invasive alternative, but its utility as a diagnostic test is limited since there is no agreement on the level that will accurately differentiate between normal and luteal phase deficiency cycles. The purpose of this study was to determine whether there was an optimal time in the menstrual cycle when serum progesterone measurement could improve the diagnostic accuracy of the test. The results demonstrate that this time period is day 25 to day 26 and not the midluteal phase as has previously been suggested. The discriminatory level was found to be 21 nmol/L and provided a test with sensitivity of 81% and specificity of 73%.