Natalie, these terms are referring to stages of the menstrual cycle. Follicular is during the 1st half through ovulation, and luteal is the second half. As you can tell by the numbers, there is a surge of 17 ohp after ovulation and through the second half of the cycle. So when 17 ohp is checked for diagnostic purposes, it’s supposed to be done during the first week of the cycle, when levels are lowest. The levels also are highest in the morning and drop during the day, so they are supposed to be checked early morning, follicular stage.