There are no official guidelines to assist in answering this question. I will search for relevant research papers instead.
The FDA Drug Labels for progesterone (PO) do not address this question.
From the Research
Purpose of Day 21 Progesterone Test
The day 21 progesterone test is used to:
- Confirm ovulation, as serum progesterone levels rise after ovulation in the luteinisation process 1
- Diagnose luteal phase defects, which can cause recurrent spontaneous abortion, by measuring serum progesterone levels 2
- Assess the adequacy of luteal phase support in women undergoing controlled ovarian stimulation and fresh embryo transfer 3
- Detect premature luteinization during in vitro fertilization (IVF) hyperstimulation, which can be associated with adverse pregnancy outcomes 4
Key Findings
- A single serum progesterone level ≥5ng/ml is highly specific and sensitive for confirming ovulation 1
- The optimal time for serum progesterone measurement to diagnose luteal phase defects is day 25-26 of the menstrual cycle, with a discriminatory level of 21 nmol/L 2
- A progesterone concentration ≥16.5 ng/mL on pregnancy test day is associated with higher ongoing pregnancy and live birth rates after controlled ovarian stimulation and fresh embryo transfer 3
- Direct progesterone immunoassays may overestimate serum progesterone levels, particularly at lower concentrations, which can affect the accuracy of clinical diagnoses 4