Hey Dana. I know that I am a little late on this but I have been busy at work and not enough time to stay on the boards. I think that everyone has answered your question but I wanted to respond to you since you were so nice to respond to my questions.
I have had the prog. level cd 21 test many times. It is to check to see if you produced enough progesterone, which shows whether or not you had satisfactory ovulatory production or not. The bad thing about this test is the timing. If the RE scheduled it on cd21 without paying attention to when you ovulated, the results will be inconclusive. With me, the test was always done too early, like six days too early. That made my results always be too low. If you ovulate around cd14 then, cd21 will be just fine for you.
Good luck!! Oh, by the way, they say they want your number to be around 10 but my new RE said that reallly any number over a 3 means satisfactory ovulatory production. When we were ttc#1, I had two months where they checked this and the first month (wasn't pg.) number came back a 6. The second month (was pg. and didn't know it) number came back a 55.
That was the beginning of my exciting journey of bringing home DD!!