The 21 day progesterone test is a blood test designed to measure your progesterone levels at day 21 of your menstrual cycle or, more precisely, 7 days post-ovulation. It checks that they have risen sufficiently to indicate a) that you have ovulated and b) that your uterine lining is developing sufficiently to sustain a pregnancy.
It's important to realise that if you don't ovulate until day 16, having the test on day 21 would be too early. It should really be taken 7 days post ovulation, which for convenience is called day 21, which is for an average 28 day cycle.
If your period does not arrive 6-8 days after the test, you probably had your dates wrong and will need to try again.
What do the results mean?
By 7 days post ovulation your progesterone levels should ideally be more than 10 ng/ml, or more than 15ng/ml on a medicated cycle. If they are less than this it could indicate an anovulatory cycle or it could mean you ovulated slightly later than you thought.
Even levels of over five can indicate some kind of ovulation, but this is not an ideal result.
It is important to note that most women do not ovulate every cycle, particularly once they are in their 30s, so one 'bad' 21 day progesterone test need not mean very much.
NB This test cannot predict pregnancy.
Read more about ovulation symptoms.