Can a low progesterone level predict an ectopic pregnancy?

According to a study done by Buckley and colleagues (Ann Emerg Med 2000 Aug;36(2):95-100), all patients with an ectopic pregnancy had a progesterone level below 22 ng/ml. Of the patients with a progesterone level below 22 ng/ml, 10% had an ectopic pregnancy, but none of the patients with progesterone over 22 had an ectopic pregnancy. Of the patients who did not have an ectopic pregnancy, 73% had a progesterone level below 22 ng/ml. This means that if your progesterone level is above 22 ng/ml, your chances of having an ectopic pregnancy according to this study was 2% or less. But even if your progesterone level is below 22 ng/ml, your chances of NOT having an ectopic is above 88%.

In another study the authors said: "Using individual prediction limits progesterone greater than 24 ng/ml would exclude an ectopic pregnancy in 99% of patients. ".