When I worked at the hospital, I remember the nurse manager saying that you can’t get a false positive on a pregnancy test, only a false negative, because the hormones are only present during a pregnancy. This is second-hand information, so don’t quote me. If the person taking the test is unsure, they should see a physician and get a serum pregnancy test.
Pregnancy tests detect the level of the hormone hcg, which is only present during pregnancy. Birth control does not affect the ability of the test to detect this hormone.
“Will a pregnancy test be accurate if you’re taking the birth control pill?
You can get accurate results from a pregnancy test while you’re on the pill. Pregnancy tests work by measuring a specific pregnancy-related hormone — human chorionic gonadotropin (HCG) — in your blood or urine. The active ingredients in birth control pills don’t affect how a pregnancy test measures the level of HCG in your system.”