“Why is the pregnancy test positive if she’s not pregnant?”
This is a question I’ve been asked several times and it’s a good one. The query usually comes from a nurse, doctor, or other healthcare provider after performing a test for human chorionic gonadotropin (hCG) and getting a positive or elevated result that they did not anticipate.
Because hCG is a hormone normally produced during pregnancy, hCG tests are used to diagnose the pregnant patient. That makes it easy to think of hCG tests as “pregnancy tests.” While that’s not an inaccurate label for them, identifying hCG tests as pregnancy tests gives the impression that is all they are supposed to do. Technically, hCG tests are designed to qualitatively detect and/or measure the hormone in urine or blood and there are other reasons besides being pregnant that can cause hCG to be present.
I can think of four different reasons why hCG could be present in a non-pregnant woman.
- Biochemical pregnancy. A biochemical pregnancy occurs when a woman becomes pregnant yet has a spontaneous loss of the fetus before she even knew she was pregnant. If hCG testing occurs before all of the hCG has been metabolized out of the body then hCG can be detected by a lab test. This situation is not as uncommon as one might think for two reasons. First, hCG tests are frequently performed in healthcare settings in order to identify the pregnant patient in order to avoid any medical interventions that are potentially harmful to a fetus. Second, hCG tests are capable of detecting very low concentrations of the hormone. The high frequency of testing combined with the analytical sensitivity of the tests means that biochemical pregnancies are easily detected.
- Pituitary hCG. Although the placenta normally produces hCG during pregnancy, it can be made by the pituitary gland. The pituitary gland is a small structure in the brain that secretes many different hormones that function to regulate many endocrine organ systems. Interestingly, three hormones normally produced by the pituitary gland (thyroid stimulating hormone, follicle stimulating hormone, and luteinizing hormone) are structurally similar to hCG. Pituitary hCG is more commonly detected in women greater than 55 years of age but can be detected in women as young as 41 years. Non-pregnant women with pituitary hCG usually have low concentrations of hCG present in the blood and urine. Importantly, concentrations of hCG produced by the pituitary gland don’t show the rapid increases that occur during pregnancy.
- Malignancy. Cancer cells sometimes make hCG. While many different types of cancer have been shown to make the hormone, it’s most commonly associated with the gestational trophoblastic diseases and certain types of germ cell tumors of the testes. Because testicular tumors occur only in men, the question of detecting hCG in the absence of pregnancy is clearly not relevant.
- Interfering antibodies. Some women have antibodies in their blood that can interfere with hCG tests and cause a positive or elevated result in the absence of hCG. Only hCG tests performed on blood can be affected by this problem because the interfering antibody molecules aren’t normally present in the urine. This can be a serious problem because some women have been mistakenly diagnosed with cancer due to the false-positive hCG test result and have undergone unnecessary treatments for it. The frequency of this problem is difficult to know but it’s probably very low. Over the last several years, the manufacturers of hCG tests have worked to minimize possible interference from these antibodies but nothing can be done to completely eliminate the problem. When alerted, the laboratory can help to determine if an hCG test result is falsely positive due to this issue.
So, just because an hCG test result is interpreted as positive doesn’t automatically mean that a woman is pregnant. There are very valid reasons for detecting hCG in the absence of pregnancy. That said, when the hCG test result doesn’t match the clinical picture, the laboratory should still be asked that question! When alerted to the discrepancy, the lab can help to investigate the problem and perhaps shed some light on the cause.
There is quite a bit more to say on each of those four causes but I’ll save those comments for future posts.