In an earlier post, I wrote about the different types of tests used to detect hCG. While the results of These test are usually very accurate, false positive results can and do occur. Here I discuss some reasons why a urine pregnancy test might give a false positive result.
First, though, a few comments about what a false positive result actually is. Pregnancy tests are designed to detect hCG;the hormone produced by the placenta during pregnancy. If the hormone is truly present in the urine and the test detects it, then that result is a true positive result. In other words: hCG present = true positive test result. However, because hCG is usually only produced during pregnancy,many people consider a positive result to mean that a woman is pregnant. There's a big difference between those two meanings! To summarize:
- Pregnancy tests detect hCG
- hCG is usually (but not always) produced during pregnancy
- A positive pregnancy test indicates that hCG is present (even if the hCG is being made by something other than the placenta)
With that in mind, let's explore the causes of false positive urine pregnancy test results.
- Missed reaction time: The result of a pregnancy tests should be read when the manufacturer instructions say to read it. If the result is read after that time, a faint line is sometimes visible (possibly due to evaporation of the urine) that might be interpreted as a positive result. This would truly be a false positive result because hCG is not present.
- Biochemical pregnancy: A biochemical pregnancy is a real pregnancy but is one that ends very early after conception. If a pregnancy test is performed near the time of the expected period (or even several days later), a positive result can be obtained. This is actually not a false positive result because hCG is present and it was detected by a test designed to detect it! Even though the pregnancy has ended, hCG can still be detected in the urine for a few, or even several, more days. Biochemical pregnancies are quite common although many aren't even detected because the miscarriage occurs before the woman even knew she was pregnant and so had no reason to perform a pregnancy test. Indeed, the term "biochemical pregnancy" came into use only after urine pregnancy tests became sensitive enough to detect hCG close to the day of the expected period. Prior to that, the tests were not able to detect that rather low amounts of hCG that are associated with a biochemical pregnancy.
- hCG from sources other than the placenta: Although hCG is most commonly made only during pregnancy, there are other times that it is produced. Certain cancers sometimes make hCG; most notably a family of tumors collectively called trophoblastic tumors. The pituitary gland in the brain can also produce hCG although this is more common in women who have gone through menopause. Finally, people taking hCG as a weight-loss aid (a waste of money, by the way) or as part of fertility treatments can also have detectable hCG in their urine.