We say pregnancy tests, but what most patients and physicians mean are those substances that we can test for in the urine or in the blood. Probably it would be more rhetorically (grammatically?) accurate to say that a real pregnancy test is one where we see the pregnancy itself…but that requires a bit more patience, as it takes a while for those tests to be positive.
Mostly we are testing for HCG or human chorionic gonadotropin. Thisis a substance made by placentas. It is secreted into the blood stream and then excreted out into the urine. And companies have developed extraordinarily accurate tests to determine the presence of HCG as early as possible in a pregnancy. And typically blood testing can pick up very minute levels and is very accurate.
But now and then individuals (yes, rarely, men too) can have a circulating substance that shows a positive HCG test that is not HCG at all. It has been nicknamed “phantom” HCG. It is another antibody that is similar enough to the HCG molecule that it binds to the test agents and reads positive. Usually it only reads falsely positive in blood. These molecules tend to be large, and may not actually be spilled intact into the urine.
So, oddly, the negative urine test is more accurate in this case, and can reveal the true nature of the so-called “phantom” HCG, or a positive "pregnancy" test when in fact there is no pregnancy. How common is this? I used to say rare, but some studies say 1-2% of the population may have these antibodies that may read this way on a test.
So don’t just assume that blood tests are “better,” some interpretation is important for every test you ever take!