When is a pee test better than a blood test?

In Marianna Wheel's popular Harpers book of 1914 called Before the Baby Comes she cautioned that "cold and change of climate (among other things) can cause you to skip your period, but if not that, and you skip two successive menstrual periods [you] may be fairly positive of your condition [pregnant]!" But skip ahead a century, and now we have tests, but there are still questions as to which test! Women will tell you they've often taken that pee test to see if they are pregnant! And though women want to rely on the information they get from their home tests, what they ask is it more accurate to actually take a blood test at the gyno office to see if they really are pregnant. I get a lot of questions: like 'I looked later and saw a faint second line' and that is not according to your package instructions on the urine pregnancy tests. So the question patients ask is which test to get and for the most part the gynos just say, be sure to get any pregnancy test. But what most patients and physicians mean is getting a test for 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…like an ultrasound...because that is the baby, not just a substance made by the placenta, which is what we are testing in the pee or the blood. But that requires a bit more patience as it takes awhile, at least a month or more into the pregnancy, for those ultrasound tests to be positive. Mostly though, the first test of pregnancy in your pee or your blood is a test for HCG or human chorionic gonadotropin. This is a substance made by placentas. HCG 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 circulating substance that shows a positive HCG test that is not HCG at all. It has been nick named “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 positive ‘pregnancy’ test when in fact there is no pregnancy. How common is this? I used to say rare, but some studies quote rates of 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!


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