1. They are more accurate and reliable. A negative test ( HCG level of less than 10 mIU/ml) can reliably rule out a pregnancy with 100% accuracy. Urine tests are not as reliable or accurate. This is because they can have false negatives. A false negative is the situation when a woman is pregnant, but the urine pregnancy test comes back as negative. This can be because her HCG levels are low, so that they cannot be picked up by the urine test ( which becomes positive only after the blood HCG levels cross more than about 30-50 mIU/ml). Other reasons the urine test can be incorrectly negative is when it’s not done properly by the patient ( who is not an expert, after all, and can make errors !); and if the urine is too dilute ( so that the concentration of the HCG in the urine is too low for it to be picked up by the urine test). This is why an IVF clinic will never rely on a urine pregnancy test and will insist that the patient do a blood test to check their HCG level.
2. They are quantitative. The other major advantage of a blood test over a urine test is that a urine test is only qualitative – it just gives a yes or no ( positive or negative) result. The blood test, on the other hand, provides a number. This is especially useful when the doctor needs to do serial tests to monitor the health of the pregnancy. Thus, in a healthy pregnancy, the HCG level should double every 48 hours. If it fails to do so, this gives the doctor an idea that there maybe a problem with the pregnancy. Similarly, a very high level can help the doctor to suspect a multiple pregnancy !
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