Blood Pregnancy Tests
Blood pregnancy tests (also called serum tests) can detect the hCG at much lower levels that the urine tests. To give an idea about the difference, the blood tests can detect levels as low as 2-5 IU/L (IU/L = International Units/liter, don't worry about what that means, this is just for comparison). Most home urine pregnancy tests detect levels of hCG as low as 100 IU/L , but the range is 25-250 IU/L. However, blood pregnancy tests are only done for serious medical reasons, such as suspected ectopic pregnancy. As anxious as a woman may be to know if she is pregnant or not, a blood test will not be done for a routine pregnancy.
In general, the lower the level of hCG the test can detect, the earlier it can detect a pregnancy. Even though these very sensitive blood tests can detect levels of hCG in the single digits, the standard is not to report the result positive unless the level is 25 IU/L or above. Levels between 5-25 IU/L are considered inconclusive. Only levels below 5 IU/L are truly negative.
The level of hCG rises steadily throughout the first ten weeks of pregnancy. It doubles about every 1.4 to 3.5 days between weeks 4 and 6. This is the time when women often suspect they are pregnant. Given that pattern and the fact that blood tests can give an absolute number to the amount of hCG, it is possible to repeat the test between two to four days and compare all the values. If there is a rise in the level of hCG that fits into the upward pattern expected of a normal pregnancy, then perhaps the first test was simply performed too early. When significant levels of hCG are detected, but they don't follow the normal upward path, there may be an abnormal pregnancy, an impending miscarriage, and sometimes an ectopic pregnancy. Still, the hCG levels are only one clue; the different patterns of the different conditions overlap quite a bit. It will be necessary to examine the woman, ask her questions, and do other tests to decide which one applies to her.
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