just found this for you:
A non-pregnant women usually has 0-2 mIU/mL? of hCG in her blood, with 0.5? being the average. (Peri- and post-menopausal woman can have up to 12 mIU/mL!)? There are two types of pregnancy blood tests: quantitative, which measures the quantity of hCG in the blood in numbers, and qualitative, which measures the quality of hCG (either positive reaching a predetermined cutoff point set by the lab, or negative.) A level of 5 or less is usually deemed "not pregnant" by labs. Some labs use a cutoff point of 20 or 25 as a "positive" threshold. In other words, they do something very similar to peeing on a stick except they use a test designed for blood instead of urine.
Most pregnancy tests are more sensitive than advertised (though not all tests, even among the same brand, are equally sensitive. If there are 3 tests in a box, they will all detect AT LEAST the advertised sensitivity say, 25 mIU/mL but one may detect 17, another 13, another 6.) Consumer Reports once found that First Response Earliest Result was capable, on occasion, of detecting as little as 4.5 mIU/mL. In other words, home pregnancy tests are capable of detecting very, very small amounts of hCG and detecting pregnancies very, very early.
The important thing to remember in this scenario is that when an embryo secretes hCG, it is metabolized into the blood first, and takes another day or two to reach the urine. Likewise, when hCG dissipates, it leaves the blood first. So, when a miscarriage occurs, the hormone will linger in the urine for a short while after it is gone from the blood.
So, it is entirely possible for a pregnancy test to detect a pregnancy and turn positive, followed by a negative blood test-- simply because the embryo has produced a small amount of hCG before dying. That small amount is enough to trigger a positive HPT, and small enough to dissipate from the blood stream by the time a blood test is conducted.