Blood tests, just like urine tests, work by detecting HCG levels. Blood tests are much more precise and can tell you the exact number of HCG mIU/ml in the blood. Even with the most sensitive blood test, it is still possible to be pregnant with a negative test.
There is a lot of confusion an ambiguity about blood pregnancy tests. Although they are very precise and more sensitive than urine tests, they still require one thing - HCG to be present in the women's blood stream. Blood tests can detect very low levels of HCG and can confirm pregnancy with as little as 5 mIU/ml in the blood. (Although most tests are looking for a number 25 mIU/ml or higher.)
Implantation and HCG levels
HCG is produced at least in small quantities from the moment of conception, so why can't you get a positive result on a pregnancy test right away? In order to get a positive pregnancy test, blood or urine, HCG has to be present in the woman's blood stream. Once the egg implants it burrows into the woman's uterus connecting the baby directly to the woman's blood stream. Until implantation occurs, your developing baby, the fertilized egg is not directly connected to your blood stream. Only when implantation occurs will their be enough HCG present in your blood stream to confirm pregnancy.
Implantation time varies, averaging between 6 to 12 days past ovulation. Some women may notice slight bleeding, known as implantation bleeding, around the time that the egg implants. If your baby implanted late in your cycle and your test was performed early, you could get a negative test result on a pregnancy test when you actually are pregnant.
Miscalculating your period
Another reason you might get a negative pregnancy test is miscalculating when your period is due and/or when ovulation occurred. If you miscalculated your ovulation date by even a few days, you could be testing too early and get a negative test result.
If you think you are pregnant and received negative pregnancy test results, the best thing to do is wait a week and retest either with a home pregnancy test or a follow up blood test.
Think you might be pregnant? Use JM's Due Date Calculator to determine your due date.