It depends on when you are due for AF and when you ovulated, most blood pg tests detect pregnancy 8-10 dpo.
This was copied from a doctors FAQ site..
A blood test is more accurate, but not necessarily more sensitive. A lot depends on the lab. A quantitative blood test, usually called a beta hCG test, measures the exact units of hCG in the blood. That means it will detect even the most minimal level. There is another type of blood test sometimes called a qualitative hCG. This is a test that simply gives a yes or no answer to whether you are pregnant. Just like urine tests, labs vary as to what is considered a positive pregnancy test. Common cutoffs for positive are 5, 10, and 25 units. A level under 5 is considered negative. A test that is only triggered at 25 units of hCG is not any more sensitive than several of the urine HPTs.
Implantation needs to occur before hCG is produced, and that generally happens between 6-12 days post ovulation, usually by 10 days after. For this reason, it makes economical sense to wait until 10-12 days after ovulation, but even then a significant percentage of women who are indeed pregnant will still show a negative result.
check out that site...and ...GOOD LUCK