How soon is a blood hcg test should be done after a missed period?

After there is a...: After there is a question about the reliability of a urine pregnancy test, one can do a blood pregnancy test. If the urine test result looks accurate (they are very accurate 1 week after missing a regular period, or 3 weeks after sex), a blood test is not needed. Sometimes, blood tests are done to see if the bHCG level is going up or down, and by how much between test days.