How early can a blood test detect pregnancy? Please answer only if you know the definitive answer. Obgyns?

Date of conception. A quantitative blood test can tell approximately the date it was conceived. The beta subunit of HCG or human Chorionic Gonadotropin can be detected in urine or blood after implantation or 6-12 days after fertilization. The test which can give the quickest result is a rosette inhibition assay for early pregnancy factor (epf) which can tell within 48 hours of fertilization, but it is expensive.