usually quantitative blood test
can detect pregnancy
more accurately 8 to 10 days after fertilization, in case of qualitative blood test that may be like urine test to detect pregnancy and not that much sensitive as quantitative test,
anyhow menorrhagea can be seen because of disorders affecting hormones etc that can be mistaken for miscarriage
, so if your bleeding continues better to go for ultrasound
once to rule out the possibility of miscarriage.