My friend told me that after miscarrying you have to have a blood test to detect a new pregnancy is this true?

Not true: No, that's not true. A urine pregnancy test is just as accurate.

MISCARRIAGE: After a miscarriage, usually a few weeks later, ovulation can begin again, and since ovulation is necessary for pregnancy to occur, you could become pregnant without having a period, so you would not "know" if your period was "late". Get a serum HCG test (very accurate) be certain, if your period has not started in 9-10 weeks after a miscarriage.