When does a blood pregnancy test become positive?

A blood pregnancy test usually becomes positive within 3-4 days after implantation, or several days before your expected menstrual period, when hCG levels have reached a high enough level.

The blood pregnancy test becomes usually positive about 10-11 days after ovulation and fertilization. 

Typical pregnancy symptoms usually do not show up until after the blood hCG has reached significant enough levels.

How are hCG levels measured?

An hCG level below 5 mIU/ml is considered negative, and a level above 25 mIU/ml is positive. You can't be sure that a blood hCG level between 5 and 25 mIU means it's a positive test because an hCG level between 5 and 25 mIU is considered 'equivocal,' which means we don't know if it's negative or positive.

At a blood hCG level between 5 and 25, most doctors probably repeat the test 2-3 days later. A rise above 25 mIU means that you are pregnant.