Pregnancy blood test on cd21 too early to confirm pregnancy?

SheaLe
My cd ranges from 34-35 and I was supposed to have labs on cd25 but ended up needing an emergency CT. I was on a medicated cycle with Clomid days 5-9. I'm currently on cd33 and no early symptoms of my next cycle like I normally get. They told me the blood test showed negative but I ovulate later than most due to PCOS. I'm curious if I should ask for another test or if the blood test that early could be 100% accurate?

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