Are pregnancy blood tests accurate early on?

Blood tests look for hCG, the hormone produced during pregnancy, and can detect as few as 5 mIU of hCG. Since a pregnant woman produces an average of 25 mIU only ten days after ovulation, the only possible way to get a false negative with a blood test would be by testing too early (as in like a day of two after ). Since three weeks had gone by between the date in question and the test, if she were indeed pregnant she would have had an hCG count of over 100 mIU. While you can have spotting during pregnancy, and even bleed like a period during pregnancy, a negative blood test means you are not pregnant.