Can a blood test detect a pregnancy? The answer is “yes” but it is much rarer to take a blood test for pregnancy these days now that accuracy can be achieved with sensitive home pregnancy tests. Sensitive urine tests cost less than blood tests (unless it is free under your insurance plan) and can tell you your results several days before you miss your period in the comfort of your own home. The earliest you can test for pregnancy is after implantation has occurred and even then the test can be inconclusive if you test too early or have an ectopic pregnancy that naturally has low levels of HCG.
Can a blood pregnancy test be wrong? A blood test can only be wrong if you have some kind of condition, such as an HCG-secreting tumor that secretes abnormal HCG levels from the tumor instead of a pregnancy. This type of false positive pregnancy test is very rare so you can assume for the most part that, if you have a positive pregnancy test to determine pregnancy status, you are really pregnant until proven otherwise. Most of the time an ultrasound can confirm that you are in fact pregnant at some point in early pregnancy with accurate dates.