It’s not all that common, and they are more reliable than your standard home pregnancy test. Blood tests are extremely accurate but they can be wrong; the test may give an inaccurate result if the test is done too early, as the levels of HCG in the blood may be too low to register on the test. There’s always a chance that it could be wrong, but I think it’s probably right. last month my period cycle was 25 days it is irregular every month. Dr. Tracey Leaver-Williams answered. But, your results are all about timing and the lab that runs your test. View 1 more answer. The hook effect will give you a false negative, when you should have a positive result. 0. I found this on the net: False positive results can result from diseases like choriocarcinomas, IgA deficiencies, heterophile antibodies, enterocystoplasties, gestational trophoblastic diseases (GTD), gestational trophoblastic neoplasms (GTN), and testicular germ cell malignancies. last period was 13 sept. and today is 10 oct. Dr. Robert Vanderbrook answered. Blood tests are super accurate. but we ended up having an actual intercourse 45 mins ish or so after i took the pill on the day of. 1 thank. i stopped him after about 10 minutes cuz it felt unsafe and risky.) Send thanks to the doctor. Like 98% or something like that. 0 comment . So, pregnancy blood tests can be wrong. 21 years experience Family Medicine. I believe the high 90s %. It is also possible that ... Read More. Can a blood pregnancy test be wrong if done too early? can blood pregnancy test be wrong? 17 years experience Obstetrics and Gynecology. If one thing is off, your pregnancy blood test result could be wrong. Sometimes: Tests performed too early in the pregnancy, before there is a significant hCG level, may give false-negative results. 1. Yes: If you have a blood pregnancy test done very early and it is a "qualitative" test it could be negative as it was too early. I am freaking out because we had unprotected sex on 5/11 (he didn't ejaculate; i had taken a morning after pill just to be safe after having an anal sex the day before. If you have been sexually activ ... Read More. Go to your ob/gyn and have more tests because you might have something else wrong with you. Just like any other mass-manufactured product, pregnancy tests can arrive at the store damaged, expire on the shelves or in the back of your bathroom cabinet, get exposed to … Technically, you can have a hook effect with any kind of medical lab test: blood, urine, and saliva. Can a blood pregnancy test be wrong?