If they refuse to redo them for free, you could sue them to recover the cost of having them redone. You would not proceed on a medical malpractice basis, since that would be very expensive for you (you need a medical expert for medical malpractice), but on negligence (carelessness) and breach of contract (not providing what you paid for and they agreed to provide). It would be best to sue in small claims court, acting as your own attorney, where fees are small, since all you can recover is the cost of the blood tests.