Accuracy

The accuracy of DNA testing is extremely high. It cannot be affected by drug use, illness or disease. However, blood samples should not be collected from a person who has had a blood transfusion within 90 days of sample collection, or from a person who has had a stem cell or bone marrow transplant.

Because the blood has been "contaminated" with different DNA, a buccal swab sample should be used instead. DNA testing can exclude an alleged father 100% or determine to a 99.996+% probability that the man is question is in fact the biological father.

Even when a child's DNA and the alleged father's DNA is an exact match, there will always be a theoretical possibility that someone else fathered the child. But in order for that to be the case, someone else with similar genetic markers had to have a sexual relationship with the mother.

While this is a rarity, it can happen. The alleged father having an identical twin would make the test unreliable if the mother had a sexual relationship with both men. Twins have the same DNA and therefore, if they are alleged to be the father of a child, the results can only determine that neither twin is the father of the child in question or that one of the twins could be the father but it would not be possible through DNA testing to determine which twin is the father. In this case more extensive testing must be done.

Once the lab determines a probability of paternity to such a high degree, the likelihood that someone other than the alleged father is indeed the biological father is so small that most courts tend to disregard that possibility, and in the absence of evidence to the contrary, consider the test reliable proof of paternity.

No test yet available will give a 100% determination of parentage. In some states a 97% result is sufficient to establish paternity. Many labs will however grant a retest if the outcome is less than 99%. The person asking for the retest is responsible for all costs involved.