The last decade has seen a dramatic increase in DNA testing, allowing courts throughout the United States to determine paternity with up to 95-99% accuracy. Prior to DNA testing, courts had to rely on blood tests. Initially, by analyzing and comparing blood types, one could not necessarily determine paternity, but could determine the possibility of paternity, utilizing the ABO blood typing system.
Humans can have one of four blood types: A, B, AB, or neither (Type O). By comparing blood types of the mother, child, and possible father, they could determine who could potentially be a father, but not who definitively the father is. In the 1970's, a stronger test utilizing white blood cell antigens called the Human Leukocyte Antigen was developed, allowing testers to exclude up to 95% of falsely accused fathers. But until DNA testing became available, determining paternity was still problematic.
Deoxyribonucleic Acid, or DNA, is unique to every human being, except identical twins, and is found in every human cell. Characteristics of a child's DNA can be compared with that of the mother and fathers, to determine the father of the child. If DNA coding of a child is not a match with a mother, it must have come from the father. If the father's DNA characteristics do not match, he cannot be the father. Hence, by comparing a child's DNA with that of the mother and potential father, if the characteristics match that of both the mother and the father, the child's parents can be determined.
There are two simple ways to collect DNA for a test. A Restriction Fragment Length Polymorphism (RFLP) blood test or a Buccal scrap test, using a swab to rub the inside of a person's cheek. Both tests can be done to children at any age, and have up to a 99.9% rate of accuracy, making paternity testing simple, safe and accurate.
Source: findlaw.com, "Paternity Tests: Blood Tests and DNA", Accessed Apr. 21, 2015