Blood testing was the first method used to determine paternity with some methods dating back to the early 1900’s.
Today, scientists can compare DNA to determine paternity with a 99.99% accuracy rating. A blood sample is one of many ways to collect this DNA, allowing a trained technician to compare samples from the child to that of the alleged father using a procedure known as PCR (Polymerase Chain Reaction). Since the child’s DNA will be a fairly equal combination of the DNA of his parents, excluding the mother’s DNA gives us a genetic picture of the biological father.
But before science discovered the ability to isolate DNA, blood samples were used in a variety of testing methods to help exclude potential fathers with varying rates of accuracy:
Blood Typing: Around the 1920’s, scientists were able to distinguish one blood type from another. And as with DNA, a child’s blood type is a combination of that of his/her parents. Your blood type is determined by proteins in your red blood cells known as ABO antigens. These antigens are the reason that you must have a specific type of blood during a transfusion and also limit the blood type outcomes for offspring. Using this logic, it is therefore possible to exclude potential fathers if their blood type doesn’t match.
For example, if a mother’s blood is Type B and the child’s blood is Type AB, then the father must have Type A or Type AB. A man with Type O blood then could not be the father and would be excluded. Obviously blood typing alone cannot establish paternity since unlike DNA, blood types are not individually unique. It does however, allow a specialist to narrow down the list of putative fathers by excluding up to 30% of potential candidates.
Serological Testing: Following ABO comparison, science discovered other proteins in the blood that are more group-specific than the ABO antigens themselves. Known as Rh, Kell and Duffy blood groups, these proteins give scientists the ability to exclude potential fathers at a slightly higher accuracy rate of about 40%. As with ABO testing, serological testing cannot identify the father with certainty but it can exclude those that do not possess the correct blood groups.
HLA Testing: In the 1970’s, science made a breakthrough and isolated another set of proteins called HLA (Human Leukocyte Antigens). Unlike basic blood type proteins and grouping proteins mentioned above, the HLA proteins are found in all of your cells except the red blood cells. These antigens are found primarily in your white blood cells and are used by the body’s immune system to detect foreign bodies and fight off infections.
Like blood types, some HLA types are rarer than others so the exclusion percentage will vary depending upon the individual’s HLA makeup. While still not as reliable as DNA testing, HLA can exclude potential fathers with up to an 80% accuracy rating. Unfortunately however, HLA requires a larger blood sample, preventing the test from being performed on small infants.
Paternity testing these days is generally accomplished through DNA testing due to its accuracy, its increasing availability and its decreasing cost. Blood sample comparison can still be very relevant though – where a child’s blood type clearly does not match a putative father’s, it will be clear without the need for an expensive and emotional paternity suit and DNA test that another man must be the child’s father.