Popular belief is that DNA testing is 100% accurate. Whether used to determine the father of a child or pinpoint an individual who committed a crime, the results of DNA testing are widely considered infallible. I’ll tell you the truth. The accuracy of a DNA test depends greatly on how the samples are collected, how they are tested, and how they are reported. The results of a DNA test are sometimes misleading or wrong.
For the purposes of this article, I will stick to a standard DNA paternity test rather than expanding on forensic DNA testing. The first requirement of an accurate DNA test is proper collection of the samples. DNA can be collected from blood, skin cells, saliva, semen, and a number of other sources. The key to the DNA collections is contamination. When I say contamination I mean that the person collecting the sample accidentally includes a sample of their DNA in addition to the intended person being collected. This is called a mixture. DNA mixtures make it very difficult for a lab to differentiate between the two samples. Another issue that could go wrong with the collection is that the DNA sample is collected from the wrong individual.
The second aspect of an accurate DNA test is how the test is completed. DNA tests are completed by comparing the genetic code at a number of different loci. The more loci that are compared, the more accurate the test will be. The gold standard in today’s DNA testing laboratories is 16 genetic markers. When the laboratory gets inconclusive results after testing 16 markers they can run additional testing on more genetic loci. When 3 or more loci do not match between two individuals it means they are not biologically related. Some DNA laboratories test between 6 and 10 genetic loci. This can lead to false positives. A false positive happens when a potential father matches at 6 loci, so the laboratory reports that he is the biological father when in fact he is not.
When a DNA laboratory uses proper collection and testing procedures the probability of wrong test results are very low. Only identical twins share the exact same DNA. This means that if both twins are tested against a child, if one is the father then the results of both tests will be positive. In situations where siblings are both potential fathers the laboratory should recommend testing both siblings.
To conclude, there are two possible scenarios in a DNA paternity test. First, if the potential father is not the father, then the probability of paternity is 0%. There is a 100% chance that the man is not the father. If the father cannot be excluded as the father, then the probability of paternity is 99.99% or greater. There is never a 100% probability of paternity, because DNA testing is based on statistics.