DNA paternity testing is 100% accurate when done properly.
All paternity tests will generate a result lower than 100%. To produce a 100% probability, a laboratory would have to test every man in the world. Because this is not feasible, a paternity test uses a population database to calculate the Probability of Paternity. All tests are tested twice independently to ensure error free results through the proper handling, testing, and analysis of samples.
Results of a DNA test will either be an inclusion of 99.9% or higher or an exclusion of 0% meaning the alleged father does not match and there is statistically no chance that the two are biologically related.