A paternity test is a laboratory test conducted to determine whether a particular man could be the biological parent of a child.  Paternity testing is done by comparing the DNA of the mother, child and potential father.  A DNA sample can be taken from a swab inside the mouth or from a small blood sample (a few drops from a finger prick). 

The laboratory will compare the DNA sequence of the child to the mother and potential father.  Everyone gets their DNA from both their mother and their father – half from each one.  The parents create a unique combination of DNA in the child.  The laboratory will compare specific sequences in the DNA to each parent.  The child will have some sequences that are identical to the sequences in the DNA of their biological mother and biological father.

If the results of the paternity test show that the DNA of the potential father does not match the DNA of the child, the person is excluded.  This means there is a 100% chance that he is not the biological parent of the child.  If there is a DNA match the potential father is considered not excluded.  There is more than a 99% chance that he is the biological father of the child, but a paternity test cannot tell with 100% certainty since a close male relative (such as a brother) could have the same DNA sequences as the person in question and could be the biological parent of the child.  This is a remote possibility, but because it exists a paternity test cannot guarantee 100% that the man tested is the biological father of the child.