Understanding Your Paternity Test Result
When you perform a DNA paternity test the result will be conclusive in most cases with the result either confirming that the father is ‘not excluded’ from being the biological father or alternatively it will confirm that he is ‘excluded’ with 100% certainty. All our DNA tests are performed by analyzing 21 genetic loci. These loci are analysed individually and the resulting index of relationship used to obtain a paternity index. Once all the indices are available they are analysed together to determine the Combined Paternity Index (CPI).
What is the combined paternity index?
The Combined Paternity Index (CPI) is calculated from the Paternity Indices generated from each individual locus and gives an indication of the likelihood of Paternity between the alleged father and child as compared to analysis with the general population. In our tests we generate 20 different paternity indexes; one index is calculated for every one of the 20 genetic loci we analyse. We use these indexes to:
- calculate the combined paternity index
- calculate the probability of paternity
The 21st genetic locus is the amelogenin sex gene and we do not generate a paternity index for this genetic marker. This is because we consider testing this a locus a simple measure to make sure that no swabs have been misplaced. Sometimes clients absentmindedly swtich swabs; they might place the mother’s DNA samples in the blue envelope we allocate for the samples of the alleged father. Once in the laboratory, we wil immediately know whether the swabs contain male DNA or female DNA and hence, whether there has been any mix up.
The Probability of Paternity or POP
The probability of paternity is simple a percentage. It is the most straightforward part of your paternity test and the quickest way of knowing whether the tested man is the biological father of the child. The probability of paternity can express the following:
A 99.99% would signify that the alleged father is the biological of the child.
A 0% probability of paternity would signify that the alleged father is not the biological father of the child.
In the sample result templates we have provided, you can see that the DNA test report will show on the left hand column each of the 21 loci analsyed. You can also see the paternity indexes, the combined paternity index and the probability of paternity.
The first thing you will notice when you look at the table is the 20 genetic markers tested on the far left. You will also notice that in most cases, for each person tested, every genetic marker will be represented by 2 numbers. This is because of the genetic markers is inherited from the father and the other from the mother. If you only notice one number rather than 2, this means that the person has inherited the same genetic marker from both parents.
The biological father will need matching numbers for every one of the 20 genetic markers tested if he is to be included as the biological father. If you see two or more genetic markers that do not match, it means that the tested man is not the biological of the child. There are rare exceptions: a single mismatch can occur but this may not necessarily mean that the tested father is not the biological father. We will need to include the mother’s sample in such cases or carry out some further tests to extract more genetic markers. This will help us provide an accurate result. We do not issue paternity inclusions with results lower than 99.9%.