How to Read Paternity Test Results – DNA Match Rates
Wondering how to read your paternity test results? This short guide shows you how to find the probability of paternity and the combined parentage index in simple steps. You will learn to spot errors, trust your report, and make confident decisions fast with clear examples and plain language explanations.
Finding the Paternity Probability
When you open your paternity test results, one of the first things to look for is the paternity probability. This number shows how sure the lab is that the tested man is the biological father of the child. It is the main answer you want from a DNA test.
You can find the paternity probability in the summary part of the report. It is usually written as a percentage. For example, you may see 99.9% or 0%. A high percentage means the man is very likely the father, while zero means he is not the father.
What the Numbers Tell You
The paternity probability comes from comparing the child’s DNA with the man’s DNA. If they share many matching markers, the score goes up. Labs also give a Combined Paternity Index, but the percentage is the easiest part to read. Most courts accept a result of 99% or higher as proof of fatherhood.
A probability over 99% is strong proof that the man is the father.
Look at the table below to see common results and what they mean for your family.
| Probability Score | Meaning |
|---|---|
| 99.99% | The man is the biological father. |
| 0% | The man is excluded as father. |
| 90% or less | Test may need a repeat with more markers. |
To make sure you read the report right, follow these simple steps:
- Find the section titled “Probability of Paternity” or “Result Summary”.
- Check the percentage and compare it with the table above.
- If the number is under 99%, call the lab for advice.
Reading your paternity test results does not have to be hard. The probability number gives you a clear answer about the father-child link.
What the CPI Value Means
When you read your paternity test results, you will see a number called CPI. CPI means Combined Paternity Index. This number tells you how many times more likely the tested man is the father than a random man who is not related.
Think of it like a score. If the CPI is 50, the tested man is 50 times more likely to be the dad than a stranger. A higher CPI gives you more confidence. Labs usually want a CPI above 1000 to call the result a match.
The CPI is a simple score that shows how strong the DNA link is between the man and the child.
Quick Look at CPI Numbers
The table below shows what common CPI values mean in plain words. Use it to check your own report.
| CPI Value | Plain Meaning |
|---|---|
| 1 | No extra proof, same as a random man |
| 100 | 100 times more likely to be the father |
| 1000 | Strong match, over 99.9% chance of paternity |
Always check the CPI before the percent on your paper. A big CPI means the lab found a clear DNA pattern.
- Find the CPI on the first page of your report.
- Compare it to the table above.
- Look at the matching percent to confirm.
If your CPI is low, talk to the lab for a new test. Clear numbers help you make safe choices for your family.
STR Loci on Your Report
When you look at a paternity test report, you will see a list of STR loci. STR means short tandem repeat. These are small pieces of DNA that repeat a number of times. The test counts the repeats at many spots on the chromosomes to see if a child and a possible father match.
Each person has two numbers for every STR locus, one from the mother and one from the father. A child must have one number at each locus that matches the father’s numbers. If many loci show a match, the chance of paternity is very high. This is the core of how to read your paternity test results.
Reading the Locus Table
The report shows a table with locus names and pairs of numbers. The numbers are called alleles. You can check the child’s alleles against the father’s alleles. If the child has a 15 and the father has 15 and 20, that is a match for one side.
Most labs test at least 15 STR loci to give a solid answer.
Here is a small example of how a match looks on paper. The child got one allele from the mother and one from the father.
| Locus | Child | Alleged Father | Mother |
|---|---|---|---|
| D3S1358 | 15, 18 | 18, 20 | 15, 21 |
| TH01 | 6, 9 | 9, 9 | 6, 7 |
In the first row, the child has 15 and 18. Mother has 15, so that came from her. The 18 came from the father because he has 18. This is a clear match. You can do this for every row.
To make it easy, follow these steps when you check your report:
- Find the locus name on the left of the table.
- Look at the child’s two numbers.
- See if one of those numbers is in the father’s pair.
- If yes for all loci, the test says he is the father.
Some reports also show a Combined Paternity Index. This number is built from all the STR loci. A big number like 1,000,000 means the test is very sure. Always read this part to know the final answer.
Signs of Paternal Exclusion
When you open your paternity test report, you want to know if the man is the dad. A sign of paternal exclusion is simple: the test says he is not the father. This happens when his DNA does not match the child’s DNA in key spots.
The report will show a conclusion like “excluded as father” or a probability of 0%. You will also see a list of markers where the child has a gene that could not have come from him. These are the clear signs you need to look for.
How to Spot the Markers
Labs test about 15 to 20 genetic markers. At each marker, the child gets one gene from the mother and one from the father. If the alleged father lacks the gene the child has, that is a mismatch. One mismatch can happen by chance, but many mismatches mean exclusion.
A single clear mismatch at three or more markers is enough to say the man is not the father.
For example, if the child has gene 12 at marker D3S1358 and the alleged father only has genes 8 and 9, that is a mismatch. When this happens at many markers, the lab will exclude him.
- Conclusion says “Excluded”: The report states he is not the father.
- 0% probability: The chance he is the dad is zero.
- Multiple mismatches: Three or more marker mismatches.
- No shared paternal allele: Child’s gene not found in his pair.
Partial Matches and Next Steps
A partial match on a paternity test means the child and the tested man share some, but not all, of the genetic markers we check. This does not prove he is the father, but it can show they are related in some way.
If you see a partial match, the first step is to stay calm and look at the test report again. Many times a weak sample or a mix-up at the lab causes this result, so a new test may give a clear answer.
A partial match often means the lab needs a fresh sample or another family member tested.
Common Causes and Clear Actions
Below are the main reasons you might get a partial match and what to do about each one. Use this list to plan your next move and avoid extra worry.
- Low quality sample: Ask the lab to send a new cheek swab kit and collect it carefully.
- Close relative tested: The man might be an uncle or brother of the real father. Test the suspected father directly.
- Lab error: Request a repeat test at no charge if the company allows it.
To help you read numbers, here is a simple table of match types and meaning:
| Match Level | What It Means |
|---|---|
| Full match | Man is the father |
| Partial match | More testing needed |
| No match | Not the father |
Always talk to the testing company if you feel stuck. They can explain your report in plain words and help you pick the right next step.
Confirming Your Paternity Result
After reviewing the probability of paternity and the combined parental index, you should verify that the report originates from an accredited laboratory. Confirming your paternity result requires checking for certifications such as AABB or ISO to ensure the analysis meets legal standards.
If the outcome appears inconclusive or you seek further certainty, a repeat test with a fresh sample from an independent accredited facility is advised. This second verification step solidifies the biological conclusion presented in the initial report.
References
- Ancestry – Ancestry
- 23andMe – 23andMe
- DNA Testing Centers – DNA Testing Centers