How do you read paternity test results?

A paternity test report has two parts. A table lists the DNA markers the lab tested and each person's result at every marker. A summary gives the combined paternity index (CPI), the probability of paternity and a conclusion. Most results land at one end or the other. Cleveland Clinic says results usually show a probability of paternity of 0 or 99.9, and Labcorp says its results usually show 0.00% or 99.99% or greater.

What the markers (loci) are

Each row of the table is a marker, also called a locus (plural loci), with a name such as D8S1179 or TH01. A marker is a spot in the DNA where a short sequence repeats. People differ in how many times it repeats, which is what makes markers useful for comparing relatives. Cleveland Clinic says the industry standard is 16 markers, and some tests check more.

Why there are two numbers at each marker

Everyone has two copies of each marker, one from each parent, so each person has two numbers per row. Each number is how many times the sequence repeats. A decimal such as 9.3 means one repeat is incomplete. If both numbers are the same, that person got the same version from both parents.

How the lab finds the father's number

At each marker, the child shares at least one number with the mother. The child's other number must have come from the biological father. Labs call it the obligate paternal allele. If the man tested has that number too, the marker fits him being the father. If the child and mother share both numbers, either could be the father's, and he needs only one of them.

The table below is a made-up example, not a real report. A real report lists many more markers.

At D8S1179, the child got 14 from the mother, so 16 came from the father, and the man has a 16. At D21S11, the child has two 30s, one from each parent, and the man has a 30. At TH01, the father's number is 7, and the man has two 7s. At FGA, the child matches the mother at both numbers, so the father gave either 21 or 24, and the man has a 24. He fits at all four markers.

What is the paternity index?

The paternity index (PI) at each marker is a likelihood ratio. It compares how likely the child's result is if the man tested is the father with how likely it is if a random, unrelated man is. A PI of 5.0 means the result is five times more likely if he is the father. Labs base it on how common the father's number is in the population, so a rarer shared number gives a higher index.

What is the combined paternity index?

The combined paternity index (CPI) is every marker's PI multiplied together. In the example, 4.6 × 2.0 × 5.0 × 1.6 = 73.6. Each extra marker that fits usually pushes the total higher, so a real report for a man who is the father shows a much larger number.

What is the probability of paternity?

The probability of paternity turns the CPI into a percentage. Labs usually start from a neutral assumption that the man is as likely to be the father as not (a prior probability of 0.5). The formula is then:

Probability of paternity = CPI ÷ (CPI + 1)

• A CPI of 100 gives 99.0%.
• A CPI of 1,000 gives 99.9%.
• A CPI of 10,000 gives 99.99%.

In the example, 73.6 ÷ 74.6 gives 98.7%. Four markers aren't enough for a firm answer, which is why real tests check many more. An inclusion never reaches exactly 100%, because the formula always gives a number below 1.

Federal law lets each state set the probability at which a test creates a legal presumption of paternity. In Texas, for example, it is at least 99% with a CPI of at least 100 to 1. See court ordered paternity tests for how results are used in a case.

What does 0 probability of paternity mean?

A probability of 0% means the man tested is excluded: he is not the biological father. Labs report an exclusion when he lacks the father's number at more than one marker. A single mismatch isn't treated as an exclusion, because DNA can occasionally change between generations. The New York City medical examiner's protocol, for example, requires two or more mismatches. A 0% result doesn't say who the father is.

Included, excluded or inconclusive

• Included, sometimes worded as "not excluded": the man has the father's number at every marker, or at all but one that the lab explains as a mutation, and the probability is high.
• Excluded: he lacks the father's number at two or more markers, and the probability is 0%.
• Inconclusive: the numbers don't settle it either way. Labs then test more markers or more relatives.

If a result surprises you, see whether a paternity test can be wrong.

If you want an answer for your own knowledge, see our at-home paternity test. Home results are for personal use and can't be used in court. Every adult tested must agree, and a parent or legal guardian consents for a child.

Questions

What does 99.99% probability of paternity mean?

It means the CPI is about 10,000 or more: the DNA results are at least that many times more likely if the man is the father than if a random unrelated man is. It is an inclusion.

Is a combined paternity index of 100 good?

A CPI of 100 equals a 99.0% probability of paternity, the minimum Texas uses for a legal presumption. Labs often report much higher figures; Labcorp says its results are usually 99.99% or greater.

Why doesn't my result say 100%?

The calculation compares the man with a random unrelated man, so the probability for an inclusion always stays just below 100%.

Sources

• Cleveland Clinic: DNA paternity test
• Labcorp: How DNA testing works (dna.labcorp.com)
• The Tech Interactive: How do paternity tests work? (thetech.org)
• NIST: Biology and genetics of new autosomal STR loci useful for forensic DNA analysis (Butler and Hill, PDF)
• NYC Office of Chief Medical Examiner: Kinship and parentage analysis protocol (PDF)
• Texas Legislature: Family Code chapter 160
• Cornell Law School LII: 42 U.S. Code section 666