Interpreting Paternity and DNA Test Results
Receiving your paternity DNA test results can be an emotional moment, especially when you're waiting for answers about a biological relationship.
Although DNA testing is highly accurate, the numbers, genetic markers, and scientific terminology in a laboratory report can sometimes be confusing.
At Validity Genetics, we want our customers to understand what their DNA results mean and how the laboratory reaches its conclusions.
This guide explains how to read a paternity DNA test report, including the probability of paternity, Combined Paternity Index (CPI), genetic markers, and the difference between an inclusion and an exclusion.
What Do Paternity DNA Test Results Show?
A paternity DNA test compares genetic markers from a child and an alleged biological father to determine whether the DNA evidence supports or excludes the proposed father-child relationship.
The laboratory examines specific locations in the DNA, known as loci, and evaluates whether the child's genetic markers are consistent with inheritance from the alleged father.
A standard paternity DNA test generally produces one of three types of conclusions:
Inclusion: The DNA evidence supports the alleged father being the biological father.
Exclusion: The DNA evidence excludes the alleged father as the biological father.
Inconclusive: The available DNA evidence is insufficient to reach a definitive conclusion.
Most routine paternity tests provide either a strong inclusion or an exclusion.
What Does 99.99% Probability of Paternity Mean?
A result showing a 99.99% probability of paternity means the DNA evidence provides extremely strong statistical support for the alleged father being the biological father.
For example, a report may state:
Conclusion: The alleged father is not excluded as the biological father of the child.
Probability of Paternity: 99.99%
This indicates that the observed genetic evidence strongly favors the biological father-child relationship over the alternative hypothesis used in the calculation.
It does not mean that 99.99% of the child's DNA matches the alleged father.
Instead, the percentage is a statistical calculation based on the genetic evidence and the assumptions used in the analysis.
Depending on the strength of the evidence, a laboratory may report probabilities greater than 99.99%.
What Does 0% Probability of Paternity Mean?
A 0% probability of paternity is generally reported when the alleged father is excluded as the biological father.
This occurs when the DNA analysis identifies genetic inconsistencies that cannot reasonably be explained by normal inheritance under the laboratory's interpretation criteria.
For example, the report may state:
Conclusion: The alleged father is excluded as the biological father of the child.
Probability of Paternity: 0%
A properly supported exclusion indicates that the tested individual is not the biological father.
Laboratories consider the possibility of genetic mutations and other technical factors when evaluating inconsistencies. A single unexpected genetic marker does not necessarily establish an exclusion.
What Are DNA Markers, Loci, and Alleles?
Paternity DNA test reports often include a table of genetic information from the individuals tested.
Three terms are especially useful to understand.
DNA Markers
DNA markers are identifiable regions of genetic material used to compare individuals.
Paternity testing commonly examines markers known as short tandem repeats (STRs).
Loci
A locus is a specific location on a chromosome. The plural is loci.
Each row of a paternity DNA report typically represents a particular genetic marker or locus.
Alleles
Alleles are the genetic values reported at a particular marker.
For most autosomal STR markers, a child inherits one allele from the biological mother and one from the biological father.
The laboratory compares these inherited values to evaluate whether the alleged father's DNA is consistent with the child's genetic profile.
Example of a DNA Marker Comparison
The following is a simplified illustration, not an actual laboratory report.
Example of how DNA markers from a child, mother, and alleged father are compared during paternity testing. This simplified illustration shows genetic inheritance at three DNA markers.
In this example, the child has genetic values consistent with receiving one allele from the mother and one from the alleged father at each marker shown.
However, matching alleles at a few markers cannot establish paternity by themselves.
The laboratory evaluates the complete genetic profile and calculates the statistical strength of the evidence.
What Is the Combined Paternity Index (CPI)?
The Combined Paternity Index, or CPI, is one of the most important statistical measurements in a paternity DNA test report.
At each genetic marker, the laboratory calculates a Paternity Index (PI), which compares how likely the observed genetic evidence would be if the alleged father were the biological father versus if an unrelated man were the biological father.
The individual paternity indexes are then combined to calculate the CPI.
For example:
Combined Paternity Index: 10,000
Under the statistical assumptions used, this means the observed DNA evidence is 10,000 times more likely if the tested man is the biological father than if an unrelated man is the biological father.
Generally, the higher the CPI, the stronger the statistical support for paternity.
How Is the Probability of Paternity Calculated?
The laboratory uses the CPI together with an assumed prior probability to calculate the probability of paternity.
When a 50% prior probability is used, the calculation is:
Probability of Paternity = CPI ÷ (CPI + 1) × 100
For example, a CPI of 10,000 produces a probability of approximately 99.99%.
The reported probability therefore depends on both the genetic evidence and the statistical assumptions used in the calculation.
Can Paternity DNA Test Results Be Inconclusive?
Yes, although inconclusive results are less common in routine paternity testing than in more complex relationship tests.
An inconclusive result may occur when:
The submitted sample does not contain enough usable DNA.
The DNA profile is incomplete or degraded.
The genetic evidence requires additional analysis.
A close biological relative of the alleged father may also be a possible father.
Additional genetic markers or family reference samples are needed.
If a result is inconclusive, the laboratory may recommend additional testing or new samples.
If two potential fathers are closely related, such as brothers, inform the laboratory before testing so the appropriate analysis can be considered.
Are Home and Legal Paternity DNA Test Results Different?
Home and legal paternity DNA tests can use the same laboratory DNA analysis methods.
The primary difference is how the samples are collected and how the participants' identities are verified.
A home paternity test allows individuals to collect their own cheek swab samples privately. These results are intended for personal knowledge.
A legal paternity test requires verified identities, authorized sample collection, and documented chain of custody.
Legal testing is designed for official purposes, subject to the requirements of the court or requesting authority.
Learn more about Home vs. Legal DNA Testing.
Frequently Asked Questions About Paternity DNA Test Results
Does 99.99% mean the alleged father is the biological father?
A 99.99% probability of paternity provides extremely strong statistical support for the alleged father being the biological father, based on the genetic evidence and the laboratory's statistical assumptions.
Can a paternity DNA test show 100%?
Paternity reports generally express positive results as probabilities below 100%, even when the DNA evidence is extremely strong.
Why does a paternity test show 0%?
A properly supported exclusion generally produces a reported probability of paternity of 0%, indicating that the tested man is excluded as the biological father.
Can two brothers both match a paternity DNA test?
Closely related men share more genetic markers than unrelated men. If two brothers are potential fathers, the laboratory should be informed so it can determine whether additional testing is necessary.
How will I receive my paternity DNA test results?
Validity Genetics delivers completed DNA test results to the purchaser by email as a PDF report.
Standard paternity DNA testing generally takes 2–3 business days after all required samples are checked in at the laboratory.
Understanding Your Paternity DNA Results With Validity Genetics
A paternity DNA test report provides more than a simple positive or negative answer. It includes genetic comparisons and statistical calculations that support the laboratory's conclusion.
Understanding the probability of paternity, Combined Paternity Index, and genetic markers can help you interpret your results with greater confidence.
For additional information, see our Example Paternity DNA Test Results or read How Accurate Are Home Paternity Tests?.
If you're considering testing, explore our Home Paternity DNA Test or contact Validity Genetics with questions about your results or testing options.
