A positive paternity test result is a document, usually one to three pages, that shows a probability of paternity of 99.9% or higher. The report includes a table of DNA markers comparing the child’s genetic profile to the tested man’s, a statistical calculation called the Combined Paternity Index, and a written conclusion stating the man “cannot be excluded” as the biological father. If you’re looking at a result for the first time, here’s how to read every part of it.
The Conclusion Statement
Most people skip straight to the bottom of the report, and that’s actually the right instinct. The conclusion is the plainest part of the document. A positive result will say the tested man “is not excluded as the biological father” or “cannot be excluded as the father.” Some labs phrase it more directly, but the legal standard uses this double-negative language because DNA testing can never prove paternity with 100% certainty. It can only show that the evidence overwhelmingly supports it.
Right next to or below that statement, you’ll see the probability of paternity. For a positive result, this number is 99.9% or higher. Cleveland Clinic notes that paternity test results typically land at one of two values: 0% (excluded) or 99.9% (not excluded). There’s rarely anything in between. If the tested man is the biological father, the genetics make it obvious. If he isn’t, the mismatch is equally clear.
The DNA Marker Table
The center of the report is a table, and it’s the most intimidating part if you’ve never seen one. The table lists genetic markers (also called loci) down the left column. Modern tests examine 20 or more markers, each one a specific location on a chromosome where people carry two copies of a gene, one inherited from each parent. The standard set used in the U.S. includes 20 core locations, though some labs test additional markers for extra certainty.
Across the top, you’ll see columns for the child, the mother (if she was tested), and the alleged father. Each row shows two numbers per person, representing the two versions of the gene they carry at that location. These numbers indicate the size of the DNA fragment, not something you need to interpret yourself.
What matters is the pattern. At every single marker, the child should share at least one number with the tested man. That’s because a biological child inherits one copy from each parent. For example, if the child shows values of 12 and 15 at a given marker, and the mother contributed the 12, then the father must have contributed the 15. If the tested man has a 15 at that same marker, it’s a match. When this pattern holds across all 20-plus markers, the result is a positive identification.
A single mismatch at one marker doesn’t automatically mean exclusion, since rare mutations can cause a one-locus discrepancy. But if mismatches appear at multiple markers, the man is excluded as the father.
The Combined Paternity Index
Below or alongside the table, you’ll find a number called the Combined Paternity Index, or CPI. This is an odds ratio that expresses how much more likely the genetic evidence is if the tested man is the father versus if a random unrelated man is the father. A CPI of 1,000 means the genetic evidence is 1,000 times more likely under the assumption of paternity than under the assumption of non-paternity.
For a positive result, the CPI is typically in the thousands, hundreds of thousands, or even millions. The National Institute of Justice notes that the accepted threshold for confirming paternity can be as low as 100 in the United States and is set at 1,000 in Europe. Most genuine father-child pairs produce a CPI far above either threshold. If the CPI falls between 0 and 1, the genetic evidence actually favors non-paternity.
The CPI is then converted into the probability of paternity percentage you see in the conclusion. A CPI of 1,000 translates to a probability of 99.9%. Higher CPIs push that number to 99.99% or beyond, but labs often simply report 99.9% as the standard positive threshold.
What Appears on the Rest of the Report
Beyond the table and conclusion, the report includes several standard elements: the name and address of the laboratory, a case identification number, the names of all tested parties, the dates samples were received and analyzed, a description of the testing method used, and a signature from the lab director or analyst who reviewed the results. The date the report was issued will also appear, usually near the top or bottom.
If the test was performed for legal purposes, the report will contain chain-of-custody documentation. This means every step of sample collection and handling is recorded, showing that samples were collected at a certified facility under supervised conditions and weren’t tampered with. Legal reports may also include notary signatures or witness information from the collection appointment.
At-home paternity tests produce a similar-looking report with the same table and probability of paternity, but they lack chain-of-custody documentation. That’s the key visual difference. A home test result won’t have the signatures, witness names, or collection facility records that courts require. The science is identical, but the paperwork isn’t.
Lab Accreditation Marks
A credible report will display the lab’s accreditation status somewhere on the document. The most important credential for paternity testing in the U.S. is AABB accreditation, issued by the Association for the Advancement of Blood & Biotherapies. Many state courts require AABB accreditation for a test result to be admissible, and the federal government requires it for any DNA test used in immigration proceedings, visa applications, or citizenship cases.
AABB accreditation is voluntary, so not every lab has it. You can verify whether a lab is currently accredited by checking the list at aabb.org/dna. If the lab isn’t listed, the result may still be scientifically accurate, but it could face challenges in court. Some labs also carry ISO 17025 accreditation, an international quality standard for testing laboratories, and may display that logo as well.
What a Negative Result Looks Like by Comparison
A negative result is visually almost identical in format. You’ll see the same table, the same markers, the same columns. The difference is in the data. Multiple rows will show no shared alleles between the child and the tested man, and these mismatched loci are sometimes highlighted or flagged. The CPI will be 0, the probability of paternity will be 0%, and the conclusion will state the tested man “is excluded as the biological father.” There’s no ambiguity in the language.
An inconclusive result is rare but possible, usually when a sample was degraded or insufficient. In that case, the report will state that no conclusion can be drawn and recommend retesting. You won’t see a probability percentage at all.