A paternity test report is dense. The headline number (probability of paternity) is on the final line, but the two or three pages leading up to it contain the actual biological data: who was tested, which markers were analyzed, what alleles each person carries at each marker, and how the per-marker evidence combined into the final result. Reading the report from top to bottom, section by section, makes the headline number much more interpretable than reading it in isolation.
This guide walks through each section of a standard paternity test report, explains what the field means, and notes what pattern to look for in each one. If you have your report in front of you, follow along section by section.
Section 1: Order and participant identification
The top of the report shows the lab's identification of the order and participants. Typically includes:
- Order number (unique to your test, referenced in all lab communications)
- Collection date (the date the samples were collected, not received)
- Receipt date (when the samples arrived at the lab)
- Report date (when the result was finalized)
- Participant names and roles: "Alleged father," "Mother" (if included), "Child"
Verify the names and roles match what you expected before reading further. If a participant is labeled with the wrong role (child vs. alleged father swapped, for example), the lab's analysis is based on the labeling, and a labeling error would produce a misattributed result. If anything looks wrong here, call the lab before interpreting the rest.
Section 2: Sample receipt confirmation
Many labs include a brief statement confirming that the samples arrived intact, were properly sealed, and matched the order's participant list. This section is usually a single line or two confirming "all samples received and in good condition for analysis."
If there was any issue at intake (missing sample, broken seal, mislabeling that was resolved before processing), it would typically be flagged here. For a standard clean collection, this section is confirmational boilerplate.
Section 3: Marker analysis table (the core of the report)
This is the longest section and the most informative. It is formatted as a table with one row per marker analyzed and one column per participant.
A typical row looks like:
D3S1358 14, 17 14, 15 15, 17 PI = 4.8
Breaking this down:
- D3S1358: the marker name. These are standardized identifiers for specific STR locations on the autosomes. The 23-marker modern panel includes markers like D3S1358, vWA, FGA, D8S1179, D21S11, and others. Each marker name corresponds to a specific chromosomal location.
- 14, 17: the alleged father's two alleles at that marker. Every person carries two alleles at each autosomal marker, one inherited from each biological parent. The numbers are counts of short tandem repeats at that marker. In this example, the alleged father has 14 repeats on one chromosome and 17 on the other.
- 14, 15: the mother's two alleles at the same marker.
- 15, 17: the child's two alleles. The child must have inherited one allele from each biological parent. The 15 could have come from the mother (who has a 15). The 17 could not have come from the mother (who has 14 and 15), so it must have come from the biological father. The alleged father has a 17, so he is consistent with being the biological father at this marker.
- PI = 4.8: the per-marker Paternity Index. The observed child's genotype is 4.8 times more likely if the alleged father is the biological father than if he is unrelated. Rare alleles produce higher PI values; common alleles produce lower ones.
Reading down the column, each row works the same way. The reader can trace, marker by marker, how the child's inherited alleles were split between the mother and the biological father, and whether the alleged father is consistent with contributing the paternal allele at each marker.
For technical details on the STR markers themselves, the NIST STR DNA Database publishes the reference specifications used across the industry.
Section 4: Patterns to look for in the marker table
A few patterns tell you a lot about the result:
Consistent inclusion: every row has a per-marker PI greater than 1, and the ratios are substantial (ranging from 2 to 50 or higher depending on allele frequencies). This indicates the alleged father is consistent with being the biological father at every marker. The Combined Paternity Index will be very high.
Consistent exclusion: multiple rows (typically 3 or more) show PI = 0, meaning the alleged father's alleles are inconsistent with being the biological parent at those markers. The Combined Paternity Index will be near zero.
Single-marker mismatch: most rows show PI greater than 1, but one row shows PI = 0. This can be a rare STR mutation (which happens at a background rate of around 0.1 to 0.3% per marker per generation). Reputable labs re-run the specific marker or add additional markers rather than call an exclusion on one data point. If the final report calls this an inclusion with a note about the single-marker mismatch, that is correct behavior.
Partial data: some rows show blank or "n/a" in the alleles column. This indicates the lab could not read that marker reliably, usually because of low DNA yield from one of the samples. If several markers are missing, the result is less decisive than a complete read. A fresh collection with cleaner samples resolves this.
Section 5: Combined Paternity Index
Below the marker table, the report shows the Combined Paternity Index: the product of all per-marker PI values multiplied together. On a true inclusion, this is typically in the millions. The exact number depends on which specific alleles were observed and how common they are in the reference population database the lab uses.
A CPI of 1,000,000 means the observed DNA is one million times more likely if the alleged father is the biological father than if he is an unrelated random man. Larger numbers are more decisive. Smaller numbers (below 10,000 on a 23-marker test) are unusual and may indicate partial data or an edge-case situation.
Section 6: Probability of paternity
Derived from the Combined Paternity Index using the formula: CPI divided by (CPI plus 1). This expresses the same information as a percentage. A CPI of 10,000 produces a probability of paternity of 99.99%. A CPI of 1,000,000 produces 99.9999%. The probability of paternity never reaches 100% because the formula never does, even with astronomically high CPI values.
The probability of paternity is calculated assuming a 50% prior probability (meaning no outside evidence either way before the test). This is the standard reporting convention. The 50% assumption is a mathematical convenience; it does not reflect what the family thinks is likely before the test.
For a deeper explanation of why the result is almost always either 99.99%+ or 0% with little in between, see the guide on why DNA results are either above 99.99% or below 0.1%.
Section 7: Interpretation statement
Most reports include a plain-English interpretation line below the probability of paternity. Common formats:
- "The alleged father cannot be excluded as the biological father of the child."
- "The alleged father is excluded as the biological father of the child."
- "The results are inconclusive. Additional testing is recommended."
The language is often cautious. "Cannot be excluded" paired with a 99.99%+ probability of paternity is the standard way reputable labs confirm paternity, because the test fundamentally compares the alleged father against random chance rather than proving positive identity.
Section 8: Lab accreditation and signatures
Reputable labs include their accreditation details, the lab director's signature or electronic signature, and the date of the final report. These confirm the result was reviewed by a qualified professional and that the lab operates under recognized quality standards.
For legal paternity tests (not applicable to home tests), additional sections document the chain of custody: who collected the samples, when, at what facility, with what identification verification. These sections do not appear on home test reports because home tests do not have chain of custody. The guide on what chain of custody actually adds to a legal DNA test covers the chain-of-custody documentation separately.
What to do with the report once you have read it
For most families, the report confirms what the family already expected and the practical use is just filing it for personal records. For families with a surprising result, reading the report line by line is the first step before taking any further action. Understanding the per-marker evidence is often more useful than any secondary opinion, because it answers the question "how do we know this is right" with specifics from your own test.
If any line on the report is unclear, call the lab's customer service. Reputable labs have trained staff who will walk through the specific report with you. The explanation is authoritative and free, and it is more useful than guessing or asking non-expert friends to interpret the numbers.
USDC's home paternity test ($79) produces a report following the format described above. For families whose situation requires chain-of-custody documentation for court or government use, the legal paternity test uses the same science with additional documentation sections (coming soon at USDC).
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