What 'Probability of Paternity' Numbers Really Mean on a Result Report

You collect the swabs, send them in, and a few days later an email arrives with your results. You open the report and find a number, something like 99.98% or, if the tested man is excluded, a flat 0%. That figure is called the probability of paternity, and it is the single most important number on your result sheet. Yet it is also one of the most misunderstood. This guide explains exactly what that number means, how the lab calculates it, and what to do if your result falls into the rare inconclusive range.

Why a Percentage Instead of a Simple Yes or No?

DNA testing cannot produce a purely binary answer for inclusion the way it can for exclusion. If the tested man's DNA profile does not match the child's at even one genetic marker, he is with certainty. Exclusion is absolute. Inclusion, however, is a matter of probability because any two unrelated people could theoretically share matching alleles by chance. The probability of paternity expresses how much more likely it is that the tested man is the biological father compared with a randomly chosen unrelated man from the general population.

That comparison is built into the math through a tool called the Combined Paternity Index, or CPI.

The Combined Paternity Index: The Engine Behind the Percentage

At every genetic marker the lab examines, the analyst calculates a Paternity Index (PI), a ratio comparing two probabilities:

• The probability that the tested man would pass the child's inherited allele if he is the biological father.
• The probability that a random unrelated man from the general population would pass that same allele.

A PI of 1.0 at a single marker means the marker is uninformative. A PI of 50 means the tested man is 50 times more likely to be the source of that allele than a random man. The lab then multiplies the individual PI values across all markers tested to produce the Combined Paternity Index. The more markers that favor the tested man, the higher the CPI climbs.

From CPI to Percentage: Bayes' Theorem

The CPI is converted to a probability of paternity using Bayes' Theorem. The formula requires a starting assumption called the prior probability, which represents how likely paternity was before any DNA evidence was considered. The forensic science community has standardized this prior at 0.5, or 50/50, meaning the lab assumes no inherent advantage for or against the tested man before the DNA speaks. That neutral starting point is then updated by the CPI to produce the final probability of paternity percentage printed on your report.

The relationship looks like this: Probability of Paternity = CPI / (CPI + 1). A CPI of 999 produces a probability of 999/1000, or 99.9%. A CPI of 99,999 produces 99.999%. This is why very high percentages are common in true-paternity cases: the DNA evidence accumulates rapidly across many markers.

How Many Markers Does USDC Test?

Marker count matters enormously. The more markers included in the analysis, the more statistical power the result carries. US Diagnostics Center tests up to 28 genetic markers, which exceeds the industry-recognized minimum of 20 or more markers recommended by forensic standards bodies. The standard panel covers 23 autosomal STR loci. In cases where the initial analysis returns an inconclusive Combined Relatedness Index between 0.1 and 10, our lab can extend the panel by up to 5 additional markers to push the result toward a conclusive range.

For context on why marker counts and population databases matter in forensic DNA interpretation, see the NIST Forensic DNA Resources.

Reading the Three Possible Outcomes on Your Report

Included: 99% and Above

The scientific and legal communities have adopted 99.0% probability of paternity as the minimum threshold for a positive inclusion finding, with most quality labs aiming for 99.9% or higher. If your report shows a figure in this range, the tested man is reported as the biological father. CPIs supporting these conclusions are typically in the thousands to hundreds of thousands, meaning the DNA evidence is overwhelmingly consistent with paternity. A result of 99.9% does not mean there is a 0.1% chance he is not the father in the casual sense. It means that compared with a random man from the population, he is 999 times more likely to be the biological father.

Excluded: 0%

When the tested man's DNA does not match the child's at one or more genetic markers in a way that cannot be explained by mutation, the probability of paternity is reported as 0%. Exclusion is a definitive finding. No prior probability assumption affects this result. It simply means the tested man cannot be the biological father.

Inconclusive: Between 0% and 99%

An inconclusive result is uncommon but possible, particularly in cases involving closely related alleged fathers, cases without a mother's sample, or situations where the child's alleles are common in the general population. If the result falls into an inconclusive range, the lab may analyze additional markers to clarify the outcome. Including the mother's DNA sample in the test significantly reduces the chance of an inconclusive result by allowing the lab to subtract the maternal contribution and isolate exactly which alleles the child must have inherited from the biological father.

A mother's sample kit option is available during checkout.

Why the Mother's Sample Strengthens Your Result

A paternity test can be performed with just the child and the alleged father, but adding the mother's sample improves the precision of the analysis in meaningful ways. The child inherits one set of alleles from the mother and one from the biological father. When the mother's profile is known, the lab can apply a process called maternal subtraction. This isolates exactly which alleles in the child's profile had to come from the father, which removes ambiguity at markers where common alleles might otherwise inflate or suppress the paternity index. The result is typically a higher CPI and a more definitive probability of paternity percentage.

The AABB (Association for the Advancement of Blood and Biotherapies) publishes relationship testing standards that outline best practices for maternal inclusion in parentage cases. These standards are widely referenced by accredited labs across the industry.

At-Home Testing Versus Legal Testing: Same Science, Different Chain of Custody

A common question is whether a home DNA test produces the same quality result as a court-admissible test. The underlying lab science, including the autosomal STR methodology and the statistical framework described above, is the same in both contexts. The critical difference is not the science. It is the chain of custody.

Court-admissible paternity results require witnessed sample collection, photo identification, and documented chain of custody at every step, from collection through lab analysis to reporting. Home kits rely on self-collected samples with no independent witness, so they are designed for personal knowledge, not legal proceedings. If your situation involves child support, inheritance, immigration, or any legal matter, you will need to pursue a chain-of-custody collection through the appropriate channels.

For immigration-related parentage cases, the U.S. Citizenship and Immigration Services (USCIS) outlines DNA evidence requirements on its official site. For child support matters, your state's family court can direct you to approved collection facilities.

To explore the full range of relationship tests available for personal knowledge, see our complete guide to understanding DNA relationship testing.

Turnaround Time: When Will You See Your Results?

Once your samples arrive at the lab, standard processing takes 2 to 3 business days. Total turnaround from the time you order to the time you receive your results is typically 7 to 10 business days, accounting for shipping time in both directions. Expedited result options are available during checkout for those who need answers sooner. You will receive an email notification as soon as your report is ready in your secure online account.

For a deeper look at how different types of relationship tests work and which one fits your situation, visit our step-by-step paternity test guide.

Frequently Asked Questions

What does a 99.99% probability of paternity actually mean?

It means the tested man is statistically 9,999 times more likely to be the biological father than a randomly selected unrelated man from the general population. It does not mean there is a 0.01% chance he is not the father in an absolute sense. The figure reflects how powerfully the DNA evidence supports paternity compared with the alternative hypothesis of a random, unrelated man being the source of the child's paternal alleles.

Can a probability of paternity ever reach 100%?

In practice, no. The statistical framework used for DNA paternity testing is built on Bayesian probability, which approaches but never mathematically reaches 100% for an inclusion result. Results above 99.99% are routinely reported and are considered conclusive, but the formula does not allow an absolute 100% inclusion. Exclusion, by contrast, is absolute and reported as 0%.

What is an inconclusive result and what happens next?

An inconclusive result occurs when the Combined Paternity Index (or Combined Relatedness Index for kinship tests) falls between 0.1 and 10, a range that is neither a strong inclusion nor a clear exclusion. When this happens, the lab may analyze up to 5 additional genetic markers beyond the standard panel to generate a more definitive result. Including the biological mother's sample in a retest can also significantly improve resolution.

Does including the mother's sample change the probability of paternity?

Including the mother's DNA sample typically increases the precision of the result. It allows the lab to apply maternal subtraction, isolating the alleles the child must have received from the biological father. This often raises the Combined Paternity Index, resulting in a higher probability of paternity percentage when the tested man is the biological father, and can help resolve inconclusive cases.

Is the probability of paternity on a home test the same as on a court-admissible test?

The laboratory methodology and statistical calculations are identical. The difference between a home test and a court-admissible test is not the science. It is the sample collection process. Court-admissible testing requires witnessed collection, photo identification, and a documented chain of custody. Home kits are designed for personal knowledge and cannot substitute for that chain-of-custody process regardless of the probability percentage they produce.

How many genetic markers does USDC use to calculate the probability of paternity?

USDC's standard paternity panel tests 23 autosomal STR markers. In cases where the initial result is inconclusive, the lab can extend the analysis to up to 28 genetic markers total by adding up to 5 more loci. Testing more markers increases statistical power and reduces the likelihood of an inconclusive outcome.