What Does Combined Paternity Index Mean in a DNA Test?
Meta description: Learn what a combined paternity index means, how laboratories calculate it, how it differs from probability of paternity, and what can affect a DNA test result.
A combined paternity index (CPI) is a statistic reported in many DNA paternity tests. It summarizes how strongly the tested genetic markers support one relationship hypothesis over another: that the tested man is the child’s biological father rather than an unrelated man. A CPI can look like a probability, but it is not one by itself. Understanding what it measures—and what it leaves out—helps make a DNA report easier to interpret.
What a Combined Paternity Index Means
In a paternity test, a laboratory examines specific locations in DNA, called genetic markers or loci, in the child, the mother when available, and the alleged father. At each marker, the child inherits one copy of the relevant genetic information from each biological parent. The laboratory evaluates whether the alleged father could have passed on the genetic variant that the child shares with him.
The combined paternity index is the product of the individual paternity indexes calculated across the tested markers. Each individual index expresses how much more likely the observed genetic evidence is if the tested man is the father than if he is an unrelated man from the relevant population. The combined value brings the evidence from multiple markers together.
For example, a CPI of 1,000 means the observed genetic results are 1,000 times more likely under the tested-father hypothesis than under the unrelated-man hypothesis, given the assumptions and methods used. It does not mean there is a 1,000-to-1, or 99.9%, probability that the man is the father. A probability requires additional assumptions, including a starting probability before the DNA results are considered.
How Laboratories Calculate the CPI
At each tested locus, the laboratory compares the child’s genetic profile with the profiles of the mother and alleged father, if the mother’s sample is included. The laboratory accounts for the inheritance pattern and the frequency of relevant genetic variants in a reference population. A variant that is uncommon in that population may provide stronger evidence than one that is common, assuming the tested profiles fit the inheritance hypothesis.
The lab calculates an individual paternity index for each usable marker. It then multiplies those indexes to obtain the CPI, subject to its validated procedures and statistical model. This calculation assumes, among other things, that the markers are appropriately selected and that population-frequency data are suitable for the analysis. Laboratories use established methods, but specific panels, procedures, and reporting practices can vary.
Modern paternity testing commonly examines many short tandem repeat (STR) markers. STRs are repeated DNA sequences that vary among people. Testing multiple markers helps distinguish between individuals and builds a larger body of evidence than a single marker could provide. A report should identify the result and explain the laboratory’s interpretation; questions about the calculation can be directed to the lab.
CPI and Probability of Paternity Are Different
Many reports convert the CPI into a probability of paternity, sometimes called the probability of inclusion. The conversion uses Bayes’ theorem and requires a prior probability—a starting assumption about the chance that the tested man is the father before considering the DNA results.
A common reporting convention uses a prior probability of 50%. Under that convention, the probability is calculated as CPI divided by CPI plus 1, then multiplied by 100. If the CPI is 1,000, this convention produces a probability of about 99.9%. That percentage depends on the 50% starting assumption; it is not a direct measurement of the real-world chance of paternity and does not account for every possible circumstance.
For that reason, a high probability on a report should be read alongside the CPI, the laboratory’s assumptions, and its conclusion. The CPI describes the strength of the genetic evidence under specified competing hypotheses. The probability of paternity combines that evidence with an assumed starting probability.
| Report term | What it describes | Important limitation |
|---|---|---|
| Individual paternity index | Evidence from one genetic marker | It does not summarize all tested markers. |
| Combined paternity index | Combined evidence across tested markers | It is a likelihood ratio, not a probability. |
| Probability of paternity | A probability calculated from the CPI and a prior assumption | It depends on the prior probability used. |
How to Read a Paternity Test Result
Reports commonly reach one of two broad conclusions: the tested man is not supported as the biological father, or the genetic results strongly support that he is the father. Exact wording and thresholds differ among laboratories, so the report’s own interpretation matters more than a general rule of thumb.
When the alleged father is excluded, the child’s genetic profile may contain markers that cannot be explained by inheritance from him. A single apparent mismatch is not always enough to establish exclusion: a rare mutation, a technical issue, or other biological factors may need consideration. Laboratories generally evaluate the full set of markers and follow their validated procedures before reporting an exclusion.
When the alleged father is not excluded, the tested markers are consistent with him being the father, and the CPI indicates how strongly the results favor that explanation over the unrelated-man alternative. “Not excluded” does not mean that no other biological relationship could explain the results. If the alleged father has a close male relative—such as a brother, father, or son—who could also be the biological father, the laboratory should be told. Additional testing or a different analysis may be appropriate.
What Can Affect a Combined Paternity Index?
Whether the mother is tested
A standard trio test includes the child, mother, and alleged father. The mother’s DNA helps identify which genetic variants the child likely inherited from her, making it easier to evaluate the variants inherited from the biological father. A test without the mother is often called a duo test. It can still provide useful evidence, but interpretation may be less informative in some cases, and a laboratory may recommend testing more markers or additional relatives.
Close relatives and alternative candidates
The usual comparison may be against an unrelated man. If the possible father is closely related to the tested man, that assumption may not fit the situation: close relatives share more DNA than unrelated people do. A laboratory needs accurate information about possible alternative fathers and their relationships to the tested man. In some cases, testing the other candidate or a close relative can help clarify the result.
Mutations and unusual findings
Genetic markers can change through mutation, so a child and biological parent may occasionally differ at a marker. Laboratories account for mutation using established methods. A mutation does not automatically mean the alleged father is excluded, but it can affect the analysis. Rare situations, such as a bone marrow transplant or certain medical histories, may also require the lab to review the sample and circumstances.
Population data and laboratory methods
Marker frequencies in reference populations are part of the statistical calculation. The applicable population data and the laboratory’s analytical method can affect the reported index. A lab should use validated procedures and appropriate frequency data, and it should explain how its result is interpreted. Two reports may differ if their tests, marker panels, assumptions, or reporting conventions differ.
Why More DNA Markers Usually Strengthen the Evidence
Each suitable marker contributes evidence to the analysis. When the markers provide independent information under the laboratory’s model, multiplying their individual indexes can produce a much larger combined value. This is why a result across many markers is generally more informative than a result from only a few.
However, a large CPI should not be interpreted without context. The number depends on the genetic profiles, marker frequencies, test design, and statistical assumptions. It is not a universal score that can be compared casually across every laboratory or test. The report’s stated conclusion and any limitations are essential parts of the result.
Legal and At-Home Paternity Tests
A DNA test purchased for personal information may use a cheek swab collected at home. Such testing can provide a paternity result, but it may not meet legal requirements for court, child support, custody, birth-certificate, or immigration matters. The issue is often not the genetic analysis alone; it is also whether the identities of the people tested and the chain of custody can be documented.
For a legal test, the ordering party should confirm the requirements with the relevant court, agency, or attorney before samples are collected. A legal collection typically involves identity verification, witnessed sample collection, and documented handling. Laboratories accredited by the AABB for relationship testing may be required or preferred for certain legal uses, but requirements vary by jurisdiction and case. Accreditation does not replace checking the specific rules that apply.
Questions to Ask About a CPI Report
- What does the report’s conclusion say? Read the laboratory’s interpretation, not just the CPI or percentage.
- Was the mother tested? Find out whether the analysis was a trio or duo test.
- Could the tested man be related to another possible father? Tell the lab about close relatives who may be candidates.
- What prior probability was used? This matters if the report includes a probability of paternity.
- Is the test suitable for the intended use? Confirm legal collection and accreditation requirements before testing.
- Can the lab explain unusual markers or limitations? A qualified laboratory representative can clarify its methods and report language.
When to Seek Clarification
If the report seems inconsistent with family information, includes a marker mismatch, or does not clearly explain the conclusion, the testing laboratory is the best first contact. It can explain the CPI, the probability calculation, the marker panel, and any limitations specific to the case. A clinician or genetic counselor may also help explain biological and family-history questions, though legal questions should be directed to an attorney or the relevant agency.
In short, a combined paternity index measures how strongly DNA findings favor the tested man being the father over an unrelated-man alternative. It is a likelihood ratio, not a standalone probability. The laboratory’s conclusion, assumptions, sample collection method, and the possibility of close-relative candidates all matter when interpreting the result.