A grandparent DNA test report can look intimidating the first time you open it. There are marker tables, decimal numbers, and a statistic called the Combined Relationship Index (CRI) that most people have never seen before. Grandparent tests do not use the same probability percentage as a standard paternity test. Instead of a Probability of Paternity, you get a CRI value, and that number is read against different thresholds. As a general benchmark used across the industry, a CRI above 10 supports a biological relationship, a CRI between 0.1 and 10 is inconclusive, and a CRI below 0.1 does not support the claimed relationship. This guide walks through the report one section at a time so you can read it correctly and understand what it does and does not tell you.
Step 1: Identify the Test Type on the Report Header
Start at the top of the first page. Every credible DNA report includes a header block that names the test performed, the accession or case number, the date the samples were received, and the date the report was issued. For a grandparent test, the header should say something along the lines of "Grandparentage DNA Analysis," "Grandparent DNA Test," or "Kinship Analysis - Grandparent." If the header instead says "Paternity Test" or "Avuncular Test," you are looking at the wrong report. Call the lab before going any further.
The header also lists the participants and the biological relationship being evaluated. On a grandparent test you will typically see three named parties: the child, one or both alleged paternal grandparents, and, when included, the biological mother. The mother's DNA is not required, but her sample sharpens the calculation because the lab can subtract her contribution to the child's genome and focus on the paternal half. If the mother was tested, the report will list her as a "known contributor" or "reference sample."
Confirm the names and dates of birth match the people who submitted samples. Transcription errors happen and they invalidate the report. If anything is wrong, contact the lab and request a corrected version before you interpret any of the numbers.
One more thing to check in the header: whether the report is labeled for personal knowledge or for legal use. A home grandparent DNA test, including USDC's home grandparent DNA test, is designed for personal knowledge only. It is not court-admissible because the samples are collected at home rather than under a documented chain of custody. That does not make the science less accurate. It simply means the result cannot be used in custody, inheritance, immigration, or Social Security proceedings. If you need a court-admissible result, you need a legal-grade test with witnessed collection.
Step 2: Find the Combined Relationship Index (CRI) Number
Once you have confirmed the test type, look for the single most important number on the report: the Combined Relationship Index, usually abbreviated CRI. It is sometimes labeled "Combined Grandparentage Index," "Grandparentage Index," or just "Relationship Index." On most reports it appears near the top of the results page, either as a standalone figure or inside the conclusion block.
The CRI is a likelihood ratio. It compares two hypotheses: that the tested grandparent is the true biological grandparent of the child, versus that the tested grandparent is an unrelated person from the same general population. A CRI of 50 means the DNA evidence is 50 times more likely under the grandparent hypothesis than under the unrelated hypothesis. A CRI of 0.05 means the evidence is 20 times more likely if the tested person is unrelated.
This is a critical distinction from paternity testing. A standard paternity test produces a Combined Paternity Index (CPI) that is then converted into a Probability of Paternity, typically expressed as a percentage such as 99.99%. Grandparent testing does not produce a percentage in the same way. If you want a side-by-side explanation of how the paternity index works, our guide to the Combined Paternity Index covers it in detail, and the plain-language explainer on what the CPI number means shows how a likelihood ratio becomes a probability. The CRI on a grandparent report is a likelihood ratio too, but it is read against different thresholds and is generally not converted into a percentage because the statistical model behind grandparent testing is less definitive than paternity testing.
Write the CRI down before you move on. Every remaining step interprets that one number.
Step 3: Interpret the CRI Against the Standard Thresholds
The industry uses three interpretive bands for kinship indices, and they are the same bands used across grandparent, sibling, and aunt/uncle tests. These thresholds are drawn from the AABB Standards for Relationship Testing Laboratories and are widely adopted by accredited relationship testing labs.
- CRI greater than 10: The DNA evidence supports the claimed biological relationship. The higher the number, the stronger the support. A CRI of 100 is stronger than a CRI of 15, and a CRI of 10,000 is very strong.
- CRI between 0.1 and 10: Inconclusive. The evidence does not clearly support the relationship and does not clearly rule it out. This is a genuinely common outcome for grandparent tests, especially when only one grandparent is tested and the mother is not included.
- CRI less than 0.1: The DNA evidence does not support the claimed biological relationship. The lower the number, the stronger the exclusion. A CRI of 0.001 is a strong exclusion.
Read your number against these bands and note which band it falls in. That is your headline result. Everything else on the report is either supporting detail or context.
Two nuances matter here. First, an inconclusive result is not the same as a negative result. It means the test could not give a clear answer with the samples and markers available. Second, the thresholds above are generally accepted industry benchmarks. Individual labs may publish slightly different interpretive language, and some reports will state "supports" or "does not support" in words rather than making you compare your number to a table. Read the lab's own interpretation section carefully.
Step 4: Check Whether One or Both Paternal Grandparents Were Tested
Grandparent testing is much stronger when both paternal grandparents participate. A grandchild inherits roughly 25% of their DNA from each grandparent. When only one grandparent is tested, the lab is looking at half of the paternal contribution and comparing it to a database of unrelated people. That produces a wider range of possible outcomes, and inconclusive results are common in the middle CRI band.
When both paternal grandparents are tested, the lab can see the full paternal genetic contribution to the child. In practice, this means CRI values tend to be much higher when a real biological relationship exists, and much lower when one does not. The math is simply better supported.
Turn to the participants section and confirm how many grandparents were tested. If only one grandparent was included and your CRI landed in the inconclusive band, adding the second grandparent to a retest is often the single most useful thing you can do. If the second grandparent is unavailable, the biological mother's sample is the next best addition because it lets the lab remove her contribution from the child's DNA and focus on the paternal half.
You should also confirm which grandparents were tested. Grandparent DNA testing evaluates the paternal side because the goal is usually to establish whether an alleged father, who cannot be tested directly, is the biological father. Testing maternal grandparents does not answer a paternity question. If the report shows maternal grandparents were tested, the analysis may still be valid for a different purpose, such as confirming a maternal line, but it will not answer the paternity question most families ask. Our grandparent DNA test knowledge base covers the family structures the test is designed for.
Step 5: Look at the Individual Marker Matches
Behind the CRI number is a table of the individual genetic markers analyzed. USDC's home grandparent DNA test analyzes up to 28 genetic markers, which is above the industry benchmark of 20 or more markers used by most accredited relationship testing labs. Each row in the marker table represents one location in the genome, usually a Short Tandem Repeat (STR) locus. Standard STR loci used in relationship testing include markers like D3S1358, vWA, FGA, D8S1179, and D21S11, among others. The full set of core loci is described in publications from the NIST Short Tandem Repeat DNA Internet Database.
Each row typically shows:
- The marker name (for example, D3S1358)
- The alleles detected in the child (a pair of numbers, one from each parent)
- The alleles detected in each tested grandparent
- The alleles detected in the mother, if she was tested
- A per-locus index value contributing to the CRI
For a biological grandparent, you expect to see shared alleles at most loci. The child inherited one allele at each locus from the father, and that paternal allele had to come from one of the paternal grandparents. When the tested grandparent shares an allele with the child's paternal contribution at many loci, per-locus index values are high and multiply into a high CRI. When the tested grandparent shares no alleles with the child's paternal contribution at many loci, index values are low or zero and pull the CRI down.
A single mismatch does not exclude a relationship. Rare genetic mutations can cause a mismatch even between a true biological grandparent and grandchild. Accredited labs allow for a small number of mismatches before calling an exclusion, and mutation rates are built into the statistical model. Multiple mismatches across several loci are a different story and usually indicate the tested person is not the biological grandparent. If you see mismatches on more than two or three loci and your CRI is below 0.1, the exclusion is well supported.
You do not need to hand-verify every row. The lab has already done the math. Skimming the marker table is useful because it shows you where the CRI number came from and gives you confidence that the analysis was based on actual data rather than a black-box calculation.
Step 6: Read the Biological-Relationship Conclusion Statement
Every relationship report closes with a written conclusion. This is the plain-language sentence that translates the CRI into an answer. For a grandparent test, the conclusion will usually read something like:
- "The DNA analysis supports the claimed biological relationship between the tested grandparent(s) and the child." (CRI above 10)
- "The DNA analysis is inconclusive. Additional testing is recommended." (CRI between 0.1 and 10)
- "The DNA analysis does not support the claimed biological relationship between the tested grandparent(s) and the child." (CRI below 0.1)
Read this statement carefully and match it to your CRI. If the conclusion statement disagrees with what the CRI band would suggest, do not try to reconcile it yourself. Call the lab and ask for clarification. It is possible the report writer applied additional context you cannot see, but it is also possible there was a clerical error.
Watch out for hedging language. Phrases like "cannot be ruled out" or "not excluded" without a supporting CRI above 10 usually mean the result was inconclusive rather than supportive. The word "supports" is doing important work in a conclusion statement. Its absence tells you as much as its presence.
The conclusion statement should also be signed by a qualified laboratory director or supervisor, with credentials listed. The International Society for Forensic Genetics publishes guidelines for how kinship reports should be structured and signed, and accredited labs follow those norms.
Step 7: Decide What the Result Means for Your Family Situation
The final step is deciding what to do with the result. A DNA report is a piece of evidence. It is not a plan. What comes next depends on why you tested and what the number showed.
If your CRI is above 10 (supportive): The DNA evidence supports a biological grandparent relationship. For most families, this is enough to move forward with whatever conversation, decision, or relationship building prompted the test. If the result will inform any legal matter, such as inheritance, custody, immigration, or Social Security benefits, a home test is not sufficient. You would need to repeat the test as a legal-grade test with documented chain of custody. Home grandparent tests, including USDC's home kit, are for personal knowledge only.
If your CRI is between 0.1 and 10 (inconclusive): The test could not give a clear answer with the samples provided. This is where retesting usually makes sense. Adding a second paternal grandparent, adding the biological mother, or in some cases testing an aunt or uncle on the paternal side, will often move the result into a clearly supportive or clearly exclusionary range. See the retest section below.
If your CRI is below 0.1 (not supportive): The DNA evidence does not support a biological grandparent relationship. This is a hard result to receive, especially if there was an emotional expectation attached to it. Take time before making decisions or announcements. If the exclusion is unexpected, one option is to repeat the test at a different lab with fresh samples to rule out any procedural error. A confirmed exclusion at two labs is very strong evidence.
Regardless of the result band, remember that a home grandparent DNA test is not court-admissible. Do not present the report as legal evidence, and do not rely on it in proceedings that require documented chain of custody. If a legal answer is needed, plan for a legal-grade test through a lab that offers witnessed collection. Our broader overview of kinship DNA testing covers the other test types that can help when grandparent testing alone is not conclusive.
When to Retest or Add a Second Grandparent
Retesting is worth considering in a few specific situations, and not worth the money in others. Here is how to think about it.
Retest when your first result was inconclusive with only one grandparent tested. This is the highest-value retest. Adding the second paternal grandparent almost always sharpens the CRI because the lab can now see both halves of the paternal genetic contribution. If the second grandparent is willing and available, this is the first move.
Retest with the mother's sample added when the second grandparent is unavailable. The mother's DNA lets the lab subtract her contribution from the child's genome. This effectively isolates the paternal half of the child's DNA, which is what the grandparent comparison is trying to evaluate in the first place. Adding the mother often converts an inconclusive result into a decisive one.
Consider an aunt or uncle test instead. If both paternal grandparents are unavailable but a full sibling of the alleged father is available, an aunt/uncle (avuncular) test is another kinship option. It uses the same CRI framework and the same interpretive thresholds. Aunt/uncle testing is not stronger than grandparent testing, but it is another route when grandparents cannot participate.
Retest at a different lab when a result contradicts strong prior evidence. If a supportive result contradicts a documented paternity exclusion, or if an exclusion contradicts documented paternity inclusion, run the test again at a second accredited lab. Concordant results from two labs are much harder to dispute than a single result from either.
Do not retest just because you do not like the answer. If both parties were tested, samples were collected properly, and the CRI is clearly in the supportive or exclusionary band, repeating the same test with the same participants will produce a similar result. Retesting only helps when you can change something meaningful, such as adding a participant, correcting a collection issue, or verifying at a second lab.
Before retesting, call the lab that produced the original report and ask them what would change the outcome. They will tell you honestly whether adding a specific person would help or whether the existing result is already as informative as it can be.
Frequently Asked Questions
What is a good CRI number on a grandparent DNA test?
A CRI above 10 is generally interpreted as supportive of the claimed biological relationship. Higher numbers indicate stronger support. A CRI of 100 is stronger than a CRI of 15, and a CRI in the thousands is very strong. A CRI between 0.1 and 10 is inconclusive, and a CRI below 0.1 does not support the claimed relationship. These thresholds are drawn from AABB Standards for Relationship Testing Laboratories and are widely used by accredited labs.
Why does a grandparent test give a CRI instead of a percentage like a paternity test?
Grandparent testing is a kinship analysis, not a direct parent-child comparison. Paternity testing compares the alleged father's DNA directly to the child's DNA, which produces a Combined Paternity Index that can be converted into a percentage. Grandparent testing compares grandparent DNA to a grandchild who is one generation removed, which introduces more genetic variability. The resulting Combined Relationship Index is a likelihood ratio and is generally read against interpretive bands rather than converted into a percentage. This is why kinship tests, including sibling and aunt/uncle tests, use CRI thresholds instead of a Probability of Paternity figure.
Can a home grandparent DNA test be used in court?
No. Home grandparent DNA tests are designed for personal knowledge only. Because the samples are collected at home rather than under a documented chain of custody, the result is not admissible in court, immigration, Social Security, or other legal proceedings. If a legal-grade result is required, you need a test with witnessed sample collection through a lab that offers legal relationship testing. Home tests use the same laboratory science and produce the same kind of CRI number. The difference is procedural, not scientific.
My CRI was inconclusive. What should I do next?
The most useful next step is almost always adding another participant. If only one paternal grandparent was tested originally, adding the second paternal grandparent will usually resolve an inconclusive result in one direction or the other. If the second grandparent is unavailable, adding the biological mother's sample allows the lab to isolate the paternal half of the child's DNA and often sharpens the CRI significantly. In cases where no paternal grandparents are available, an aunt/uncle test using a full sibling of the alleged father is another kinship option. Call the lab that produced your original report and ask what would most improve the analysis with the participants you have available.
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