What Is a Combined Relationship Index (CRI) and How Do You Read One?

Combined Relationship Index (CRI) — how to read your kinship DNA test result: not supportive below 0.1, inconclusive between 0.1 and 10, supportive above 10

You ordered a kinship DNA test — maybe to find out if you and a half-sibling share the same father, or to see whether your grandchild is biologically your grandchild — and the report came back with a number labeled "Combined Relationship Index" or "CRI." Somewhere on the page it says the result is "supportive" or "inconclusive." And now you're trying to figure out what that actually means for your family.

You are not alone. CRI is one of the least-explained numbers in consumer DNA testing. It's not a percentage, it's not a yes/no, and it doesn't behave like the "probability of paternity" that most people have heard about. This guide walks through what a CRI is, how to read yours, why kinship tests use it instead of a simple probability figure, and what to do if your number lands in the inconclusive middle band.

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What is a CRI?

CRI stands for Combined Relationship Index. It is a likelihood ratio. In plain English, it compares two possibilities using the DNA markers that were tested:

  • How likely is the observed DNA pattern if the tested people are related in the way the test is checking for (full siblings, half siblings, grandparent, aunt/uncle, etc.)?
  • How likely is that same DNA pattern if the tested people are unrelated, drawn at random from the general population?

The CRI is the first number divided by the second. If the tested relationship is a much better explanation of the data than being unrelated, the CRI is a big number. If being unrelated is a much better explanation, the CRI is a very small number (a fraction). If neither explanation is clearly better, the CRI ends up somewhere in the middle.

The word "Combined" is doing real work here. Each individual genetic marker produces its own small likelihood ratio, and those ratios are multiplied together across all the markers tested to produce the final Combined Relationship Index. This is the same basic framework used across forensic and family-relationship genetics, and it is described in guidelines published by the International Society for Forensic Genetics (ISFG) DNA Commission.

How to read your CRI number

USDC kinship reports use three interpretation bands. These are the ones you'll actually see referenced next to the number on your report:

  • CRI greater than 10 → Supportive of the tested relationship. The DNA evidence is at least 10 times more consistent with the tested relationship than with being unrelated. Higher numbers (100, 1,000, 10,000+) mean progressively stronger support.
  • CRI between 0.1 and 10 → Inconclusive. The data does not lean strongly in either direction. Not a "no" — a "not enough signal yet."
  • CRI less than 0.1 → Not Supportive of the tested relationship. The DNA evidence is at least 10 times more consistent with the tested people being unrelated than with the tested relationship.

A useful way to think about it: a CRI of 100 means the DNA is 100 times more consistent with the tested relationship than with being unrelated. A CRI of 0.01 means the opposite — the DNA is 100 times more consistent with being unrelated. A CRI of 1 would mean the two explanations are equally likely, which is why anything close to 1 lands in the inconclusive zone.

Why kinship tests use CRI instead of "probability"

Most people have heard of "probability of paternity" — the 99.9%+ number reported on paternity tests. Kinship reports don't use that figure, and there's a good scientific reason.

A biological parent and child share exactly half of their DNA. That relationship is genetically clean: one person contributed one copy of every chromosome to the other. When you compare a father and child, the math is direct and the resulting probability is usually very high or very low with little middle ground.

Kinship relationships — siblings, grandparents, aunts and uncles — are different. Full siblings share on average 50% of their DNA, but the actual amount varies from roughly 38% to 62% because of how chromosomes recombine each generation. Half siblings, grandparents, and aunts or uncles all share around 25% on average, again with meaningful variation from one pair to another. The MedlinePlus overview of inheritance from the U.S. National Library of Medicine explains why: recombination shuffles the deck a little differently for every child.

Because there is real biological variability in how much DNA any specific pair of relatives shares, the test can't produce a single tidy probability the way a paternity test can. What it can produce, honestly, is a likelihood ratio: how much better does the tested relationship explain what we see, compared to the alternative? That's the CRI. It is the mathematically honest way to report a kinship result. A useful technical background is available from the National Center for Biotechnology Information (NCBI), which hosts peer-reviewed literature on kinship likelihood calculations.

What to do if your CRI is inconclusive

An inconclusive CRI (in the 0.1 to 10 range) is frustrating, but it is a normal outcome in kinship testing — especially for half-sibling and aunt/uncle comparisons, where the shared DNA is thinner to begin with. There are two productive next steps, and they can be combined.

1. Add more genetic markers. The standard kinship analysis uses a core set of markers. When a result lands in the inconclusive band, USDC's lab can extend the analysis to up to 28 genetic markers — this adds another 5 markers on top of the standard panel. More markers means more independent points of comparison, and often sharpens an inconclusive result into either "supportive" or "not supportive" territory.

2. Add another relative to the test. This is often the single most powerful move for kinship testing. For example:

  • In a half-sibling test, adding the known mother of one sibling gives the lab a genetic anchor. The math can then "subtract out" that parent's contribution and look more directly at what came from the potential shared father.
  • In a grandparent test, adding both grandparents (rather than just one) roughly doubles the resolution.
  • In an aunt/uncle test, adding a second aunt or uncle from the same side of the family gives the analysis more genetic reference points.

Testing more people costs more, but it's the difference between a definitive answer and going through this again a year from now. If your first result is inconclusive, USDC's support team can walk you through which additional participant would most improve your specific case.

Three real-world examples

Two adult sisters asking whether they share a father. Their mother is deceased and their possible father is not available for testing. They send in cheek swabs from both women. The initial CRI comes back at 4.2 — inconclusive. The lab extends the analysis to the full panel of up to 28 genetic markers and the revised CRI rises to 34, which lands cleanly in the supportive band. They now have a defensible answer for their family knowledge.

A grandmother testing whether her son's child is her biological grandchild, with the child's paternal grandfather deceased. Only one grandparent is available. This is the harder version of the grandparent test statistically, because the "signal" is coming from just one grandparent instead of two. The first result comes back with a CRI of 8 — inconclusive. Adding the child's mother to the test provides a genetic reference that lets the lab isolate the paternally-inherited DNA. The recalculated CRI climbs into strongly supportive territory.

An aunt-uncle test. A man wants to know whether his late brother's teenaged child is biologically his niece. He and the child both swab. The CRI comes back at 0.7 — right in the middle of inconclusive. Extending to the full marker set brings the CRI up to 12, just over the supportive threshold. In this case the family decides that's enough for their personal knowledge; they don't need further testing.

Where USDC's kinship kits fit

USDC offers three at-home kinship tests, all at the same price point:

  • Home Full Sibling / Half Sibling Test — $139. For two or more people who want to know whether they share one or both biological parents.
  • Home Grandparent Test — $139. For confirming a biological grandparent-grandchild relationship on either side of the family.
  • Home Aunt/Uncle Test — $139. For confirming a biological aunt/uncle-niece/nephew relationship.

Turnaround: Once your samples arrive at the lab, standard processing takes 2 to 3 business days. From placing your order to seeing your results, expect 7 to 10 business days total under standard shipping. Expedited processing options are available during checkout if you need results faster.

All three kits use cheek-swab collection. You can read more about how these tests are chosen and combined on USDC's kinship DNA testing page.

Some readers land on this article because a CRI number is going to be discussed in a legal setting — a probate case, an inheritance dispute, an immigration filing, a custody or child support matter. It's important to understand a hard rule about at-home DNA testing: an at-home test is designed for personal knowledge. Because the samples are collected without a supervised, documented chain of custody, an at-home result — no matter how strong the CRI number — is not treated as court-admissible evidence.

This is not about the science being different. The lab work is the lab work. It's about who watched the swab go into whose mouth, and whether that chain from collection to result is documented in a way a court will accept. For those situations, work with a provider specifically credentialed for court-admissible testing, where a neutral third party performs sample collection and every step from ID verification onward is logged.

The AABB (Association for the Advancement of Blood & Biotherapies) maintains a directory of laboratories that meet its relationship-testing standards, which is a reasonable starting point when researching accredited providers for legal-use testing.

Frequently asked questions

What is a CRI? A Combined Relationship Index is a likelihood ratio comparing how well the tested relationship explains the DNA data versus how well "unrelated" explains it.

What does an inconclusive CRI mean? That the markers analyzed so far don't yet lean strongly toward either the tested relationship or being unrelated. It's a signal to add markers or add another relative, not a final answer.

How do I improve an inconclusive CRI? Two options: extend the analysis to up to 28 genetic markers, or add another relative (a known parent, a second sibling, a second grandparent) to give the math more reference points.

Is CRI the same as probability of paternity? No. Probability of paternity is used for direct parent-child comparisons. CRI is used for kinship relationships (siblings, grandparents, aunts/uncles) where the shared DNA is smaller and more variable.

What CRI number is "strong enough"? The bands used on USDC reports: greater than 10 = supportive, 0.1 to 10 = inconclusive, below 0.1 = not supportive. The higher the CRI, the stronger the statistical support.

Byline

— Dr. Todd Lewis

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