The technical terminology used in the DNA Sibling Test Report comes from regulatory and legal requirements. To demystify some of this terminology, here's how to read and understand the different parts of your test report.
Overview
Your DNA Sibling Test Report will consist of 3 pages for each sibling pair tested:
- Page 1 is a cover page with the Case Number and the first names of the alleged siblings.
- Page 2 is where you will find your test results and a chart with the genetic markers for each person tested (DNA profile).
- Pages 3 and 4 include interpretation guidances for your DNA Sibling Test.
The Science
To understand the details of your test report, it is important to be familiar with the terminology and science behind how relationship DNA testing works.
In a relationship DNA test, such as the DNA Sibling Test, the laboratory generates a DNA profile of specific genetic markers (also known as short tandem repeats, or "STR") at a set of known locations ("loci", or singular, "locus") in your DNA. At each DNA locus, you will have 2 genetic variants ("alleles"), one of which is inherited from your mother, and the other from your father.
The laboratory technique used to amplify and study DNA is called polymerase chain reaction (PCR). In a DNA Sibling Test, PCR analysis is used to determine the specific alleles that you have at each DNA locus, aka your DNA profile. If two individuals share at least one allele at the same DNA locus, that means the two individuals "match" at the particular DNA locus.
Unlike the DNA paternity test, which expects a match at every DNA locus, true full- or half-siblings are expected to share some matching alleles. This is because siblings do not always inherit the same DNA markers from their parents, otherwise every sibling would be identical.
Your Results
In your test report, you will find a chart displaying the DNA profiles for each person tested. The data displayed in the DNA profile chart may look something like:
| DNA Locus | Putative Sibling 1 | Putative Sibling 2 | Sibling Index (Full) |
Sibling Index (Half) |
| D8S1179 | 13,14 | 14 | 1.32 | 1.57 |
| D21S11 | 28,30 | 28,30 | 4.25 | 1.72 |
The first column, DNA Locus, lists the genetic markers that were analyzed for your DNA Sibling Test (e.g. D8S1179).
The numbers in the Putative Sibling 1 and Putative Sibling 2 columns represent the two alleles that you and your potential sibling received from your parents at the corresponding genetic marker. You may have two identical alleles at a marker (e.g. “14”), or you may have two different alleles (e.g. “13,14”). If the same number is present in both columns, that means you “match” at the corresponding genetic marker.
The final two columns, Sibling Index (Full) and Sibling Index (Half), represent the statistical strength of a match at the corresponding genetic marker. These values are calculated based on the match type typically expected for true full- and half-sibling pairs.
Combined Sibship Index
The Combined Sibship Index ("sibship index") is calculated by multiplying the the sibling index values for each DNA locus, and it is the final result of a DNA Sibling Test. The sibship index represents how many times more likely it is that the tested individuals are full-/half-siblings than not related.
| Sibship Index | Explanation |
| 10 | The individuals are 10x more likely to be true siblings than not siblings. |
| 100 | The individuals are 100x more likely to be true siblings than not siblings. |
| 0.01 | The individuals are 100x more likely not to be true siblings than siblings (1/0.01 = 100). |
In general, a larger sibship index (>1.0) corresponds to a higher likelihood of relationship. The higher that the sibship index is above 1.0, the higher the likelihood that the two individuals are related as siblings.
Likewise, a smaller sibship index (<1.0) corresponds to a lower likelihood of relationship. The lower that the sibship index is below 1.0, the lower the likelihood that the two individuals are related as siblings.
Recommended Interpretation of the Combined Sibship Index (Full/Half)
| Sibship Index | Interpretation |
| Greater than 10 | The genetic evidence is supportive of relationship. |
| 0.1 to 10 | The genetic evidence is inconclusive of relationship. |
| Less than 0.1 | The genetic evidence is not supportive of relationship. |
Are we half or full siblings?
To determine whether it is more likely that two individuals are full vs. half siblings, you can compare the sibship index values obtained for the full and half sibling relationships.
If the Full Sibship Index is greater than the Half Sibship Index, then the individuals are more likely to be full siblings. Likewise, if the Half Sibship Index is greater than the Full Sibship Index, then the individuals are more likely to be half siblings.
To calculate the likelihood of the full vs. half sibling relationship, the sibship index values can be directly compared. For example, if a sibling pair has a Full Sibship Index of 100 and a Half Sibship Index of 2, then they are 50 times more likely to be full siblings than half siblings.
Similarly, if a sibling pair has a Full Sibship Index of 1.5 and a Half Sibship Index of 20, then they are 13.33 times more likely to be half siblings than full siblings (20 ÷ 1.5 = 13.33).