How to Read the Result Report (General Kinship Testing; STR)

Last revised: December 21, 2024

A detailed explanation of how to read the STR (Short Tandem Repeat) type table shown in a DNA paternity testing result report. We explain, with concrete examples, how paternity is affirmed or denied based on the sharing of genotypes at each locus.

How to Read the Result Report (General Kinship Testing; STR)

In our general kinship testing, we analyze specific regions of DNA (STR loci) to scientifically determine the biological relationship between test subjects. The first page of the result report uses the following kind of wording to state the conclusion of the test (using a paternity test as an example).

When affirmed: A is not excluded as the biological father of B.
When denied: A is excluded as the biological father of B.

This wording follows international forensic science convention and is a scientifically rigorous way of stating the result. However, for customers seeing this wording for the first time, the phrases "is not excluded" and "is excluded" can feel somewhat hard to grasp [ref:1]. In fact, we do receive a certain number of inquiries about how to read the result report.

When we receive such inquiries, we usually direct customers to the wording and the probability of paternity (%) found in the "Analysis of Test Results" section on the third page of the report. The probability of paternity is a value obtained by statistically comparing the likelihood that a paternal relationship exists against the likelihood that it does not, and the international standard is that a value of 99.99% or higher is considered to mean that "a paternal relationship effectively exists" [ref:2].

Meanwhile, even among customers who do not contact us with questions, many check their results by looking only at this "Analysis of Test Results" section. However, since many customers also want to accurately understand how the result in front of them was actually derived, this page provides a detailed explanation to help you understand the result report.

Correctly Understanding General Kinship Testing Results

To correctly understand the results of general kinship testing, it is essential to grasp how to read the STR (Short Tandem Repeat) table attached to the results.

What Is STR (Short Tandem Repeat)?

STR refers to special regions within the DNA sequence where a short unit sequence of about 2 to 6 bases repeats consecutively. The number of these repeats (the repeat count) varies from person to person, and this variation is what makes DNA typing possible [ref:3].

To emphasize this important point again: the number of STR repeats differs from person to person. This individual variation is precisely what makes it possible to investigate kinship relationships and to identify perpetrators in criminal investigations [ref:4].

This STR type is generally inherited half from the father and half from the mother. This is a basic principle based on Mendelian inheritance. Human chromosomes come in pairs, one from each parent (22 pairs of autosomes), and at each STR locus a child inherits one allele from the father and one from the mother. Therefore, DNA paternity testing checks, at every locus examined, whether the genotype has been correctly passed down from the father or mother to the child [ref:5].

About the Number of Loci Used in Testing

DNA paternity testing typically analyzes 13 or more loci. This conforms to the standard CODIS (Combined DNA Index System) criteria established by the U.S. FBI, and some testing laboratories analyze 20 or more loci. The more loci examined, the higher the accuracy of the test, and the lower the probability of a coincidental match [ref:6].

Below, to aid understanding, we use a simplified table showing about six loci to explain concretely how to read the results.

How to Read the STR Test Result Table (Concrete Example)

※The values above are given at random for the purpose of this explanation.

Understanding the Basic Structure of the Table

• Leftmost column (locus): Lists the names of the STR loci examined. This column tells you how many loci were checked.
• Right-hand columns (STR type of each subject): Shows, as numeric values, the STR type results for each subject, such as Father A and Child B.
• Meaning of the numbers: Each number indicates the number of repeats (allele value) at that STR locus. For example, "15 – 16" means that the two chromosomes carry 15 and 16 repeats, respectively.
• When only one number is shown: As with Child B's D3S1358, when only "15" is listed, this indicates that the paternal-derived and maternal-derived alleles have the same value (a homozygote); it is shorthand for what should strictly be written as "15 – 15."

When Paternity Is Affirmed (Father A and Child B)

First, let's look at the relationship between Father A and Child B.

1. Locus D3S1358: Father A's genotype is 15 and 16; Child B's genotype is 15 (= 15-15). Child B may have inherited the "15" allele from Father A, so the STR type is shared.
2. Locus vWA: Father A has 18 and 19, and Child B also has 18 and 19. It can be confirmed that one of these alleles may have been inherited from Father A.
3. Locus D16S539: Father A has 9 and 12; Child B has 10 and 12. They share the "12" allele.
4. Locus CSF1PO: Father A has 12 and 13; Child B also has 12 and 13. The alleles are shared.
5. Locus D8S1179: Father A has 11 and 15; Child B has 10 and 11. They share the "11" allele.
6. Locus D21S11: Father A has 29 and 30; Child B has 29 (= 29-29). They share the "29" allele.

In this way, when the STR type is shared at every locus, Father A is determined to be the biological father of Child B. The report states, "A is not excluded as the biological father of B."

When Paternity Is Denied (Father A and Child C)

Next, as an example of denial, let's look at the relationship between Father A and Child C.

At locus D3S1358, Father A's genotype is 15 and 16, and Child C's genotype is 16, so the "16" STR type is shared. However, looking at the vWA locus below it, Father A has 18 and 19, while Child C has a completely different STR type of 6 and 11.

Furthermore, at the CSF1PO locus Father A has 12 and 13 while Child C has 10 and 11; at D8S1179 Father A has 11 and 15 while Child C has 18 and 24; and at D21S11 Father A has 29 and 30 while Child C has 15 and 18 — mismatches in STR type are confirmed at multiple loci.

In this way, when there are multiple loci where the STR type is not shared, Father A is determined not to be the biological father of Child C. The report states, "A is excluded as the biological father of C." In general, a paternal relationship is judged to be denied when there are mismatches at three or more loci [ref:2].

About Mutation

In very rare cases, even when a parent-child relationship actually exists, the STR type may fail to match at one or two loci. This is due to a phenomenon called mutation, in which the number of STR repeats increases or decreases by one during DNA replication [ref:5].

In such cases, the testing laboratory carefully determines whether the discrepancy is a mutation or a genuine mismatch (indicating no biological relationship) by analyzing additional loci or performing statistical calculations (correction of the paternity index). Generally, if there is a complete allele mismatch at three or more loci, it is judged to be a denial of paternity rather than a mutation.

The Meaning of the Probability of Paternity (CPI)

The probability of paternity shown in the "Analysis of Test Results" section of the result report is calculated based on the CPI (Combined Paternity Index). This is obtained by multiplying together the Paternity Index (PI) calculated for each locus, and then converting that value into a probability using Bayes' theorem [ref:6].

If the probability of paternity is 99.99% or higher, it is considered that "the paternal relationship is effectively certain." On the other hand, in the case of denial, the probability of paternity is 0%, clearly reflecting the mismatches found at multiple loci.

Structure of the Result Report

The result report for our general kinship testing is generally structured as follows.

• Page 1: The statement of the test conclusion (affirmation or denial)
• Page 2: The table of STR type analysis results (allele values by locus, as explained above)
• Page 3: Analysis of Test Results (statistical calculation results and probability of paternity)

When checking your results, we recommend first confirming the conclusion on page 1, and then reviewing the STR table on page 2 and the probability of paternity on page 3 as the supporting evidence, for a deeper understanding.

Accuracy and Reliability of DNA Testing

DNA paternity testing using STR types is one of the most reliable kinship testing methods in forensic science today. By simultaneously analyzing multiple independent loci, it is possible to reduce the probability of a coincidental match to an astronomically low level [ref:4].

For example, when 20 loci are analyzed, the probability that two unrelated people would coincidentally share alleles at every single locus is extremely low — on the order of one in several billion to one in several tens of billions. This high discriminating power is why DNA testing is widely accepted as evidence in courts and government agencies [ref:3].

The above explains the general approach to reading the STR table and the result report. If you have any questions or concerns about your result report, please feel free to contact our customer support team.

Frequently Asked Questions

Q1. Does the phrase "is not excluded" mean that a paternal relationship exists?

A. Yes. The phrase "A is not excluded as the biological father of B" means that genotype sharing has been confirmed at every STR locus, and paternity has been scientifically affirmed. In forensic science, it is international convention to use the modest phrasing "is not excluded" rather than "is proven." Specifically, if the probability of paternity is 99.99% or higher, the paternal relationship is judged to be effectively certain.

Q2. What does it mean when only one number is written for an STR locus value?

A. When only one number is listed (for example, "15"), it indicates that both the paternally derived and maternally derived alleles have the same value (a homozygote). Formally, this should be written as "15 – 15," but that is abbreviated. When two different values are present (for example, "15 – 16"), this is called a heterozygote.

Q3. Can a parent-child relationship be affirmed even if not all loci match?

A. In very rare cases, even when a parent-child relationship actually exists, the STR type may fail to match at one or two loci. This is a phenomenon called mutation, which occurs when the repeat count changes during DNA replication. In such cases, additional analysis and statistical correction are performed, and a comprehensive judgment is made. Generally, if there are mismatches at three or more loci, the result is judged to be a denial.

Q4. How is the probability of paternity (%) calculated?

A. The probability of paternity is derived by multiplying together the Paternity Index (PI) calculated for each STR locus to obtain the Combined Paternity Index (CPI), and then converting that into a probability using Bayes' theorem. This calculation references a database of allele frequencies (rates of occurrence in the general population) for each locus, and computes the likelihood ratio between "the tested man is the father" and "an unrelated man is the father."

Q5. How many STR loci are used in DNA paternity testing?

A. Generally, 13 or more STR loci are used. This conforms to the CODIS standard established by the U.S. FBI. Some testing laboratories analyze 20 or more loci, and the more loci examined, the higher the accuracy and reliability of the test. We test a sufficient number of loci to deliver highly accurate results.

Q6. Where should I go if I have questions about my result report?

A. If you have any questions about the contents of your result report, please feel free to contact our customer support team. Our specialized staff will explain things carefully. Checking your questions alongside the probability of paternity listed in the "Analysis of Test Results" section on page 3 of the report will help you understand it more smoothly.

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