[Expert Explains] How Often Do DNA Testing Errors Actually Occur?

Rewritten on: March 1, 2026

DNA paternity testing is extremely accurate, but rare biological factors such as chimerism and chromosomal abnormalities can occasionally lead to a misinterpreted result. An expert explains in detail the measures used to prevent misjudgment, along with cases reported overseas.

DNA paternity testing is known as a highly accurate examination. However, because the results can affect the rest of a person's life, many people feel anxious or wonder, "Is it really correct? Could it be wrong?" In fact, DNA test results serve as extremely important evidence in legal proceedings and in confirming family relationships, so their accuracy and reliability are always subject to close scrutiny. This article introduces extremely rare cases in which a parent-child relationship truly exists, yet the result may be interpreted as "no biological relationship," and explains in detail the recommended approach to handling such exceptional cases.

DNA paternity testing is "almost certain," but not 100%

DNA paternity testing statistically evaluates the number of matches and mismatches at specific genetic loci (STR regions) to calculate the probability of paternity. STR stands for "Short Tandem Repeat," referring to regions on the DNA where a specific base sequence repeats. Because the number of repeats varies from person to person, examining whether the inheritance pattern matches between parent and child allows a highly accurate determination of the biological relationship [ref:1]. This method is standardized under internationally trusted accreditation systems, and today's DNA paternity testing offers extremely high accuracy and reliability [ref:1]. Generally, if the probability of paternity is 99.99% or higher, the relationship is determined to exist; if mismatches are confirmed at multiple loci, the relationship is determined not to exist.

On the other hand, extremely rare cases have been reported in which a parent-child relationship truly exists, yet exceptional biological factors (described below) cause the result to be interpreted as "no biological relationship." These cases are not due to misconduct at the testing laboratory or flaws in the system, but rather stem from the fact that DNA paternity testing is a probability-based examination [ref:2]. In other words, it is important to understand that DNA paternity testing does not provide a "100% definitive diagnosis," but rather a mechanism for evaluating the presence or absence of a biological relationship based on statistical probability.

For this reason, even for tests conducted under international accreditation systems, careful interpretation of results is required, and retesting—including recollection of samples—or additional review may be conducted as needed. In particular, when a mismatch is observed in the results, international guidelines recommend a comprehensive judgment that considers the possibility of mutation and other biological factors, rather than simply concluding "no parent-child relationship" [ref:6]. Accordingly, in the field of DNA paternity testing, professional caution continues to be raised regarding the way such exceptional conditions can affect the interpretation of results.

Factors that can lead to a misinterpreted result

In DNA paternity testing, there are mainly two types of factors that can cause a result to be reported as "no biological relationship" even though a parent-child relationship truly exists. The first is an operational factor—so-called human error. The second is a biological factor—the extremely rare cases addressed in this article. Even when lumped together as "misjudgment," these factors differ in kind, and each requires its own appropriate preventive measures. Below, we organize each of these factors.

A. Operational factors (human error)

• Errors from the complexity of collection, transport, and processing: This includes sample mix-ups, contamination (contamination by another sample), and mislabeling. International accrediting bodies (e.g., AABB, the American Association of Blood Banks) maintain a framework that ensures quality through procedures, records, and proficiency testing, so the occurrence rate of such errors is kept extremely low at accredited testing laboratories [ref:5].
• Misunderstanding from oversimplifying a probabilistic result: DNA paternity test results are calculated based on probability, but simplistically judging this probabilistic result as "either 0 or 100" can lead to misinterpretation. It is important to accurately understand what the figures mean and to receive a careful explanation from an expert.

B. Biological factors (extremely rare cases in which a result can be misinterpreted even when the testing process itself is correct)

Biological factors arise when the test itself is performed correctly, but the subject's own DNA has an unusual characteristic. Specifically, the following three are representative examples.

• Mutation at a specific genetic locus (STR region): During inheritance from parent to child, the number of repeats in an STR region can change. If this happens simultaneously at multiple loci, the result may appear at first glance as though there is no parent-child relationship. The mutation rate of STR regions varies by locus, but is generally estimated at around 0.1–0.4% per locus [ref:2].
• Chimerism: A condition in which multiple sets of genetic information coexist within the body, so that the DNA profile detected can differ depending on the site of collection. This includes tetragametic chimerism (fusion-type chimerism) as well as chimerism resulting from bone marrow transplantation.
• Rare chromosomal abnormalities: In exceptional cases where the standard inheritance model does not hold, such as uniparental disomy (UPD), care is needed in interpreting the results of standard STR analysis.

In the next section, we explain factor B (biological factors) in detail, based on cases actually reported overseas.

Cases reported overseas

Although the frequency is extremely low, papers and reports from international professional organizations overseas have documented cases in which, due to biological factors, a true parent and child could be interpreted as "unrelated." Below are representative examples of such cases. Knowing about these cases provides important foundational knowledge for responding calmly and appropriately in the unlikely event of such a result.

Case 1: A case involving surrogacy

In a case report related to surrogacy (where a woman other than the intended parent carries and delivers the pregnancy), tetragametic chimerism was reported as the cause of an unexpected genetic mismatch in a DNA paternity test [ref:3]. Tetragametic chimerism is considered one of the most typical and pronounced forms of chimerism, arising from the fusion of fertilized eggs. Normally, one individual develops from a single fertilized egg, but in extremely rare cases, two different fertilized eggs fuse at an early stage of development, resulting in a single individual carrying two distinct sets of genetic information. In such cases, DNA collected from blood may show a different genetic profile from DNA collected from oral mucosa or hair, which can make interpretation difficult with standard STR analysis and requires careful handling [ref:3].

Case 2: A case involving a chromosomal abnormality

An overseas case report described an instance in which uniparental disomy (UPD), an extremely rare chromosomal abnormality, caused a negative result in DNA paternity testing even though a parent-child relationship truly existed [ref:4]. Uniparental disomy refers to a condition in which a chromosome that should normally be inherited one copy from each parent is instead inherited as two copies from only one parent. For example, a child would normally inherit one homologous chromosome from the father and one from the mother, but in UPD, both copies come from only one parent. In this case, because the other parent's DNA information is not reflected on that chromosome, the standard inheritance model does not hold, and care is needed in interpreting the results of standard STR analysis [ref:4]. Although the incidence of UPD is said to be very low, its existence itself provides important insight for understanding the limitations of DNA paternity testing.

Case 3: A case of natural chimerism

A newsletter from AABB (American Association of Blood Banks) introduced, for professionals, a challenge in paternity determination involving naturally occurring chimerism [ref:5]. Natural chimerism can arise when one of a pair of twins disappears early in development and is absorbed by the other (the so-called "vanishing twin" phenomenon). In this case, although the individual develops as an entirely normal singleton in appearance, some tissues in the body may contain the genetic information of the absorbed twin. Even within the framework of international accreditation systems, it has been shown that such extremely rare biological factors can affect the interpretation of DNA paternity test results [ref:5].

Such cases are shared among experts, and countermeasures are being examined. So what specifically can be done to avoid misjudgment?

Concrete points for avoiding misjudgment in DNA paternity testing

To minimize the risk of misjudgment in DNA paternity testing, there are several important points to keep in mind, from the preparation stage before testing through to how the results are received. Below, we organize the measures that ordinary clients should keep in mind.

1. Choose an internationally accredited testing laboratory: Laboratories that have cleared international quality standards, such as AABB accreditation or ISO 17025 accreditation, keep the occurrence rate of human error extremely low. When choosing a testing laboratory, checking for accreditation is the first step [ref:7].
2. Check the number of genetic loci (STR) tested: In general, the more STR loci examined, the higher the accuracy of the test. Choosing a testing laboratory that uses 16 or more loci as standard provides greater peace of mind. Testing a larger number of loci is recommended to reduce the impact of mutations [ref:2].
3. Request a trio test (three-party test) if possible: A "trio test," conducted with the father, mother, and child together, provides significantly greater accuracy than a duo test conducted with only the father and child. Adding the mother's DNA information makes it much easier to accurately distinguish a mismatch caused by mutation.
4. Consider additional testing if there is a mismatch in the results: If a mismatch is observed at one or two genetic loci, additional STR loci or a retest using a different method is recommended to determine whether it reflects a natural mutation or a true absence of a parent-child relationship [ref:8].
5. Consult an expert about how to interpret the results: Because DNA paternity test results are expressed as probabilities, it is important to accurately understand what the figures mean. If anything is unclear, consult a genetic counselor or DNA analyst and have them explain how to read the results.

What matters most is not to base a final decision solely on the DNA paternity test result, but to understand the scientific basis and limitations behind the result and to seek expert advice as needed. With the right choice of testing laboratory and correct knowledge, the risk of misjudgment can be reduced to nearly zero.

Summary: How to avoid misjudgment

DNA paternity testing is an extremely accurate examination, but in cases involving extremely rare biological exceptions or operational/interpretive factors, additional review with broader testing conditions and careful interpretation of the results may be required. The table below summarizes the main factors and corresponding responses.

As described above, the accuracy of DNA paternity testing is extremely high, and when properly performed at an internationally accredited testing laboratory, the likelihood of an incorrect result is almost nonexistent. However, knowing that extremely rare biological factors exist allows you to respond calmly even in the unlikely event that they arise. If you have any doubts about a result, the best approach is to first consult the testing laboratory and seek additional testing or expert advice.

＼Accurate paternity testing even in the presence of mutation／

Frequently Asked Questions

Q1. Can DNA paternity testing really produce an incorrect result?

A. DNA paternity testing is an extremely accurate examination, and when properly performed at an internationally accredited testing laboratory, the likelihood of an incorrect result is extremely low. However, extremely rare biological factors such as chimerism and uniparental disomy have been reported overseas, in which a parent-child relationship truly exists but the result may be interpreted as "no biological relationship" [ref:3][ref:4]. Such exceptions are not a matter of testing accuracy, but stem from the fact that DNA paternity testing is a probability-based examination.

Q2. What is chimerism, and how does it affect DNA testing?

A. Chimerism refers to a condition in which two or more distinct sets of genetic information exist within a single person's body. A representative example is "tetragametic chimerism," in which two fertilized eggs fuse early in development to form a single individual. With chimerism, the DNA profile detected can differ depending on the collection site (blood, oral mucosa, hair, etc.), which can affect the result of DNA paternity testing [ref:3][ref:5].

Q3. If there is an STR mutation, will the parent-child relationship be denied?

A. An STR mutation is a phenomenon in which the number of repeats changes slightly when DNA is passed from parent to child. If there is a mismatch at only one or two loci, it is likely due to mutation, and the parent-child relationship is not immediately denied. In most cases, increasing the number of STR loci examined, or conducting a trio test with the father, mother, and child, allows accurate distinction between mutation and a true absence of a parent-child relationship [ref:2].

Q4. What is uniparental disomy (UPD)?

A. Uniparental disomy (UPD) is an extremely rare chromosomal abnormality in which a chromosome that should normally be inherited as one copy from the father and one from the mother is instead inherited as two copies from only one parent. As a result, the other parent's DNA information is not reflected on that chromosome, which can produce unexpected results in standard STR analysis [ref:4]. When UPD is suspected, expert review beyond standard paternity testing is conducted.

Q5. How can misjudgment in DNA paternity testing be prevented?

A. To prevent misjudgment, it is most important to choose a reliable testing laboratory that holds international accreditation (such as AABB accreditation or ISO certification). It is also recommended to choose a testing plan that examines a larger number of STR loci, to request a trio test with the father, mother, and child if possible, to consider additional testing if a mismatch is found in the results, and to consult an expert regarding the interpretation of the results [ref:6][ref:7].

Q6. What services does seeDNA offer as measures against misjudgment?

A. In addition to standard parent-child DNA testing, seeDNA Genetic Medical Research Institute offers "Special Parent-Child DNA Testing," which is enhanced to address STR mutations. It holds the international quality standard ISO 9001 and the Privacy Mark for privacy protection, and testing is conducted under a quality management system. If you have any concerns about your results, our specialist staff are available for free consultation, so please use our services with confidence.