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DNA sequencing reads the order of A, T, C, and G bases in the DNA that a test examines. That can reveal genetic differences and, when evidence supports it, help inform conclusions about health, biological relationships, or ancestry. But reading a sequence is not the same as interpreting it: a result depends on what the test covered, what it can detect, and how confidently a finding can be connected to an outcome.
What DNA sequencing actually measures
DNA sequencing determines the order of the four chemical bases—A, T, C, and G—in a DNA molecule or selected region. Researchers and clinicians can use that sequence to identify genes, regulatory instructions, and differences that may be associated with a trait or disease. The National Human Genome Research Institute (NHGRI) explains DNA sequencing as a way to read this biological order, not as an automatic diagnosis.
A sequencer produces data; interpretation asks what a particular difference means for a person. As NHGRI puts it, “Our ability to sequence DNA has far outpaced our ability to decipher the information it contains,” in its Genomic Data Science Fact Sheet. A sequence can be measured accurately while the health significance of a finding remains uncertain.
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What a result may tell you
Whether a particular genetic difference was detected
A test can report differences between the DNA it examined and a reference sequence, or check for specific variants chosen in advance. Whether a reported variant is truly present is a separate question from what it means. NHGRI’s Guide to Interpreting Genomic Reports Toolkit notes that confirmation with another method, such as Sanger sequencing, can increase confidence that a reported variant is real.
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Possible health relevance
If a variant is well studied and associated with a condition, it may help explain a diagnosis, indicate elevated risk, or inform a clinician’s next steps. The strength of that conclusion varies: some findings have strong evidence, while others are uncertain or depend on a person’s medical and family history. A variant should not be called disease-causing just because it differs from a reference sequence.
Clues about ancestry or biological relatives
Consumer ancestry reports compare DNA markers with reference data to estimate how a person’s DNA relates to populations represented in the provider’s database. These are estimates, not a complete account of identity or family history. Their accuracy depends in part on the breadth and quality of comparison data; NHGRI cautions that people from underrepresented populations may be misidentified when adequate reference sequences are unavailable. DNA testing can also reveal unexpected biological relatives or ancestry information. See NHGRI’s explanations of direct-to-consumer genetic testing and direct-to-consumer genomic testing.
Why the kind of test matters
“DNA test” can refer to tests with very different coverage. Some examine a selected gene or set of variants; some sequence larger regions or a genome; many consumer health tests use SNP arrays, which check selected markers rather than reading every DNA base. Some specialized consumer offerings use next-generation sequencing, yet may still report only a predetermined set of variants. NHGRI’s DNA sequencing glossary notes that sequencing methods have different characteristics, and its genomic-report toolkit warns that detection capability and sensitivity vary by disease and variant type.
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| Test approach | What it examines or reports | What to keep in mind |
|---|---|---|
| Targeted clinical test | A specified gene, region, or set of variants selected for a clinical question. | A result applies to the scope and detection limits of that particular assay; it is not a screen for every genetic cause. |
| Broader sequencing | A larger DNA region or, depending on the test, much or all of the genome. | More sequence does not guarantee that every relevant variant type is detected or that every finding can be interpreted. |
| Consumer SNP-array test | Selected DNA markers; health reports may focus on a preselected set of variants. | A negative report cannot rule out all disease-associated variants. Some consumer tests do not assess every variant known to cause a condition. |
| Consumer sequencing-based test | Some products use sequencing, but the report may still be limited to selected variants or traits. | The method’s name alone does not establish the report’s coverage, clinical validity, or usefulness for medical decisions. |
These descriptions are general categories, not guarantees about every product. NHGRI’s direct-to-consumer FAQ includes material tied to 2021, so it supports these qualitative distinctions but should not be read as a current survey of the consumer-testing market. Ask the provider for the specific genes or regions, variant types, and reporting rules for the test you are considering.
What a negative result can—and cannot—rule out
A negative result means the test did not report a relevant finding within the regions and variant types it assessed, at its level of sensitivity. It does not necessarily mean that an inherited condition is absent. A variant may lie outside the test’s scope, be a type the method detects poorly, or remain unknown to science. NHGRI’s genomic-report toolkit specifically cautions that a negative genomic report does not exclude an underlying genetic disease and that sensitivity varies by disease.
For that reason, a negative result should be interpreted alongside the reason for testing, symptoms, and family history. If a clinician suspects a condition despite a negative result, ask whether a different test or method would address the remaining question.
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What a positive result can—and cannot—predict
A positive result does not always mean a person will develop the condition associated with a finding. Even when risk is increased, the test may not predict whether the condition will occur, when it might appear, or how severe it would be. NHGRI’s Genetic Testing FAQ notes both that a positive result does not always lead to disease and that predicting severity can be difficult.
Risk is not destiny. A genetic result is one part of a health picture that may also depend on other genes, environment, and personal circumstances. The meaning of a particular finding should be discussed in the context of the specific condition and the evidence behind that result—not generalized from the word “positive.”
Why variant interpretations can change
Labs classify variants using the evidence available at the time. That evidence can be incomplete, and classifications can be wrong or change as more information accumulates. A result might be reported as uncertain when there is not enough evidence to say whether it affects health. The toolkit on genomic reports distinguishes detecting a variant from establishing its significance: confirming that a sequence difference is present does not prove that it causes disease.
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If a report calls a finding uncertain, do not treat it as a diagnosis or make medical decisions from the label alone. A genetics professional or clinician can explain what is known, whether the finding fits the person’s history, and whether follow-up is appropriate.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to evaluate a DNA test before relying on it
There is no universally best test; the right choice depends on the question. Before testing—or before treating a report as actionable—check:
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- Purpose: Is the test intended for ancestry, a specific clinical question, carrier status, or general health information?
- Coverage: Which genes or regions and which types of variants does it assess? Does it report only a selected list?
- Detection limits: What relevant changes can the method miss, and how sensitive is it for the condition in question?
- Evidence: How strong is the evidence linking each reported variant to the trait or disease?
- Clinical support: Is counseling available, and can a potentially actionable consumer finding be confirmed through clinical testing?
- Data practices: Who can access the results and sample, whether data may be shared or used for research, and how deletion works.
- Ancestry reference data: For ancestry estimates, which populations are represented and how are estimates updated?
Consumer results can lack the counseling and risk assessment that accompany clinical care, and raw data or third-party interpretation can produce false positives. NHGRI advises against independently changing medication based on direct-to-consumer pharmacogenomic results. If a consumer report suggests a high-risk, actionable, or carrier finding, discuss it with a healthcare professional before acting; clinical confirmation may be appropriate.
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Family implications, privacy, and legal limits
Biological relatives share DNA, so a finding relevant to one person may have implications for family members. A genetic counselor or other genetics professional can help interpret a result, consider family history, and discuss whether to share information or pursue confirmatory testing. NHGRI recommends professional guidance for people seeking help with health-related consumer results.
Genetic data are sensitive, and privacy practices vary among services. Before submitting a sample or uploading raw data elsewhere, review who may access the information, whether it can be used for research or commercial purposes, whether third parties receive it, what happens to the biological sample, and how to request deletion. NHGRI outlines these issues in Privacy in Genomics; the policies of individual companies can change and are not established by that general guidance.
For readers in the United States, NHGRI describes the Genetic Information Nondiscrimination Act (GINA) as providing protections related to health insurance and employment, but not life, disability, or long-term-care insurance. These protections are not comprehensive, and legal coverage depends on the circumstances. The genomic-report toolkit and NHGRI’s genetic testing FAQ summarize these limits.
Quick Recap
How to respond to a result
- Read the scope first. Find out what the test examined and which variant types it could detect before drawing a conclusion from “positive” or “negative.”
- Separate the finding from its interpretation. Check whether the report says a variant is confirmed, what classification it assigns, and how strong the supporting evidence is.
- Bring medical concerns to a clinician. Discuss symptoms, family history, and the report together; do not start, stop, or change medication solely on a consumer result.
- Confirm actionable consumer findings. Ask a clinician or genetics professional whether clinical testing is warranted before making healthcare decisions.
- Consider relatives and data choices. Get help deciding how to communicate a potentially family-relevant finding and review privacy settings before sharing or uploading DNA data.
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