What Is Germline Genetic Testing and How Does It Work?

Germline genetic testing reads the DNA you were born with, the version of the genetic instructions you inherited from your parents and that nearly every cell in your body shares. That makes it unlike most medical tests, because its answer is about your whole family as much as about you. It usually needs only a tube of blood or some saliva, and the result itself doesn’t change over your lifetime, though how a result is understood can.

Inherited DNA Versus Changes in a Tumor

A germline variant is a DNA change present in the egg or sperm that formed you.¹ As the embryo grows, that variant is copied into every cell in the body, including your own egg or sperm cells.² That’s how it can be passed on to your children.³

Somatic changes, also called acquired changes, are different. They happen to genes in individual cells during your life, and they are the most common cause of cancer.⁴ They are not in every cell and are not passed from parent to child.⁵ An inherited change is in all your cells, while an acquired change shows up only in the group of cells that grew from the one that changed.⁶

This is why someone with cancer may get two different genetic tests. Tumor testing looks at the cancer’s acquired changes to guide treatment. Germline testing uses blood or saliva to check for an inherited cause. A finding in a tumor test can prompt a germline test to see whether it’s inherited, and only the germline result carries meaning for relatives. A variant found only in the tumor says nothing about what you were born with, and a negative tumor test doesn’t rule out an inherited variant, because the two tests look at different DNA. Most cancers are not inherited, and an inherited variant raises risk without guaranteeing disease.

How the Test Is Done

The lab extracts the DNA and reads it with a method that reads many genes at once.⁷ Many tests are panels that read only the genes linked to a specific condition, such as hereditary cancer or heart rhythm disorders.⁸ Broader tests read all the protein-coding genes (the exome) or nearly the whole genome.⁹

Sequencing reads your DNA letter by letter, and the lab compares each stretch with a standard reference sequence to find places where yours differs. Most differences are harmless, since everyone’s DNA differs from the reference in many places. The lab’s real work is deciding which few matter. Labs typically report only variants relevant to the reason for testing, so a long list of harmless differences doesn’t reach you.

A panel is focused and easier to interpret when you have a known concern. An exome or genome is used when the cause isn’t clear, and broader tests turn up more uncertain and unexpected findings. Among children with suspected genetic disorders in hospital settings, usual testing found a diagnosis in about 43%, exome sequencing in about 58%, and whole-genome sequencing in about 64%.¹⁰

For you, the process usually starts with a doctor or genetic counselor who orders the test and discusses it first. Results often take a few weeks, and you typically go over the report with the person who ordered it. Ask which genes were tested so you know what a negative result covers.

That matters because tests have gaps. They mainly cover the coding parts of genes and their edges.¹¹ Missing or duplicated chunks of a gene can be harder to spot and may need a second confirming test.¹² A change present in only some cells is hard to detect when it is in fewer than about 10% to 20% of them, and a blood test can miss a variant that exists only in the affected tissue.¹³ A negative result means no variant was found by that test, not that none exists.

What Your Results Can Say

Labs sort each variant into one of five categories:

To decide, labs weigh evidence such as how common the variant is in healthy people, computer predictions, lab studies, and whether it tracks with disease in families.¹⁹ Hereditary cancer testing commonly turns up uncertain results.²⁰ An uncertain variant is often reclassified as benign once many healthy people are found to carry it.²¹ In one study, 91% of reclassified variants were downgraded to benign and 9% upgraded to pathogenic.²² But reclassification can take months to decades, and some variants are never resolved.²³ It’s also often unclear whether the lab, your doctor, or you is responsible for passing along an update.²⁴ So ask how you’ll learn about a reclassification, and keep your contact details current with the lab.

In the meantime, doctors generally don’t base medical decisions on an uncertain variant alone. Care is guided by your personal and family history instead, and relatives usually aren’t tested for an uncertain variant.

A negative result is most reassuring when it tested for a relative’s known variant. Without a known family variant, risk from family history may remain, so ask whether your negative result changes your screening plans.

Two labs can also reach different categories for the same variant if they’ve seen different data. If a variant’s category matters for a big decision, such as surgery, your care team may check how it’s classified in shared public databases.

Who Gets Tested and What It Can Change

Hereditary breast and ovarian cancer and Lynch syndrome are the most common inherited cancer syndromes found by testing.²⁵ Heart conditions are another major reason. In people tested for inherited heart rhythm and heart muscle disorders, about 24% had a disease-causing variant, and about 8% got a refined diagnosis that could change their care.²⁶

Your family history can point toward testing. Signs include several relatives with the same cancer, especially on one side, cancer diagnosed at an unusually young age, unexplained sudden death or heart disease in a young relative, or a relative already found to carry a variant.

A result can change care in concrete ways. A pathogenic variant in a cancer gene can lead to earlier or more frequent screening, such as breast MRI or colonoscopy, starting at ages different from the general population. Some results open up preventive options a person might otherwise not be offered, and the choice among them is personal and made with a specialist. In people who already have cancer, an inherited variant can influence treatment, including eligibility for certain targeted drugs.

In inherited heart conditions, a variant can confirm which specific disorder a person has, which matters because different disorders call for different management. It can also clarify which relatives need heart checks, since relatives without it may not need ongoing monitoring. Even so, a heart-related variant doesn’t always predict how severe or how early symptoms will be, so results are read alongside heart tests and history.

Testing can also guide medicines. Pharmacogenomic testing reads inherited variants that affect how your body handles certain drugs.²⁷ It can help predict how you’ll process a medicine and guide the dose or the choice of drug.²⁸ In cancer care, it can flag people at higher risk of severe side effects from some chemotherapy drugs.²⁹

Risk scores are a different tool. Some tests add up many common small-effect variants into a polygenic risk score.³⁰ When added to other risk models for breast cancer, such scores moved many women above or below the risk cutoff for extra MRI screening.³¹ These scores give an estimated risk, not a diagnosis, and their routine use is still debated.

What It Means for Your Relatives

Because inherited variants are shared within a family, a positive result lets parents, siblings, and children get tested for that same variant. This is called cascade testing: offering counseling and testing to at-risk relatives of someone found to have a genetic condition.³² It finds more people at risk and helps them get preventive care.³³ Yet in inherited heart disease, only about 40% of at-risk relatives were tested a year after a family variant was found, rising to 60% after 15 years.³⁴

When you tell relatives, share a copy of your report so their tester knows the exact variant. Testing for one known variant is simpler than a full panel. Each first-degree relative has about a 50% chance of sharing a dominant variant, and some clinics provide a family letter to help explain it.

A relative tested for the family variant gets a fairly clear answer: they either carry it or they don’t. One who doesn’t carry it generally doesn’t carry the extra risk it causes, which can spare them intensive screening. One who does can begin screening or prevention before any disease appears, which is the main point of testing relatives.

Not everyone will want to know. Testing is a personal choice for each relative, and some prefer to wait, for example until starting a family. Offer the information without pressuring.

Carrier Screening Before or During Pregnancy

Expanded carrier screening uses a blood sample to check whether you carry variants for conditions such as cystic fibrosis.³⁵ Everyone carries some variants that could cause recessive disease in a child if a partner or donor carries one in the same gene.³⁶ Screening identifies couples at risk of passing on recessive and X-linked disorders.³⁷

A carrier is someone with a variant for a condition who is usually healthy themselves. For a recessive condition, a carrier has one changed copy of a gene and one working copy, and the working copy usually keeps them well. Risk to a child arises mainly when both parents carry the same condition, and for two carriers, each pregnancy has a 1 in 4 chance of an affected child. Screening the partner is the usual next step.

X-linked conditions look different. A mother who carries a variant can pass it to sons, who may then be affected. That’s why a carrier result is described by the condition’s pattern of inheritance, not just by the word.

If only one partner is a carrier of a recessive condition, the child usually isn’t affected, though the child could be a carrier too. The other partner is then tested for the same gene. If that result is negative, the chance of an affected child drops a lot, though a screening test can’t rule out every variant, so a small risk may remain.

Specialists agree that couples who both carry a variant for the same condition, or a woman carrying an X-linked variant, should have face-to-face counseling.³⁸ That counseling explains the condition and the reproductive options.³⁹ One option is IVF with embryo testing, where only embryos without the family variant are transferred.⁴⁰ Testing in pregnancy is optional, and choices should fit your own values.⁴¹

Privacy and Insurance Protections

In the US, the Genetic Information Nondiscrimination Act (GINA) bars health insurers and employers from using genetic information to raise premiums, deny coverage, or make job decisions.⁴² Health insurers may still request limited genetic information to decide whether to pay for a procedure.⁴³

Many people misunderstand the limits. Even among people who felt well-informed, over half wrongly thought or weren’t sure whether GINA covers life, long-term care, and disability insurance.⁴⁴ It covers none of these.⁴⁵ Life insurers can ask about your health, including genetic results you already have, when you apply. Disability and long-term care insurers may take genetic information into account. For employers, the law covers most but not every size or kind, so people in some settings may have fewer protections. If you’re considering life or disability insurance, think about timing before testing.

Practical questions are worth asking: who will see your results, and do they go in your medical record? Some US states add protections beyond the federal law, and outside the US, protections vary by country.

Consumer DNA Kits Versus Clinical Tests

Consumer kits read some of the same DNA but aren’t designed to diagnose. In one analysis, 40% of variants reported in consumer kits’ raw data were false positives on clinical testing.⁴⁶ Some variants labeled “increased risk” by kits or third-party apps were common, harmless variants.⁴⁷ Confirm any worrying kit result with a clinical test before acting on it.

Kits check selected spots rather than reading whole genes, so a clean result can miss variants a clinical panel would find. Clinical tests also come with interpretation and counseling. A kit that reports raw data leaves interpretation to you or to a third-party app, and these apps don’t always follow the evidence standards clinical labs use.

If a kit flags something, bring the report to your doctor or a genetic counselor. Expect a fresh sample to be tested in a clinical lab, and don’t change medicines or screening based on a kit alone.