Germline vs. Tumor Genomic Testing Explained Clearly

Germline vs. Tumor Genomic Testing: What Cancer Patients Need to Know

a couple of weeks ago
/ cancer
/ By David Emerson

Germline vs. Tumor Genomic Testing: What Cancer Patients Need to Know. Germline vs. tumor genomic testing explained. Learn what each cancer genetic test looks for, why both may be important, and what results mean for treatment and family risk.

When you are diagnosed with cancer, you may hear terms such as genetic testing, germline testing, tumor genomic testing, somatic testing, molecular profiling, biomarker testing, and next-generation sequencing.

These terms can sound interchangeable. They are not.

One of the most important distinctions for a newly diagnosed cancer patient is the difference between germline genetic testing and tumor genomic testing.

The two tests examine DNA for different reasons. One primarily asks, “Did I inherit a genetic change that increases my cancer risk?” The other asks, “What genetic changes are present in my cancer that might help guide treatment?”

Sometimes the results overlap. But tumor testing cannot simply be substituted for germline testing when the goal is to determine whether a cancer-related genetic variant was inherited.

The distinction has become increasingly important as precision oncology expands.

If you undergo genetic testing, you may wonder What does a Positive Cancer Genetic Test Mean?

I am a long-term survivor of an incurable blood cancer called multiple myeloma. I have been amazed by the continual advancements in diagnostic testing that I have witnessed since my diagnosis in 1994. I consider cancer genetic testing to be yet another advancement in diagnostic testing.

I have to admit, however, that genetic testing is a complicated area 0f cancer. I wrote the post Cancer Genetic Testing: What Newly Diagnosed Cancer Patients Need to Know , and I was still somewhat confused by the two different types of genetic testing. So I wrote this post to clarify Germline vs. Tumor Genomic Testing.

Scroll down the page and post a question or a comment if there’s anything you’d like to know about cancer genetic testing.

Good luck,

David Emerson

Concepts Central to Managing Your Cancer-

Germline vs. Tumor Testing: The Short Answer

	Germline genetic testing	Tumor genomic testing
Primary question	Did I inherit a cancer-associated genetic variant?	What genetic changes are present in my tumor?
DNA source	Usually blood or saliva	Tumor tissue or sometimes blood-based circulating tumor DNA
Looks for	Inherited variants	Acquired/somatic variants and sometimes inherited variants
Implications	Patient and potentially blood relatives	Primarily the patient’s cancer treatment
Can identify hereditary cancer risk?	Yes	Sometimes suggests it, but does not replace germline testing
Can help select treatment?	Sometimes	Often
Should one replace the other?	No	No

The National Cancer Institute makes this distinction clearly: genetic testing for inherited cancer risk is different from tumor genetic testing, which is often used to help guide treatment. Tumor testing can sometimes identify an inherited variant, but it does not replace testing for inherited cancer risk.

What Is Germline Genetic Testing?

Germline testing looks for genetic changes that you inherited from a biological parent.

Because these variants are present in the germ cells from which your body developed, they are generally present throughout your body.

A germline test may use:

Blood
Saliva
Cheek cells
Occasionally another normal tissue sample

The test may examine one gene or, increasingly, multiple genes simultaneously through a multigene panel.

What can germline testing tell you?

A pathogenic or likely pathogenic germline variant may indicate an increased inherited susceptibility to certain cancers.

Examples include variants involving:

BRCA1
BRCA2
PALB2
TP53
ATM
CHEK2
MLH1
MSH2
MSH6
PMS2
EPCAM

The significance depends on the particular gene, variant, and the individual’s personal and family history.

Importantly, a germline result can have implications beyond the person who has cancer.

If an inherited pathogenic variant is identified, biological relatives may also have an increased likelihood of carrying the same variant.

That can affect decisions about cancer screening, prevention and—in some circumstances—treatment.

What Is Tumor Genomic Testing?

Tumor genomic testing examines the DNA of the cancer itself.

Cancer cells accumulate genetic alterations as they develop and evolve. Many of these changes are somatic, meaning they developed during the person’s lifetime and were not inherited from a parent.

Tumor genomic testing attempts to identify these alterations.

The goal is often to answer a very different question:

Does this cancer contain a molecular feature that could help guide treatment?

Depending on the cancer, tumor profiling may identify alterations involving genes or pathways that could potentially be targeted with:

Targeted therapies
Immunotherapies
Hormonal therapies
Other biomarker-directed treatments
Clinical trials

Tumor genomic testing may be performed on a tissue biopsy or, in some circumstances, through a blood test that analyzes circulating tumor DNA (ctDNA).

Somatic Does Not Mean Hereditary

This is one of the most important concepts for patients to understand.

Suppose tumor testing identifies a mutation in BRCA1.

That does not automatically mean the patient inherited a BRCA1 mutation.

The alteration could have developed only in the cancer.

In genetic terminology:

Somatic mutation → acquired by the tumor

Germline mutation → inherited and present throughout the body

However, some variants discovered through tumor testing can be clues that a hereditary mutation may be present.

That is why an abnormal tumor test can sometimes lead to follow-up germline testing.

ASCO’s current guideline specifically recommends germline testing for certain patients when pathogenic variants are identified through tumor testing.

Why Tumor Testing Cannot Replace Germline Testing

This distinction deserves special emphasis.

Imagine that a patient has a hereditary cancer syndrome caused by a germline pathogenic variant.

If the tumor test does not identify that variant, it does not necessarily mean the patient does not have the inherited variant.

According to the 2024 ASCO guideline, approximately 8%–10% of germline pathogenic variants may be missed by tumor testing.

ASCO therefore recommends that patients who meet criteria for germline testing should receive germline testing regardless of the results of tumor genomic testing.

In other words:

A normal tumor genomic test does not necessarily rule out an inherited cancer predisposition.

Why Would a Cancer Patient Want Germline Testing?

There are several potentially important reasons.

1. It may identify inherited cancer risk

A pathogenic germline variant can explain why a person developed cancer at a relatively young age or why certain cancers occur repeatedly within a family.

But family history is not always obvious.

A person can carry an inherited pathogenic variant even without a dramatic family history.

ASCO notes that germline testing is recommended regardless of family history for some patients with particular cancers or clinical characteristics.

2. It may influence cancer treatment

Inherited mutations can sometimes affect treatment decisions.

For example, pathogenic variants involving DNA-repair genes can have therapeutic implications in selected cancers.

ASCO specifically notes the treatment relevance of germline findings such as BRCA1/2 variants, including their relationship to PARP inhibitor therapy in certain cancers.

Therefore, germline testing is not simply about what happened to your parents or what might happen to your children.

It can sometimes matter to your current cancer treatment.

3. It may affect cancer screening after treatment

A cancer survivor with an inherited pathogenic variant may have an increased risk of developing another primary cancer.

That can change recommendations for surveillance.

The appropriate screening depends on the specific gene and the person’s personal and family history.

4. It may provide information for relatives

This is another major difference between the two tests.

A somatic mutation found only in a tumor generally does not mean that a patient’s children or siblings inherited it.

An inherited germline pathogenic variant potentially can be passed from one generation to another.

This means a germline result can become a family health issue, not merely an individual cancer issue.

Why Is Tumor Genomic Testing Important?

Germline testing and tumor testing should not be viewed as competing tests.

They answer different questions.

Tumor testing can sometimes identify a molecular alteration that gives an oncologist another treatment option.

For example, tumor profiling might identify a biomarker associated with a targeted treatment or clinical trial.

This is one reason precision oncology has become increasingly important.

Instead of asking only:

“Where did the cancer start?”

oncologists can increasingly ask:

“What is driving this particular cancer at the molecular level?”

That information can sometimes help personalize treatment.

A Tumor Mutation Can Lead to Germline Testing

The relationship between the two tests becomes particularly interesting when tumor testing identifies a potentially hereditary variant.

For example:

Tumor genomic testing

↓

Pathogenic variant identified

↓

Variant could potentially be inherited

↓

Germline testing

↓

Determines whether the variant is present in the patient’s normal cells

↓

Potential implications for:

Treatment
Cancer surveillance
Future cancer risk
Biological relatives

ASCO’s 2024 guideline provides specific recommendations for which tumor findings should prompt consideration of germline testing.

Germline Testing Should Not Depend Only on Tumor Testing

This may be the single most important practical point in this article.

If your cancer diagnosis or personal/family history means that you meet criteria for germline testing, you should not assume that tumor genomic testing has already answered the question.

ASCO recommends offering germline testing to patients who meet germline-testing criteria regardless of their tumor-testing results.

This matters because tumor testing and germline testing have different purposes and different limitations.

What About a Liquid Biopsy?

Patients may also encounter the term liquid biopsy.

A liquid biopsy can analyze fragments of tumor DNA circulating in the bloodstream.

This can be useful in certain clinical situations, but there is an important limitation:

Finding a genetic variant in circulating tumor DNA does not establish that the variant is inherited.

ASCO specifically cautions that variant allele frequency from circulating tumor DNA is not informative for determining whether a variant is germline.

If determining inherited risk is clinically important, appropriate germline testing is required.

What Does a Positive Germline Test Mean?

A positive germline test does not mean that a person is guaranteed to develop another cancer.

It means that a pathogenic genetic variant associated with increased cancer susceptibility has been identified.

The actual cancer risk varies substantially according to:

The gene involved
The specific variant
The patient’s sex
Age
Personal cancer history
Family history
Other genetic and environmental factors

This is why genetic counseling and careful interpretation are important.

What Does a Negative Germline Test Mean?

A negative test can be reassuring, but it does not necessarily mean:

“I have no inherited cancer risk.”

There are several reasons.

A patient’s family history may still indicate increased risk.
Testing technology has limitations.
Not every cancer-predisposition gene is known.
And not every genetic variant that affects cancer risk can necessarily be identified or interpreted with current testing.

ASCO emphasizes that family history remains important even after a negative or uninformative germline test.

What Is a Variant of Uncertain Significance?

Modern multigene panels can identify genetic variants whose significance is not yet clear.

These are called variants of uncertain significance (VUS).

A VUS is not the same thing as a pathogenic mutation.

Patients should be cautious about making major medical decisions based solely on a VUS.

As more scientific information becomes available, some uncertain variants may eventually be reclassified.

This is one reason ASCO cautions that the potential benefits of broader genetic panels need to be balanced against the possibility of uncertain findings.

Should Every Cancer Patient Have Both Tests?

There is no universal answer.

The appropriate testing strategy depends on the:

Type of cancer
Age at diagnosis
Stage and disease characteristics
Personal cancer history
Family history
Previous genetic testing
Treatment options
Results of previous tumor testing

Importantly, testing recommendations have expanded considerably.

ASCO’s 2024 germline-testing guideline recommends that when multiple genes are relevant, multigene panel testing should be offered rather than automatically testing one gene at a time.

The decision should still be individualized.

Questions to Ask Your Oncologist

If you have recently been diagnosed with cancer, consider asking:

About germline testing

Do I meet current criteria for germline genetic testing?
Should I have a multigene panel?
What inherited cancer syndromes are relevant to my cancer?
Should I meet with a genetic counselor?
Could the results affect my treatment?
Could the results affect cancer screening for me or my relatives?

About tumor testing

Has my tumor been tested for actionable genomic alterations?
What genes and biomarkers were included in the test?
Was the test performed on tumor tissue or circulating tumor DNA?
Did the tumor test identify anything that might require follow-up germline testing?
Are there targeted therapies or clinical trials associated with my tumor’s molecular findings?

About the relationship between the two

Does my tumor testing rule out the need for germline testing?
If a mutation was found in my tumor, do I need confirmatory germline testing?
If my germline test is negative, does my family history still indicate increased cancer risk?

The Bottom Line

Germline testing and tumor genomic testing are not the same thing.

Germline testing asks:

“Was I born with a genetic change that may increase my cancer risk?”

Tumor genomic testing asks:

“What genetic changes are present in my cancer that may help guide treatment?”

Sometimes the same gene appears on both tests.

But the tests have different purposes.

For cancer patients, understanding this distinction can prevent a potentially important misunderstanding: a tumor genomic test should not automatically be considered a substitute for hereditary cancer testing.

The 2024 ASCO guideline specifically recommends germline testing for patients who meet germline-testing criteria regardless of tumor-test results, while also recommending germline follow-up in selected patients whose tumor testing reveals potentially hereditary pathogenic variants.

For a newly diagnosed cancer patient, the practical question is therefore not necessarily:

“Should I have germline testing OR tumor testing?”

It may be:

“Do I need germline testing, tumor genomic testing, or both—and how could the results change my treatment, surveillance, and my family’s cancer risk?”

That is a conversation worth having with your oncologist and, when appropriate, a genetic counselor.

PeopleBeatingCancer Perspective

Genomic medicine is becoming an increasingly important part of modern oncology. But genetic testing is not a single test with a single purpose.

Patients should understand what is being tested, why it is being tested, what the result can actually tell them, and what it cannot tell them.

I would encourage cancer patients to obtain a copy of their genomic and genetic testing reports and ask their oncology team to explain:

What was tested? What was found? What does it mean for treatment? What does it mean for future cancer risk? And does anyone else in my family need to know?

Those questions can turn a complicated laboratory report into information that may actually help guide cancer care.

Research and Evidence

National Cancer Institute — Genetic Testing Fact Sheet
The NCI explains the difference between inherited genetic testing and tumor genetic testing, including the types of samples used and the implications of testing.

NCI: Genetic Testing Fact Sheet

American Society of Clinical Oncology — Selection of Germline Genetic Testing Panels in Patients With Cancer: ASCO Guideline
A major 2024 guideline addressing germline multigene testing, family history, tumor-testing findings and when tumor findings should prompt germline testing.

ASCO Guideline: Germline Genetic Testing Panels in Patients With Cancer

ASCO — Germline and Somatic Tumor Testing in Epithelial Ovarian Cancer
An example of how germline and somatic testing can complement one another in a specific cancer.

ASCO Ovarian Cancer Genetic Testing Guideline

Germline vs. Tumor Genomic Testing Germline vs. Tumor Genomic Testing Germline vs. Tumor Genomic Testing

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