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Molecular & Genomic Testing

Germline vs Somatic Testing: — One Is About You. One Is About Your Family.

These two tests are often ordered together but they answer completely different questions. One looks inside your tumour. The other looks at the DNA you were born with — and whether a risk runs in your family.

Medically reviewed by Dr. C. Raghavendra Reddy, Medical Oncologist, MBBS (Gold Medal) · DNB · DM (Medical Oncology, Gold Medal) · Last reviewed September 2026

  • Different samples — Somatic testing uses tumour tissue from your biopsy. Germline testing uses a blood sample or cheek swab.
  • Different questions — Somatic testing asks what is driving this tumour. Germline testing asks whether a risk was inherited from your parents.
  • Family implications — Somatic results guide your treatment. Germline results may matter to your children, siblings, and parents.
  • Both may be needed — The tests answer different questions. One does not replace the other.
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Germline testing looks at the DNA you were born with — variants inherited from your parents and potentially passed to your children. Somatic testing looks at mutations that developed only inside your tumour. Your biopsy tissue is used for somatic testing. Only germline results carry meaning for your family members.

Which test is done on your biopsy sample?

Somatic testing is done on tumour tissue — usually from the biopsy you have already had. The laboratory reads the DNA inside the cancer cells to find the mutations that are driving that specific tumour.

Germline testing is done on a blood sample or cheek swab. It reads the DNA in your normal cells — the DNA you were born with and carry in every cell of your body.

These two samples answer different questions. The tumour sample tells your oncologist what is happening inside the cancer. The blood sample tells whether a risk was inherited and whether family members need to be tested.

How do germline and somatic testing compare?

Germline testingSomatic testing
What it examinesDNA in your normal cells — present since birthDNA in the cancer cells from your tumour
Sample usedBlood test or cheek swabTumour tissue from your biopsy or surgery
What it findsInherited variants that may raise cancer riskAcquired mutations driving this specific tumour
Affects family members?Yes — relatives may carry the same variantNo — somatic mutations are not inherited
Guides treatment?Sometimes, for certain cancer typesYes — determines targeted therapy eligibility
Who orders itYour oncologist or a medical geneticistYour oncologist or pathologist

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What these words mean

Germline
The DNA you were born with, present in every cell of your body. Inherited from your parents and potentially passed to your children.
Somatic mutation
A change in the DNA that developed inside a tumour cell during your lifetime. It is not present in the rest of your body and cannot be passed to family members.
Pathogenic variant
A change in a gene known to increase the risk of disease. When found in germline testing, it confirms a hereditary risk that relatives may share.
Variant of uncertain significance (VUS)
A gene change whose clinical meaning is not yet established — neither confirmed harmful nor confirmed harmless. Its classification may change as more evidence accumulates.
Hereditary cancer syndrome
A condition caused by an inherited gene variant that significantly raises the risk of certain cancers. Identified only through germline testing, not through the tumour biopsy.

Questions families ask about these tests

Do I need both tests, or just one?

You may need one or both, and the answer depends on your cancer type and family history. Somatic testing is nearly always done because it directly guides treatment decisions. Germline testing is added when there is reason to suspect an inherited pattern — a strong family history, a young age at diagnosis, or a cancer type that is commonly hereditary. Your oncologist will tell you which applies to your situation and why.

My somatic test found a mutation in a gene linked to hereditary risk. Does that mean my family is at risk?

Not automatically, and this is where the distinction matters most. A mutation found in your tumour tissue may have developed inside the tumour during your lifetime, not been inherited from your parents. It does not confirm that the same mutation is in your blood cells, which is what would affect your family. Only a germline test on a blood sample can answer that question. Ask your oncologist whether a germline test should follow your somatic result.

Can germline testing find what is driving my cancer and guide treatment?

Not directly. Germline testing finds variants you were born with — these may have raised your risk of developing cancer, but they do not describe what is driving the tumour now. Somatic testing does that. For certain cancer types, however, a germline variant can affect which treatments or surgical options are suitable, so both results together sometimes inform the decisions ahead. Your oncologist will explain what your specific results mean.

Who else in my family should be tested if I carry a germline variant?

That decision is made with a genetic counsellor who will look at your specific variant, your family structure, and which relatives face meaningful risk. First-degree relatives — parents, siblings, and children — are usually considered first. Testing is most useful for relatives who can act on the result through surveillance or preventive steps. Your oncologist or genetics team will explain who should be referred and how to arrange it.

Is germline testing done only once, or does it need to be repeated?

Once. Your germline DNA does not change during your lifetime, so a germline result is permanent. If a variant was found, the question afterwards is not whether to retest you but whether your relatives should be tested. One exception: a variant reported as uncertain significance may be reclassified years later as more evidence accumulates. Your genetics team may contact you if a reclassification changes what your result means.

Did you know?

Most cancers are caused by mutations that develop during a person's lifetime, present only in the tumour. But a proportion arise from a variant inherited through the family — present from birth and potentially carried by blood relatives.

Somatic testing on the biopsy alone will not find an inherited cause. Only germline testing on a blood sample can.

Source: NCCN Guidelines on Genetic/Familial High-Risk Assessment; ASCO recommendations on hereditary cancer syndrome testing

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Common questions

Frequently asked questions

What is the difference between germline and somatic testing?

Germline testing reads the DNA in your normal cells — the DNA you were born with, inherited from your parents and potentially passed to your children. Somatic testing reads the DNA inside your tumour cells, which acquired mutations during the development of the cancer. Germline testing identifies inherited risk; somatic testing identifies what is driving this specific tumour and which treatments may help.

Does my biopsy cover both tests?

No. Your biopsy provides tissue for somatic testing only. Germline testing needs a separate sample of normal tissue — usually a blood test or cheek swab — because it reads the DNA in your normal cells, not the tumour. If both tests are recommended, your team will ask for that second sample separately, though some centres collect both at the same time to avoid a return visit.

Can a germline result change my treatment?

Sometimes. For certain cancer types, a germline variant can influence which treatments are suitable or which surgical options are offered. More often it is the somatic result that drives immediate treatment decisions, with germline testing done alongside for its family implications. Your oncologist will explain whether your specific germline result affects the decisions ahead.

How long does each test take to come back?

Somatic testing on tumour tissue typically takes one to three weeks, depending on the panel ordered and the laboratory. Germline testing on a blood sample usually takes a similar time, though comprehensive panels may take longer. Ask your team when your sample was sent and when the result is expected, so you have a timeline rather than waiting without one.

My oncologist ordered somatic testing. Should I also ask about germline testing?

It is a reasonable question. The answer depends on your cancer type and family history. NCCN and ASCO guidelines recommend germline testing routinely for cancer types that are more commonly hereditary. If you have close relatives who have had cancer, or you were diagnosed at a young age, mention this to your oncologist. The decision about whether germline testing is indicated belongs with your treating team.

If my germline test is negative, does that mean no one in my family is at risk?

A negative result means no pathogenic variant was found in the genes that were tested, which reduces but does not eliminate hereditary risk. Not all risk genes are included in every panel, and some family patterns do not involve known variants. If your family history is strong, a genetics counsellor can advise whether further evaluation is worthwhile even after a negative result. Share your full family history with your treating team rather than relying on the test result alone.

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