Medical Oncologist
CION Cancer Clinics
Why one test does not replace the other | CION Cancer Clinics
A tumour test reads the cancer to find which medicine is likely to work. A blood test reads your healthy cells to find a fault you were born with. They look at different DNA and answer different questions, so neither can stand in for the other. This page explains what each one sees, how they fit together, and who needs both. At CION Cancer Clinics in Hyderabad, our oncologists review your family history with you and guide you to the right genetic counselling and testing.
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Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed September 2026, next review due September 2027
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On this page
- Why can't one genetic test do both jobs?
- What does each test see that the other cannot?
- How do the two tests fit together?
- The words you will meet, in plain language
- Tumour test and blood test, point by point
- What this page cannot tell you
- Four things families assume about the two tests
- Common questions about tumour and blood tests
The short answer
Why can't one genetic test do both jobs?
Because they look at different DNA to answer different questions. A tumour test reads the cancer to find which medicine is likely to work. A blood test reads your healthy cells to find a fault you were born with. Neither can reliably answer the other's question, so some patients need both.
What the tumour test is for
A cancer collects many faults as it grows. Most of them arose during life and exist only in the tumour. Some of them can be targeted by specific medicines. Finding those is the tumour test's job, and it is covered on our targeted therapy pages.
What the blood test is for
A blood test for inherited faults looks at the DNA in your healthy cells. If a fault is there, it was there from birth and may be shared with parents, brothers, sisters and children. That answer affects the whole family, not only the person with cancer, and it does not change over a lifetime.
The tumour test serves the treatment. The blood test serves the family, and sometimes the treatment too.Different views
What does each test see that the other cannot?
The two tests overlap a little. The gaps between them are where families get caught out.
Only the tumour test sees
The changes that arose inside the cancer and drive its growth. A blood test cannot find these, because they are not in your healthy cells.
Examples
- EGFR and ALK changes in lung cancer
- Extra copies of HER2 in breast cancer
- Signs of a broken repair system in the tumour
Only the blood test settles
Whether a fault was present from birth. A tumour test usually cannot tell, and some tumour labs deliberately leave inherited-looking faults off the report.
Both may see
A fault in a gene such as BRCA1 or BRCA2. The tumour test finds it. Only the blood test tells you whether your relatives could carry it too. The same fault can also matter for treatment, which is why some patients are offered both tests close together.
Neither can do
Tell you for certain whether you or a relative will develop cancer, or explain itself. Every result needs someone qualified to read it with you. A result searched online, without the rest of your history, is easy to misread in either direction.
Not sure whether this applies to you?
Ask an oncologistIn one patient's care
How do the two tests fit together?
-
The diagnosis
A biopsy confirms the cancer. The tissue is kept, because it will be needed for tumour testing.
-
The tumour test, for treatment
Your oncologist has the tumour tested for changes that guide medicines. For some cancers this happens automatically in the lab.
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The blood test, if something points to it
Your cancer type, your age, your family history or the tumour result itself may suggest an inherited cause. A counsellor then arranges a blood test.
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The two results read together
A fault in both the tumour and the blood is inherited. A fault in the tumour alone is acquired. Your doctors use both to plan care.
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The family, if a fault is inherited
Relatives can be tested for that exact fault. The tumour result alone would never have given them that option.
On your reports
The words you will meet, in plain language
- Tumour profiling
- Testing the cancer's own DNA to find changes that medicines can target.
- Germline test
- A blood or saliva test for faults present in every cell from birth.
- Somatic
- Found only in the tumour. It arose during life and cannot be passed on.
- Germline
- Present in every cell from birth. It can be inherited and passed to a child.
- Paired testing
- Testing the tumour and blood side by side, so each fault can be sorted into inherited or acquired.
- Targeted therapy
- A medicine designed to act on one specific change in the cancer.
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Side by side
Tumour test and blood test, point by point
Tumour test
Blood test for inherited faults
Which medicine is likely to work?
Was this fault there from birth?
Needs tumour tissue or sometimes a blood sample for tumour DNA
Needs a blood or saliva sample
Results can change as the cancer changes
Results never change over a lifetime
May be repeated if the cancer grows
Usually done once
Helps the person with cancer
Helps the person and their relatives
Being straight with you
What this page cannot tell you
It cannot tell you whether you need one test, the other or both. That depends on your cancer, your age and your family. What your specific result means is a question for the counsellor or oncologist who ordered the test.
It cannot say which kind of test you already had
Reports do not always make this obvious. Some tumour tests use a blood sample to read tumour DNA, which people easily mistake for an inherited test. Ask your oncologist plainly: was this test on the tumour, or on my inherited genes? Keep a copy of every report in one folder. Families who travel from the districts to Hyderabad often find that the one report a counsellor needs was left at home.
Who this does not apply to
Most people with cancer need only the tumour test. If your cancer came at an older age, your family history is quiet and your tumour report raises no inherited flag, a blood test is unlikely to add anything. Your oncologist will say so if that is your situation.
If your family has had only one of the two tests, it is reasonable to ask whether the other one is needed.Commonly believed
Four things families assume about the two tests
"We did the big genetic test, so everything is covered."
A large tumour panel is still a tumour test. It is not designed to answer the family question, and it may leave inherited faults off the report altogether.
"My blood test was negative, so the tumour has no gene changes."
A negative blood test means no inherited fault was found in the genes tested. The tumour can still carry many acquired changes, some of which guide treatment.
"A test on blood must be the inherited test."
Not always. A liquid biopsy reads tumour DNA floating in the blood. It is a tumour test, even though the sample is blood.
"Doing both is just the hospital earning more."
Most patients need only one. When both are advised, it is because each answers a question the other cannot, and one of those questions affects your relatives.
Questions we are asked
Common questions about tumour and blood tests
Can a tumour test replace a blood test for BRCA?
No. A tumour test can find a BRCA fault but usually cannot say whether it was inherited, and it may miss some kinds of inherited fault altogether. If your relatives are to be tested, a blood test on you is needed first.
Can a blood test replace a tumour test for treatment?
No. Most changes that guide targeted medicines arose inside the cancer and are not in your healthy cells. A blood test for inherited faults will not find them. The tumour itself, or tumour DNA in the blood, has to be tested.
Can both tests be done at the same time?
Yes. Some labs test the tumour and a blood sample side by side, which sorts each fault into inherited or acquired. It can save time, but it still needs counselling for the inherited part. Your oncologist will say whether it suits your case.
Which test should come first?
Usually the tumour test, because treatment decisions cannot wait. For some cancers, such as ovarian and pancreatic cancer, the blood test is often offered straight away too. Your oncologist decides the order based on what your treatment needs first.
Is a liquid biopsy an inherited test?
No. A liquid biopsy reads tumour DNA that has leaked into the blood. It is a tumour test. It can occasionally pick up an inherited fault by accident, but it is not designed to find one, and any such finding needs confirming.
Will the inherited test need to be repeated later?
Usually not. Inherited DNA does not change. A repeat is sometimes suggested if the first test covered only a few genes, or if a newer test looks at more. The classification of a variant can also change over time.
How do we afford two tests?
Ask what each test costs and whether your insurance, Aarogyasri or Ayushman Bharat covers either. A targeted blood test for one known family fault is usually simpler than a full panel. The CION helpline can explain the papers to bring.
Who can tell me which tests I have had?
Your oncologist, who has your reports. If you are still unsure after that conversation, ask for a genetic counsellor. Call the CION helpline if you are unsure who to approach, and someone will point you to the right clinic.
Your Specialists
Meet CION's oncologists. Bring your family history or genetic report to them.
Our medical oncologists see people with a strong family history of cancer, arrange genetic counselling and testing where it fits, and plan the checks that follow.
Medical Oncologist
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Medical Oncologist
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
Medical Oncologist
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Sources
- National Cancer Institute — Biomarker Testing for Cancer Treatment
- National Cancer Institute — Genetic Testing for Inherited Cancer Susceptibility Syndromes
- National Cancer Institute — BRCA Gene Changes: Cancer Risk and Genetic Testing
- MedlinePlus Genetics — What do the results of genetic tests mean?
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
How tumour and germline testing fit togetherThe overview of both tests in one placeHaving both tests at the same timeWhen paired testing saves a second waitTumour-only testing and missing an inherited causeThe ways a tumour test can miss a family faultGetting the order of tests rightWhich test should come first
Talk to us
Not sure whether you need the other test?
Tell us which tests have been done and who in your family has had cancer. We will tell you honestly whether a second test would add anything. One helpline serves every CION centre.