Medical Oncologist
CION Cancer Clinics
How far out in the family should testing go? | CION Cancer Clinics
Testing moves outward from the first person found to carry a fault, one branch at a time. Parents, brothers, sisters and children come first. Each relative who tests positive opens the next circle of their own relatives, and each who tests negative closes that branch. In a large family this can reach cousins and beyond. Here is how far it usually goes, and where it can safely stop. At CION Cancer Clinics, our team helps carriers and their families plan checks, next steps and support after a genetic result.
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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
- How far out in the family should genetic testing go?
- Who sits in each circle, and when are they reached?
- How does testing move outward through a family?
- The family testing words, in plain language
- When should testing keep going, and when can it stop?
- What this page cannot tell you
- Four things families believe about testing relatives
- Common questions about how far testing goes
The short answer
How far out in the family should genetic testing go?
As far as the fault can travel, and no further. Testing starts with the parents, brothers, sisters and children of the person who carries it. It then follows each relative who tests positive outward to their own close relatives. It stops on any branch where a relative tests negative, because nobody can pass on a fault they do not have.
Why it moves in steps, not all at once
Testing every cousin on the first day would test many people with almost no chance of carrying the fault. Moving one step at a time means each result tells you who needs testing next. It keeps the cost down for the family, and it spares relatives on the clear side a test they never needed.
Why the side of the family matters
A fault usually comes down one side, from your mother or from your father. Once that side is known, relatives on the other side have no raised chance from this fault. Finding the side early is often the most useful single step. It usually comes from testing a living parent, or from seeing which aunts and uncles test positive.
Testing travels outward through carriers. A negative result closes that branch, for that person and for their children.The circles of the family
Who sits in each circle, and when are they reached?
Doctors describe relatives by how many steps they sit from the person who carries the fault.
First-degree relatives
Parents, full brothers and sisters, and children. Each starts with an even chance of carrying the fault. They are offered testing first, as soon as the result is confirmed.
Second-degree relatives
Grandparents, aunts, uncles, nieces, nephews, grandchildren and half-siblings. They start with a lower chance, and are usually reached once a closer relative on their side tests positive.
Usually reached through
- A parent who tests positive
- A brother or sister who tests positive
Third-degree relatives
First cousins, great-aunts and great-uncles. Their starting chance is lower again. They come in when the relative who links them to the fault has tested positive, or has died without being tested.
Beyond that
Second cousins and further. Testing can reach this far in a large family where the fault keeps appearing. It always gets there step by step, never by testing distant relatives first.
Not sure whether this applies to you?
Ask an oncologistOne branch at a time
How does testing move outward through a family?
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The first carrier is confirmed
Usually the relative who had cancer, tested on a broad panel. Their report names the exact fault the rest of the family will be tested for.
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Close relatives are offered a targeted test
Parents, brothers, sisters and adult children. A targeted test looks only for the one known fault, so the answer is usually a clear yes or no.
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Each result opens or closes a branch
A relative who tests positive becomes the next starting point. A relative who tests negative closes their branch, and their children do not need testing for this fault.
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The carrier side moves outward
Aunts, uncles and cousins on the side the fault came from are offered testing through the carrier nearest to them. The other side of the family is usually left alone.
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Testing stops where the fault stops
It ends when every branch has reached a negative result, a relative who declines, or someone nobody can contact. Those gaps are noted on the family tree, not forgotten.
Words you will hear
The family testing words, in plain language
- Cascade testing
- Offering a targeted test to relatives one step at a time, outward from the first person found to carry a fault.
- Degree of relationship
- How many steps a relative sits from you on the family tree. Parents are one step, grandparents two, first cousins three.
- Targeted test
- A test for the single fault already found in the family, rather than a broad panel of many genes.
- Carrier side
- The side of the family, your mother's or your father's, that the fault came down.
- Obligate carrier
- A relative who must carry the fault because of where it sits in the family, even without a test. For example, someone whose parent and whose child both carry it.
- Uptake
- How many of the relatives offered a test actually take it. In many families it is lower than doctors would hope.
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Side by side
When should testing keep going, and when can it stop?
Keep going on this branch
This branch can usually stop
A relative tests positive
A relative tests negative
The side the fault came from is still unknown
The fault is confirmed to come from the other side
A carrier has adult children or siblings not yet told
Everyone on the branch has already been offered a test
Parents are related by blood, so both sides need checking
Parents are unrelated and the carrier side is confirmed
Being straight with you
What this page cannot tell you
It cannot tell you how far testing should go in your own family. That depends on the exact fault, which side it came from, who is alive and who can be reached. A counsellor works it out from your drawn family tree. What your specific variant means is a question for the counsellor who ordered the test.
It cannot reach relatives for you
Nobody can test a relative who has not been told, or who does not want to know. In large families the news often stops at one household. A family letter, written by the counsellor and passed on by you, usually carries it further. It explains the fault without relying on anyone's memory of the appointment.
When the usual rule does not hold
Where parents or grandparents married within the family, a fault can come down both sides at once. The idea that one side is clear may then be wrong. Tell your counsellor about any marriage between relatives, even several generations back.
Who this does not apply to
If nobody in your family has a confirmed fault, there is nothing yet to follow outward. The first step is testing the relative who has had cancer. Most families with one older relative diagnosed will never need this page.
Commonly believed
Four things families believe about testing relatives
"Test everyone in the family at once, to be safe."
That tests many people with almost no chance of carrying the fault, and costs the family far more. Step by step testing finds the same carriers and spares those who never needed a test.
"My parent tested negative, but I should be tested anyway."
If your parent on the carrier side tests negative, you cannot have inherited the fault from them. Your counsellor will still check the other side is truly clear before telling you no test is needed.
"Cousins are too distant to matter."
Once the aunt or uncle linking you is known to carry the fault, that cousin has the same even chance as any child of a carrier. What matters is whether the path to them runs through carriers, not how far away they sit on the tree.
"Only relatives who have had cancer need testing."
Well relatives are the whole point. A well carrier found early can start screening before anything develops. A well relative who tests negative can return to ordinary screening and stop worrying about this fault.
Questions we are asked
Common questions about how far testing goes
Should my cousins be tested?
Only if the relative who links you to them, usually a shared aunt or uncle, carries the fault or cannot be tested. If that aunt or uncle tests negative, their children need no test for this fault. Your counsellor will tell you which cousins sit on the carrier side.
If the fault came from my father, does my mother's side need testing?
Usually not, for this fault. A fault from your father's side gives your mother's relatives no raised chance of carrying it. They are still watched on their own family history, and if a pattern of cancer shows up there too, that is a separate conversation.
What if the parent who carried it has died?
Their brothers, sisters and children can still be tested. The family tree and the pattern of results often show which side the fault came from. A stored tissue block from that parent's old surgery can sometimes be tested too, if the hospital still holds it.
How far back does the family tree need to go?
Usually three generations on both sides, which covers grandparents, aunts, uncles and cousins. Going further back helps when an older pattern shows which side the fault came from. Confirming the cancer type and age for key relatives matters more than naming everyone.
Who pays for each relative's test?
Each relative's test is usually paid for separately. A targeted test for a known fault is generally cheaper than the first broad panel. Ask your counsellor about current costs, and whether any scheme or insurance covers testing in your situation.
What about relatives in another state or abroad?
They can usually be tested where they live. Send them the family letter and a copy of the original report, so their laboratory tests for the exact fault. Their result can then be shared back to keep the family picture complete.
Do we have to reach relatives we are not in touch with?
Testing is offered, never forced. Many families pass the letter to one person in each household and let it travel from there. If a branch cannot be reached, the counsellor notes it. Nobody expects you to track down every relative yourself.
Where do I start?
Bring the report of the first person who tested positive, and a list of who is alive on each side. A counsellor will map which relatives to offer testing first. Call the CION helpline if you are not sure who to approach.
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 — Genetic Testing for Inherited Cancer Susceptibility Syndromes
- National Cancer Institute — Cancer Genetics Risk Assessment and Counseling (PDQ)
- MedlinePlus Genetics — What are the different ways a genetic condition can be inherited?
- NHS — Predictive genetic tests for cancer risk genes
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
Cascade testing: how a result moves through a familyThe whole process, before the question of how farTesting first-degree relativesThe first circle, and exactly who is in itObligate carriers: when testing is not neededRelatives whose status the family tree already settlesConsanguineous families and cascade testingWhen parents are related and both sides need checking
Talk to us
Not sure which relatives should be offered a test?
Bring the first positive report and a list of who is alive on each side, and a counsellor will map who to approach first. One helpline serves every CION centre.