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CION Cancer Clinics
RB1 testing for the family: who, in what order, and why | CION Cancer Clinics
Testing the family for RB1 starts with the person who had retinoblastoma. Once their exact fault is known, parents, brothers and sisters, and later their own children can be tested for that one change with a simple blood test. A negative result in a young sibling can end months of eye examinations. This page explains who is tested, in what order, and what each result means. At CION Cancer Clinics, our oncologists explain what a gene result means for you and your family, and plan the checks that follow.
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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
- Who in the family should be tested for RB1, and why?
- Which relatives are offered a test?
- How does family testing actually happen?
- The words you will meet, in plain language
- What does each result mean for a young sibling?
- Four things families tell us, and what is actually true
- What this page cannot tell you
- Common questions about testing an RB1 family
The short answer
Who in the family should be tested for RB1, and why?
The person who had retinoblastoma is tested first. If a fault is found, the relatives who could have inherited it are offered a test for that one exact change. Young brothers and sisters, and the children of a survivor, come at the front of the queue.
Why start with the person who had the tumour
Their result tells you whether there is a fault to look for at all, and which one. Testing a well sibling first can give a negative result that means very little, because nobody knew what the laboratory was looking for. Starting in the right place saves money and avoids false comfort.
Why it matters so much for young children
Babies and toddlers at risk need frequent eye examinations from birth, often under a short anaesthetic. A test showing a child did not inherit the family fault can end those checks. A test showing they did makes sure none are missed. Few genetic tests change a child's care as directly as this.
A blood test cannot find retinoblastoma. It finds the fault that raises the chance of it. Eye checks are what find tumours.Once the fault is known
Which relatives are offered a test?
Testing moves outward from the affected person, one branch at a time, and only as far as the results say it should.
Parents
Testing both parents shows whether the fault was inherited or appeared for the first time in the child. That answer shapes the chance for every other relative.
Even when both parents test negative, siblings are usually still checked for a while.Brothers and sisters
Young siblings come first, because their eye checks are happening now. Older siblings who are well still benefit. They may carry the fault without ever having had a tumour, and could pass it on.
Children of a survivor
Ideally tested at birth or soon after. Arranging the test before the baby arrives means the result is back early, often before the eye check schedule becomes heavy.
The wider family
If a parent carries the fault, that parent's brothers, sisters and their children may carry it too.
Usually offered to
- Aunts and uncles on the carrier's side
- Their children, especially young ones
- Grandparents, to confirm which side it came from
Not sure whether this applies to you?
Ask an oncologistStep by step
How does family testing actually happen?
Counselling first
A genetic counsellor draws the family tree and explains what a positive or negative result would mean, before any sample is taken.
Testing the person who had the tumour
Their blood is checked across the whole RB1 gene, including large missing pieces that ordinary reading can miss. If an eye was removed, the stored tumour can be tested too.
Targeted tests for relatives
Once the exact change is known, each relative is tested for that one change only. This is quicker and much cheaper than the first test.
A plan with every result
Each result comes with a plan: which children keep eye checks, which can stop, and who else in the family should be told.
On your report
The words you will meet, in plain language
- Index case
- The first person in the family to be tested, usually the one who had retinoblastoma.
- Cascade testing
- Offering a test to relatives one branch at a time, once the family's fault is known.
- Targeted test
- A test that looks only for the family's known change. It is cheaper and faster than reading the whole gene.
- Deletion or duplication
- A missing or doubled piece of the gene. It needs a separate method to find, so ask whether the test includes it.
- Tumour testing
- Testing the removed tumour to find both changes that caused it, then checking the blood for them.
- Mosaicism
- The fault is present in only some of a person's cells, which can make a blood test look negative.
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Side by side
What does each result mean for a young sibling?
The sibling carries the family fault
The sibling did not inherit it
Eye checks continue on the full schedule
Frequent eye checks can usually stop
Their own children will be offered testing
Their children are not at risk from this fault
Lifelong awareness of other cancers
No extra lifelong watching for RB1
Commonly believed
Four things families tell us, and what is actually true
"The blood test found nothing, so the siblings need no eye checks."
A negative blood result in a child with a tumour in one eye greatly lowers the chance for siblings. It does not remove it entirely, because a fault can hide in only some cells. The eye team decides how much checking is still sensible.
"Only the mother's side needs testing."
The fault can come from either parent. Both are tested, and the father's side matters just as much.
"Testing a baby is unfair. We should wait until they can decide."
For many inherited cancer genes, testing does wait for adulthood. RB1 is different. Tumours can start in the first months of life, so a test in infancy either spares a child frequent examinations or makes sure none are missed.
"Grandparents are too old for this to matter."
They will not need eye checks. Their result can still show which side of the family the fault came from, and that tells aunts, uncles and cousins whether they need testing.
Being straight with you
What this page cannot tell you
It cannot tell you whether your family's fault was inherited or new, or who else might carry it. Those answers come only from testing, done in the right order, with a counsellor guiding it. Nothing you read online can replace that family tree.
It cannot interpret a result
What your specific variant means is a question for the counsellor who ordered the test. RB1 results can be complex. Some faults sit in only a share of cells, and some variants are of unclear meaning. Bring the report to someone qualified to read it.
Who this does not apply to
Not every relative needs a test. If the affected child's testing found no fault, distant relatives are usually not tested at all. This page is also about inherited testing only. Tests on a tumour to plan treatment belong to the child's cancer team, and adult tumour testing is covered under targeted therapy.
Bring any old eye reports or discharge summaries, even decades-old ones. They help the counsellor draw an accurate family tree.Questions we are asked
Common questions about testing an RB1 family
Who should be tested first in an RB1 family?
The person who had retinoblastoma, whether they are a child now or an adult survivor. Their result shows whether there is a fault and which one. Relatives are then tested for that exact change, with young children first because their eye checks depend on the answer.
How is a baby tested?
With a small blood sample, or with blood from the umbilical cord at birth. The laboratory looks only for the family's known change, so the test is quick. Tell the laboratory the baby is at risk so the sample is handled promptly and the eye team gets the answer early.
Both parents tested negative. Do the other children still need eye checks?
Usually some checks continue for a while. A parent can carry the fault in only some egg or sperm cells, which a blood test will not show. The chance is much smaller, and the eye team will set a lighter schedule than for a child known to carry the fault.
What if the person who had retinoblastoma has died?
A stored tissue block from their old surgery can sometimes still be tested. If nothing is available, relatives can have the full gene test instead. A negative result is then less certain, and the counsellor will explain how much comfort it can give.
How long do results take?
A first full test of the gene usually takes a few weeks. A targeted test for relatives is usually quicker. If a newborn is waiting, eye checks start on schedule anyway, so nobody is left unchecked while the laboratory works.
Should adult brothers and sisters be tested?
Yes, if the counsellor offers it. Adults rarely need eye checks for retinoblastoma. A carrier adult still benefits from knowing, because of the raised chance of some other cancers later in life and because their own children would need testing.
Can the counselling be done in Telugu?
Yes. Ask for it when you book. Families often bring grandparents who are more comfortable in Telugu, and a family tree drawn in the language everyone speaks is usually more accurate. You can ask for the key points to be written down to share at home.
Will a result affect our children's marriage prospects?
Results are confidential medical information and who you tell is your decision. India has no dedicated law on genetic discrimination. A counsellor can help you think through when and how to share a result with a future spouse's family, which many families find hard.
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
- GeneReviews (NCBI) — Retinoblastoma
- MedlinePlus Genetics — Retinoblastoma
- National Cancer Institute — Retinoblastoma Treatment (PDQ) – Health Professional Version
- MedlinePlus Genetics — RB1 gene
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
Screening siblings and children of a survivorWhat the eye checks involve while you waitIs this retinoblastoma inherited?How testing answers the first questionRB1 testing cost in HyderabadWhat the first test and relatives' tests usually costRB1 and family planningOptions for carriers planning their own children
Talk to us
Not sure who in your family should be tested?
Tell us who had retinoblastoma and what testing has been done so far. We will arrange counselling and help you plan testing in the right order. One helpline serves every CION centre.