Medical Oncologist
CION Cancer Clinics
FISH testing: what it looks for and when it is used | CION Cancer Clinics
FISH is a laboratory test that checks one exact spot on a chromosome using a glowing probe, rather than reading a gene's full spelling. It is chosen when a specific structural change is already known or suspected in a family. This page explains what FISH can and cannot show, how it differs from sequencing, and when your team is likely to order it. 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
The short answer
What does a FISH test actually look for?
FISH stands for fluorescence in situ hybridisation. It does not read the spelling of a gene the way sequencing does. Instead it uses a small glowing probe that sticks to one exact spot on a chromosome, so a technician can see under a microscope whether that spot is present, missing, doubled, or sitting somewhere it should not be.
Built to answer one narrow question
A FISH test is ordered when a laboratory already knows which chromosome change it is looking for. It is not a general search through your genes. If a family carries a known rearrangement, FISH can check a relative for that exact change quickly and cheaply, without sequencing the whole gene again.
Where it sits alongside other tests
Most inherited cancer testing today is done by reading the letters of several genes at once. FISH is kept for the structural questions that letter-reading answers less well: whether a whole section of a chromosome has been lost, copied extra times, or swapped with another chromosome. Your report will usually say why FISH was chosen over a panel.
FISH tells you whether a known landmark is where it should be. It does not tell you what the genes around it say.When a team reaches for FISH
In which situations is FISH the right tool?
FISH is chosen for a specific job, not as a first step. These are the situations where it usually comes up.
A known family rearrangement
If sequencing has already found a large piece of a chromosome missing or duplicated in one relative, FISH can check whether another family member carries the same change, without repeating the full original test.
Confirming an unusual sequencing result
Sequencing sometimes flags a pattern that looks like a large piece of a gene is missing or doubled. FISH is used to confirm that finding directly, on the actual chromosome, rather than relying on the sequencing pattern alone.
Alongside a chromosome study
When a broader chromosome analysis raises a question about one particular region, FISH can zoom in on that single spot far faster than repeating the whole study.
Not the everyday choice
Most people having inherited cancer testing for the first time will have a panel or single-gene test, not FISH. It is a follow-up tool more often than a starting point.
Not sure whether this applies to you?
Ask an oncologistIn the laboratory
What happens to a sample sent for FISH?
A blood sample is taken
For inherited testing, an ordinary blood draw is enough. The laboratory grows the white cells for a short period so there are enough dividing cells to look at.
The glowing probe is added
A short strip of laboratory-made genetic material, carrying a fluorescent tag, is designed to stick only to the exact chromosome spot in question.
The probe finds its match
Where the probe binds, it lights up under a special microscope. Two normal signals usually mean the region is present in both copies.
A scientist reads the pattern
A trained analyst counts the signals across many cells and records whether the pattern is normal, missing a copy, showing an extra copy, or rearranged.
On your report
The words a FISH report uses
- Probe
- The lab-made piece of genetic material carrying the glowing tag, built to stick to one exact chromosome location.
- Locus
- The specific address on a chromosome that the probe is designed to find. Each FISH test targets one locus, or occasionally two together.
- Deletion
- A piece of a chromosome, including the region the probe targets, is missing from one copy.
- Duplication
- An extra copy of that same region is present where only one should be.
- Translocation
- A piece of one chromosome has become attached to a different chromosome. FISH can show whether the two pieces the probes target now sit together.
- Mosaic pattern
- Some cells show the change and others do not, in the same sample. It changes how a result is interpreted and is always explained by the counsellor.
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Side by side
FISH and gene sequencing, compared
FISH
Sequencing (panel or single-gene)
Looks for one known structural change at a chosen spot
Reads the actual spelling of a gene, letter by letter
Needs the change to already be known or suspected
Can find a fault nobody knew to look for
Fast and inexpensive once the target is fixed
Broader search, usually a longer turnaround
Cannot detect a small spelling change in a gene
Can miss some large structural rearrangements without extra analysis
Being straight with you
What a FISH result cannot tell you
A normal FISH result only rules out a change at the one spot tested. It says nothing about the rest of the gene, or about any other gene that might raise your risk. Families sometimes hear "FISH was clear" and assume the whole question is closed. It usually is not.
It does not replace counselling
Deciding which spot to test, and what a positive or negative result changes for you and your relatives, is a conversation with a genetic counsellor, not something the report answers on its own. The same finding can mean different things depending on which relative already has cancer and at what age.
Who this does not apply to
If nobody in your family has a known chromosome rearrangement, FISH is very unlikely to be your first test. Most people are better served by a panel that reads several genes' spelling at once, with FISH kept in reserve for a specific question that panel raises.
If you are unsure which test fits your situation, call the helpline and describe your family history before booking anything.Commonly believed
What families get wrong about FISH testing
"FISH checks my whole genetic code for cancer risk."
It checks one exact spot, chosen in advance. A clear FISH result says nothing about the thousands of other places a fault could sit.
"It is the newest test, so it must be the best one."
FISH is an older, narrower technique kept for a specific job. For a first-time search across several genes, sequencing is the more thorough choice.
"If FISH is negative, my relative does not need testing either."
A negative FISH result only rules out the one change tested. A relative may still need a broader test if the family history suggests something FISH was never designed to find.
"FISH results and sequencing results always agree."
They answer different questions, so a doctor sometimes orders both. A gene can read normally on sequencing while a structural change is still present, which is exactly the gap FISH is built to close.
Questions we are asked
Common questions about FISH testing
Is FISH the same as a genetic panel?
No. A panel reads the spelling of several genes at once. FISH looks at one fixed spot on a chromosome to see if a known structural change is there. They answer different questions and are sometimes used together.
Do I need to fast or prepare before a FISH blood test?
No special preparation is usually needed. It is an ordinary blood draw, sent to a laboratory equipped to handle it. Your clinic will tell you if your specific situation needs anything different.
How long does a FISH result take?
Turnaround varies by laboratory and by how many cells need to be counted for a reliable pattern. It is often quicker than a full sequencing panel, but ask your clinic for a timeframe specific to your sample.
Can FISH be done on old tissue, not just fresh blood?
Yes, FISH can often be performed on stored tissue blocks as well as blood, which is one reason it is sometimes chosen when a fresh sample is not available.
Will my report tell me which chromosome was checked?
Yes. A FISH report names the exact chromosome location tested and states plainly whether the expected pattern was seen, whether a copy was missing, doubled, or rearranged.
Why would my doctor order FISH instead of sequencing?
Usually because a specific change is already known or suspected in the family, or because an earlier test raised a structural question that FISH can answer more directly than repeating the sequencing.
Can a FISH result be a variant of uncertain significance?
Not in the same way sequencing results can. FISH generally reports the pattern as present, absent, or rearranged, though an unusual or mosaic pattern can still need expert interpretation.
Should the whole family have FISH if one relative did?
Only relatives with a reason to be tested for that exact change usually need it. A genetic counsellor can help you work out who in the family that applies to, and in what order.
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
- MedlinePlus Genetics — How is genetic testing done?
- GeneReviews (NCBI Bookshelf) — GeneReviews: Medical Genetics Information Resource
- Cancer Research UK — Inherited cancer genes and increased cancer risk
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.
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Tell us what has already been found in your family and we will explain, honestly, whether FISH, a panel, or another test fits your situation. One helpline serves every CION centre.