Fluorescence in situ hybridization (FISH) testing is a molecular pathology test that helps identify specific genes in a person’s cells. It can show how genes have changed and whether there is an increased risk of specific cancers or other genetic conditions.

Human cells contain chromosomes, which are threaded structures that contain genes made of deoxyribonucleic acid (DNA). These tell cells how to divide and which proteins to make.

Some conditions change the genetic material of a cell. FISH testing allows doctors to see how genetic information has changed and “map” a person’s genetic material to diagnose conditions, personalize treatment plans, and advance research.

Read on to learn more about FISH testing, including how doctors use it and what to expect during a FISH test.

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FISH testing shows where a gene sits on a person’s chromosome.

A lab technician makes a short DNA sequence called a “probe” that slots into a specific place on the chromosome. This makes use of the way DNA attaches itself to the chromosome. Two strands of molecules make up each piece of DNA, and they stick to each other like magnets.

However, the DNA probes only have a single strand, meaning they will attach to their matching strand wherever it is on the chromosome. The technician tags DNA probes with a colored marker of fluorescent dye, showing technicians and researchers where they are on the chromosome.

Different types of FISH probes are available that work in different ways:

  • Locus-specific probes: These connect to a targeted area of a chromosome. This type of probe is useful if scientists have identified one part of a gene and want to know which chromosome houses the gene. It can also show how many times a gene has been copied.
  • Alphoid or centromeric repeat probes: Clinicians make these probes from sequences in the middle of each chromosome that repeat themselves. They can show how many chromosomes a person has. A clinician can also combine these with locus-specific probes to show whether genes are missing from a chromosome.
  • Whole chromosome probes: These are collections of smaller probes that bind to different points along the edge of a chromosome. These provide a full, color-coded map of a chromosome called a spectral karyotype. Clinicians use them to assess abnormal chromosomes.

FISH can help doctors identify several cancers and genetic conditions, including:

FISH testing identifies a cancer by showing if cells have extra copies of genes associated with cancer growth. For example, people with breast cancer have more copies of a gene called HER2. More HER2 genes in a FISH test mean more HER2 receptors in breast cells with which they can bind. These receptors can trigger cancer growth.

Learn more about the types of cancer.

A clinician needs a tissue sample to perform FISH testing. They often collect a tissue sample during a biopsy. They may also use tissue that they collect after removing a tumor during surgery.

The type of biopsy will depend on the cancer for which the clinician is testing. A person’s doctor can advise on how they will collect the sample and what it involves.

FISH test results show the following:

  • Duplication or amplification: The results show copies of chromosomes, in part or in full. This might also occur with individual genes.
  • Deletion: Part or all of a chromosome is missing.
  • Translocation: Part of one chromosome has joined up with another chromosome.

By highlighting the specific gene or chromosome that has undergone these changes, clinicians can refer to existing databases of gene abnormalities and pinpoint certain features regarding cancer or other conditions.

FISH testing allows for effective and efficient case handling by medical teams investigating complex genetic conditions, according to a 2017 review.

It helps clinicians identify a course of treatment and monitor cancers after treatment. FISH testing can help with the following elements of cancer care:

  • Specific diagnosis: Some cancers and genetic anomalies affect specific genes or chromosomes. Identifying this with a FISH test helps clinicians determine the type of cancer.
  • Outlook: A cancer’s genetics partly indicate how fast a cancer is likely to grow. FISH testing may help a clinician predict how far a cancer will spread and how much it will grow.
  • Treatment response: Certain genetics can mean that some therapies may be more effective at treating cancer than others.
  • Risk of recurrence: Clinicians use FISH testing to track a cancer after testing and assess whether it has a high risk of returning. Chromosomes may return to normal after treatment. However, if they show abnormal results in testing later on, it could mean some cancer remains.

A FISH test can be enough to provide a diagnosis on its own for many cancers. For example, when testing for HER2 gene copies in breast cancer cells, the result comes back either positive or negative/zero. This would help guide the treatment.

However, another test called an immunohistochemistry (IHC) test is more widely available but less accurate, according to the National Breast Cancer Foundation. In these cases, clinicians may administer a FISH test after an IHC.

Here are some frequently asked questions about FISH testing.

What does a positive FISH test mean?

The implications of a positive FISH test will depend on the specific medical condition a doctor is assessing for. For people with breast cancer, a positive HER2 test means that breast tissue has more HER2 receptors. This means the cancer is more likely to respond to trastuzumab (Herceptin) treatment.

What is the FISH test for genetic disorders?

Genetic disorders are conditions that occur as a result of genetic mutations, or changes in genes. FISH testing focuses on identifying various aspects of chromosomes, which contain genes.

This means it can help with verifying specific diagnostic features of cancers, which typically occur due to gene mutations, and confirming mutations responsible for inherited genetic conditions such as Prader-Willi syndrome.

How long does a FISH test take?

FISH tests may take 2–5 working days to generate a result. However, the time from testing to consulting with a doctor to find out may be longer depending on hospital administration times, sample postage, and other factors.

FISH testing lets clinicians map out a cell’s chromosomes and genes. By adding a “probe” DNA strand that attaches to a specific place on the chromosome, technicians can pinpoint cancer features and genetic health problems.

This testing method lets cancer doctors identify optimum cancer treatment strategies to achieve the best possible outcomes.

A person’s doctor can advise on whether they recommend FISH testing, how they will take the sample, and what they can expect from test results.