Medically Reviewed by Carol DerSarkissian, MD on November 06, 2021

Fluorescence in situ hybridization (FISH) is a test that "maps" the genetic material in human cells, including specific genes or portions of genes.

Because a FISH test can detect genetic abnormalities associated with cancer, it's useful for diagnosing some types of the disease. When the type of cancer has previously been diagnosed, a FISH test also may provide additional information to help predict a patient's outcome and whether he or she is likely to respond to chemotherapy drugs.

In breast cancer patients, for example, a FISH test on breast cancer tissue removed during a biopsy can show whether the cells have extra copies of the HER2/neu gene. Cells with extra copies of the gene have more HER2 receptors, which receive signals that stimulate the growth of breast cancer cells. So patients with extra copies of the gene are more likely to respond to treatment with trastuzumab (Herceptin), a drug that blocks the ability of HER2 receptors to receive growth signals.

Other treatment options for HER2 include:

Because FISH testing is expensive and not widely available, it's not as commonly used as another breast cancer test: ImmunoHistoChemistry (IHC).

How FISH Works

During a FISH test using a sample of the patient's tissue, special colored dyes are attached to specific parts of certain chromosomes in order to visualize and count them under a fluorescent microscope and to detect cancer-promoting abnormalities.

Abnormalities found in cancer cells include:

  • Translocation. Part of one chromosome has broken off and relocated itself onto another chromosome.
  • Inversion. Part of a chromosome is in reverse order, although it is still attached to the correct chromosome.
  • Deletion. Part of a chromosome is missing.
  • Duplication. Part of a chromosome has been copied and the cell contains too many copies.

Translocations can help doctors identify some types of leukemia, lymphomas, and sarcoma. Duplications in breast cancer cells can help doctors choose optimal treatments.

Compared to standard cytogenetic (cell gene) tests, one advantage of FISH is that it can identify genetic changes that are too small to be seen under a microscope. Another advantage is that FISH doesn't have to be performed on cells that are actively dividing. Because other tests cannot be performed until cancer cells have been growing in lab dishes for about two weeks, the process usually takes about three weeks. FISH results are usually available within a few days.

Examples of FISH Tests for Cancer

Although the FISH test is often used to analyze genetic abnormalities in breast cancer, it also can provide important information about many other types of cancer.

In the diagnosis of bladder cancer, for example, FISH testing of urinary cells may be more reliable than a standard test that looks for abnormal cells. In addition, FISH may detect bladder cancer recurrences three to six months earlier.

FISH also can identify chromosomal abnormalities in leukemias, including chronic lymphocytic leukemia (CLL) cells, some of which are associated with aggressive forms of the disease. Patients with more aggressive forms of CLL may need urgent treatment, while those with less aggressive forms may only require observation.

Show Sources

SOURCES: "Fluorescence In Situ Hybridization (FISH)." "FISH Test (Fluorescence In Situ Hybridization)."
American Cancer Society: "Special studies in cancer diagnosis." "Targeted Therapy for Breast Cancer."
Mayo Clinic: "Liver Cancer Diagnosis;" "Ureter Cancer Diagnosis;" and "Chronic Lymphocytic Leukemia." "Cytogenic and molecular genetic diagnostic tools."     

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