FISH Test (Fluorescence In Situ Hybridization)

Fluorescence in situ hybridization (FISH) is a test that “maps” the genetic material in a person’s cells. This test can be used to visualize specific genes or portions of genes. FISH testing is done on breast cancer tissue removed during biopsy to determine whether the cells have extra copies of the HER2 gene. The more copies of the HER2 gene that are present, the more HER2 receptors the cells have. These HER2 receptors receive signals that stimulate the growth of breast cancer cells.

The FISH test results will tell you that the cancer is either “positive” or “negative” (a result sometimes reported as “zero”) for HER2. A positive result suggests that the cancer is likely to respond to treatment with Herceptin (chemical name: trastuzumab), a treatment that blocks the HER2 receptors from receiving growth signals.

Generally, the FISH test is not as widely available as another method of testing for HER2, called ImmunoHistoChemistry, or IHC. However, FISH is considered more accurate. In many cases, a testing facility will perform the IHC test first, ordering FISH only if the IHC results don’t clearly show whether the cells are HER2-positive or negative. FISH is best performed on tissue that has been preserved in wax or chemicals, rather than on fresh or frozen tissue.