FISH (Fluorescence In Situ Hybridization) Test

A FISH test is used to figure out a breast cancer’s HER2 status. It measures the number of HER2 genes in breast cancer cells. If a breast cancer cell has too many copies of  the HER2 gene, it makes higher than normal numbers of HER2 proteins. These proteins act as receptors on breast cancer cells and receive signals that stimulate the growth of the cancer.

When a breast cancer has too many HER2 receptors, it’s called HER2-positive. Knowing if the breast cancer is HER2-positive or HER2-negative can help you and your doctor make treatment decisions.

FISH testing is usually done if the first HER2 status test, the IHC test, is inconclusive. Not all breast cancers need FISH testing.

FISH stands for fluorescence in situ hybridization. The test uses special labels that attach to the HER2 genes. The special labels have chemicals added to them so they change color and glow in the dark when they attach to HER2 genes. Because the genes are glowing, a pathologist can then count the genes to see if there are more than normal.

The FISH test results come back as a score of either positive or negative (some labs and hospitals call a negative test result "zero”).

A positive result means the cancer is HER2-positive. A negative or zero result means the cancer is HER2-negative.

Many breast cancers considered HER2-negative have some HER2 proteins on the surface of their cells. Doctors consider these cancers HER2-low.

HER2-low breast cancer has a 1+ score on an IHC test or a 2+ score on an IHC test plus a negative FISH test.

There are targeted therapies that treat HER2-positive and HER2-low breast cancer.