FISH is an acronym for Fluorescence In Situ Hybridization. The FISH test is used to detect chromosomal abnormalities in cells and is one of the most commonly used molecular cytogenetic techniques.

FISH can be performed on any number of specimens: tissue, bone marrow, peripheral blood, and fluids. It is usually performed on the specimen that is thought to harbor the cell line of interest. For most of the leukemias, the marrow is typically used, but other specimens that are sometimes evaluated include cells from the cerebrospinal fluid and the lymph nodes.

In CLL the FISH test is used to analyze lymphocytes for chromosome defects. It is estimated that more than 50 percent of patients with CLL will have detectable chromosomal abnormalities, some of which are associated with disease progression and survival.

Examples of chromosomal abnormalities commonly found in CLL include, trisomy 12 and deletions in chromosomes 6, 11, 13 and 17. These are not the only chromosomal abnormalities that can occur in CLL, and combinations of abnormalities are also possible.