CLL by FISH (Fluorescence In Situ Hybridization) is a laboratory test used to detect genetic abnormalities in the cells of patients with Chronic Lymphocytic Leukemia (CLL). FISH is a technique that uses fluorescent probes to bind to specific chromosomal regions, allowing for the identification of genetic changes such as deletions, duplications, or translocations in the chromosomes of CLL cells. This test helps in diagnosing CLL, assessing its prognosis, and guiding treatment decisions, as certain chromosomal abnormalities are associated with more aggressive disease progression.