The CD57 test is a flow cytometry-based assay used to measure the number and percentage of CD57-positive lymphocytes, primarily a subset of natural killer (NK) cells and certain T lymphocytes, in peripheral blood. CD57 is a cell surface marker associated with immune cell maturation and function. The test helps evaluate immune system status and may be used in the assessment of immune deficiencies, chronic infections, autoimmune conditions, and certain hematologic disorders. Results are reported as absolute cell counts and/or percentages of CD57-positive cells within the lymphocyte population.