A Lymphocyte Subset Panel is a diagnostic test that quantifies and assesses the distribution of lymphocyte subsets in blood, specifically T cells, B cells, and natural killer (NK) cells. These lymphocyte subsets are crucial immune cells in cell-mediated, humoral, and innate immunity. These cells are assessed by fluorescent-labeled surface marker antibodies such as CD3, CD4, CD8, CD19, and CD56 markers to label T/B/NK cells on flow cytometry. CD4:CD8 ratio is also included as a calculated parameter.