The CD15 test is a flow cytometry-based immunophenotyping assay used to detect the expression of CD15 (Lewis X antigen) on hematopoietic and certain non-hematopoietic cells. CD15 is a carbohydrate antigen commonly expressed on mature granulocytes and myeloid lineage cells, and it is also seen in specific pathological conditions, including some malignancies such as Hodgkin lymphoma and select solid tumors including gastrointestinal stromal tumors (GISTs) in supportive diagnostic panels.