tartrate-resistant acid phosphatase

TRAP is a lysosomal enzyme expressed in osteoclasts and certain hematopoietic cells, including the malignant B cells seen in hairy cell leukemia. Unlike other acid phosphatases, TRAP retains its enzymatic activity even in the presence of tartrate, which inhibits most other acid phosphatase isoforms—hence the diagnostic utility of the TRAP stain. Hairy cell leukemia classically presents in older men with pancytopenia (due to bone marrow fibrosis and splenic sequestration), massive splenomegaly (due to red pulp infiltration), and 'dry tap' on bone marrow aspiration due to associated marrow fibrosis. Peripheral smear shows characteristic 'hairy' cytoplasmic projections on lymphocytes. TRAP positivity supports this diagnosis, though flow cytometry (CD11c, CD25, CD103, CD123) and BRAF V600E mutation testing are now more commonly used for definitive diagnosis. Treatment typically involves purine analogs like cladribine or pentostatin, which have excellent response rates. TRAP is also used as a marker of osteoclast activity in bone remodeling studies, reflecting osteoclast number and function, and can be elevated in conditions with increased bone turnover.