Which laboratory finding supports a diagnosis of hairy cell leukemia?

• A. Tartrate-resistant acid phosphatase positivity
• B. Auer rods
• C. Philadelphia chromosome positivity
• D. BCR-ABL fusion

Answer: (A)

Hairy cell leukemia is a mature B‑cell neoplasm that classically shows high activity of tartrate-resistant acid phosphatase on cytochemical staining. This enzyme activity is retained in the leukemic cells despite tartrate inhibition, making the stain positive and helping distinguish hairy cell leukemia from other hematologic malignancies. Auer rods are features of certain myeloid blasts (AML), while the Philadelphia chromosome and the BCR-ABL fusion are associated with CML (and some ALL), not hairy cell leukemia. So a positive tartrate-resistant acid phosphatase staining best supports the diagnosis.