Cytochemical studies demonstrating tartrate-resistant acid phosphatase (TRAP) activity, traditionally have been used to confirm the diagnosis of HCL, although the routine use of immunophenotyping for the diagnosis of chronic lymphoproliferative disorders has diminished reliance on the TRAP stain. Immunohistochemical stains for TRAP are also available and can be used in paraffin-embedded sections to demonstrate the activity of this enzyme. Similar to cytochemistry, a positive immunohistochemical stain is not pathognomonic for HCL; positivity has been reported in systemic mastocy-tosis and Gaucher's disease.42 However, the presence of cells positive for both CD20 and TRAP activity confirm the diagnosis of HCL.

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