Investigation and diagnosis

• FBC: moderate to severe pancytopenia; Hb <8.5g/dL 35%.

• Blood film: low numbers of 'hairy cells' in 95%; florid leukaemic features unusual

• Hairy cells—kidney shaped nuclei, clear cytoplasm and irregular cytoplasmic projections (more notable on EM).

• WBC differential: neutropenia, <1.0 x 109/L in 75%; monocytopenia is a consistent feature.

• Cytochemistry: +ve for tartrate-resistant acid phosphatase (TRAP) in 95%; now identified by flow cytometry using antibody to TRAP.

• Immunophenotyping: typically CD11c, CD25, CD103 & HC2 +ve; differentiates HCL from other chronic lymphoproliferative disorders (Mtable, p174).

• Bone marrow: aspiration often unsuccessful—'dry tap' due to 4 BM fibrosis; trephine shows diagnostic features with focal or diffuse infiltration of HCL where cells have characteristic 'halo' of cytoplasm confirmed by immunocytochemistry with anti-CD20/DBA-44 and anti-TRAP.

• Abdominal CT for intra-abdominal lymphadenopathy (15-20%). Differential diagnosis

Confirmation of diagnosis may be difficult because of low numbers of circulating leukaemic cells and dry tap on marrow aspiration; trephine histology usually diagnostic; differential diagnosis includes myelofibrosis and other low grade lymphomas notably SLVL.

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