Accurate flow cytometric gating of the large lymphocyte region is a powerful screening method for detecting hairy cell leukemia presenting with a low tumor burden.

Hairy cell leukemia typically presents with pancytopenia and often mimics aplastic anemia. Making an accurate diagnosis is crucial, as treatment with the purine analogues cladribine and pentostatin brings about durable complete remission in the majority of patients. Surface kappa and lambda flow cytometric analyses of peripheral blood or bone marrow are a powerful screening tool, although routine gating of the entire lymphocyte region may fail to show light chain restriction due to a low tumor burden. We herein demonstrate that accurate subgating of the large lymphocyte region is essential and recommend the application of this method in all cases of pancytopenia of unknown etiology.