The diagnosis of hairy cell leukemia can be established by flow cytometric analysis of peripheral blood, even in patients with low levels of circulating malignant cells

The diagnosis of hairy cell leukemia (HCL) has traditionally been based on microscopic means. Immunophenotypic analysis of peripheral blood by flow cytometry is not widely recognized as a method for diagnosing HCL, perhaps due to the expectation of low yield of neoplastic cells in patients who are characteristically leukopenic. The abnormal coexpression of CD103, CD25, and intense CD11c and CD20 on monotypic, slightly large B-lymphocytes has previously been shown to be highly characteristic of HCL. We wished to determine if this pattern was valuable in the diagnosis of HCL in leukopenic patients with low levels of neoplastic cells in the peripheral blood. The abnormal immunopheno-type above was observed in 25 peripheral blood specimens from patients with unexplained cytopenias or suspected lymphoproliferative processes. Ten of the 25 blood samples exhibited this abnormal phenotype in less than 5% of circulating leukocytes (ranging from <1% to 4%). All 10 patients had other manifestations of HCL, including cytopenias (mean white blood cell count, 1.8 × 10³/mm³;; hemoglobin, 11.0 gm/dl; platelets, 74 × 10³/mm³), splenomegaly, and typical bone marrow morphologic changes. Eight of the 10 patients achieved an excellent response to one course of 2-CDA, with significant improvement of cytopenias (mean white blood cell count: 5.3 × 10³/mm³; hemoglobin: 14.4 g/dl; platelets: 181 × 10³/mm³) and regression of splenomegaly. One patient had a partial response to alpha interferon and a subsequent complete response to 2-CDA, and one died during treatment. In conclusion, flow cytometric immunophenotyping of peripheral blood is capable of detecting low levels of circulating malignant cells in HCL, even in leukopenic patients. As such, it can be a very useful, non-invasive tool in the diagnosis of this disorder.