Detection of hairy cell leukaemia in blood and bone marrow using multidimensional flow cytometry with CD45-PECy5 and SS gating

CD45 and right angle light scatter (SS) gating are used commonly in clinical flow cytometry to differentiate cells of various lineages (Stelzer et al., 1993). We have used CD45‐PECy5 (Clone J33) since 1998 and have noticed that malignant lymphoid cells such as hairy cells can form distinct populations. Previous studies indicate that hairy cells reside where normal monocytes are usually found in CD45/SS scatter plots (Wells et al., 1998). We studied six patients with hairy cell leukaemia (HCL) and found that hairy cells have a higher CD45 mean cell fluorescence than normal lymphocytes and monocytes. Two of the six patients presented with mild unexplained cytopenias, without the usual clinical, morphological and cytochemical findings. In both cases, CD45/SS gating of bone marrow cells showed a small population with strong expression of CD45. The presence of hairy cells was confirmed using a panel of monoclonal antibodies. In one patient with HCL variant, CD45 expression was indistinguishable from that of normal lymphocytes. We conclude that CD45‐PECy5 (Clone J33) is useful for screening peripheral blood and bone marrow and for the detection of HCL without obvious morphological involvement.