Detection of small numbers of cells characteristic for haematological disorders in peripheral blood (the deep diff).

We studied cytocentrifuge preparations of peripheral blood mononuclear leucocytes in haematological patients with nondiagnostic white cell differential counts. This approach (the 'deep diff') enabled the detection of small numbers of diagnostically significant cells in a majority of patients. We found ringed sideroblasts in 5/7 patients with sideroblastic anaemia and megaloblasts in 9/10 patients with megaloblastic anaemia. We observed centrocytes in seven patients with follicular lymphoma, mantle cells in five patients with mantle cell lymphoma and marginal zone cells in five patients with nodal marginal zone lymphoma. We detected small percentages of lymphoma cells in cytospins of mononuclear leucocytes in 12 patients with large B-cell lymphoma, Burkitt's lymphoma and anaplastic large-cell lymphoma. The deep diff was nondiagnostic in 5/6 patients with hairy cell leukaemia and in 9/10 patients with Waldenström's macroglobulinaemia and small lymphocytic lymphoma. In these cases, there were insufficient cells to detect light chain restriction. Increased counts of leukaemic cells were found in 12/13 patients with acute leukaemia with < 3% blasts in the conventional white cell differential. Increased blasts were also observed in six patients with refractory anaemia with excess of blasts (RAEB). Decreased blasts were found in five patients with aplastic anaemia and in nine patients with bone marrow aplasia after intensive chemotherapy. Increased plasma cell counts were observed in 13/14 patients with advanced plasma cell myeloma. We conclude that the 'deep diff', augmented by immunocytochemistry, may be useful in the diagnosis of haematological disorders.