Quantitation of plasma cells in bone marrow aspirates by flow cytometric analysis compared with morphologic assessment.

CONTEXT Accurate quantitation of bone marrow plasma cells is an important component in the diagnosis and posttreatment assessment of plasma cell dyscrasias. Although flow cytometry is sometimes used for this purpose and can rapidly evaluate many cells, the accuracy of flow-based plasma cell quantitation compared with morphologic assessment (currently the gold standard) is uncertain as direct comparison studies have not been previously reported. OBJECTIVE To determine how percentages of plasma cells in diagnostic aspirate smears quantitated by morphologic assessment relate to percentages of plasma cells quantified by flow cytometry. DESIGN Thirty bone marrow cases with 10% or more plasma cells and leukemia/lymphoma flow cytometry immunophenotyping studies were identified from our hematopathology database. The Wright-stained aspirate smears, marrow biopsy sections, and flow cytometry histograms were reviewed. RESULTS Morphologically determined plasma cell percentages from the diagnostic aspirate smears were consistently higher than those determined by flow cytometry. Much of this difference appeared to be related to differences in sample quality. However, the cellular processing involved in performing flow cytometry also appeared to reduce plasma cell percentages in many cases. CONCLUSIONS This study helps define the limitations of flow cytometry for quantitating plasma cell loads in marrow aspirate specimens that may significantly affect the diagnosis or assessment of treatment response.