A threshold of 10% for myeloperoxidase by flow cytometry is valid to classify acute leukemia of ambiguous and myeloid origin.

BACKGROUND: According to WHO 2008 guidelines, an important role is designated for cytoplasmic myeloperoxidase (cMPO) as measured by flow cytometry for classifying acute leukemia of myeloid or ambiguous origin (AML or MPAL). However, no threshold with respect to expression level and percentage positive cells is provided. Since the expression of solely cMPO can change the diagnosis from acute lymphoid leukemia into MPAL in the current WHO 2008, a consensus is needed for the cut-off for cMPO.
METHODS: In this study, we investigated whether or not a cut-off of 10% positivity for cMPO equally defines an acute leukemia as AML or MPAL as compared to a cut-off for cMPO of 20% and compared this with results obtained for Sudan Black B (SBB) staining by cytomorphology.
RESULTS: Cell lineage-defining markers and SBB staining were analyzed retrospectively in a cohort of 198 patients who presented with acute leukemia. Eight patients were positive for SBB (>3%), but were considered negative for cMPO (<10%); six patients were negative for SBB (≤ 3%) and positive for cMPO (≥ 10%) staining. In six patients, we found 10-20% cMPO positive leukemic cells. Five of these cases were SBB positive; the sixth patient showed a clear myeloid phenotype without positivity of any lymphoid marker. Using a 10% cut-off instead of 20% would have changed diagnosis from ALL into MPAL in two patients; both cases were SBB positive by morphology.
CONCLUSION: We conclude that a 10% cut-off is a secure lower limit for cMPO expression and can be used independently from SBB expression.