Flow cytometry thresholds of myeloperoxidase detection to discriminate between acute lymphoblastic or myeloblastic leukaemia

The World Health Organization 2008 Classification emphasizes myeloperoxidase (MPO) detection as sufficient for assigning a blast population to the myeloid lineage. Published MPO positivity thresholds are 10% for flow cytometry (FCM) but 3% for cytochemistry. Here we re‐evaluated the FCM‐MPO threshold by comparing retrospectively 128 acute lymphoblastic leukaemias and 75 acute myeloid leukaemias without maturation, all assessed by benzidine‐based cytochemistry. A 13% threshold was found to be relevant using an isotype control as background‐reference (sensitivity 95·1%, specificity 91·7%). Residual normal lymphocytes proved to be an advantageous alternative reference, a threshold of 28% yielding improved 97·4% sensitivity and 96·1% specificity.