Comparison of Cytochemistry, Flow Cytometry and Immunohistochemistry for the Detection of Myeloperoxidase in Acute Myeloid Leukemia.

Myeloperoxidase (MPO) is an unequivocal marker of myeloid differentiation and is used routinely in the diagnosis of acute myeloid leukaemia (AML). Cytochemistry (CC), flow cytometry (FCM) and immunohistochemistry (IH) are three routine techniques used to demonstrate MPO positivity. As FCM techniques have improved, IH and CC are being performed less frequently. However, very few reports comparing combinations of these modalities have been published, and to our knowledge none comparing all three. We have reviewed 158 bone marrow biopsies of patients diagnosed with AML between 2000 and 2007 at the Royal Melbourne Hospital, Victoria, Australia. A comparison of MPO positivity using CC, FCM and IH was performed. In 101/158 biopsies all three methods were performed, while the remaining 59 cases had only 2 of the 3 performed. 76 (75%) of these 101 cases showed MPO positivity by all three modalities, defined as at least 3% MPO positive cells on cytochemistry or IH, and at least 10% MPO positive cells by FCM. A further 2 biopsies were negative for MPO by all three techniques. In 23/101 (23%) biopsies in which all three methods were performed, discrepancies occurred between the different modalities (Table 1). The largest group (11/23, 48%) was positive by FCM and IH but negative by CC. A further 7/23 (30%) cases were positive by IH only. Four (17%) cases were positive by CC and IH in the presence of negative FCM, and there was a single case in which FCM was positive when CC and IH were negative. In three cases of AML minimally differentiated (FAB-M0), IH was the only modality able to identify MPO positivity. CC was the least sensitive with no cases of detection of MPO positivity in the presence of negative results by FCM and IH. However there were still 4 cases where FCM was negative in the presence of a positive CC result. These results confirm the importance of employing more than one method of detecting MPO, particularly in cases where FCM and CC are negative or discrepant. In our study, IH was the most sensitive marker for demonstration of MPO. Despite the widespread use of FCM for typing AML, CC and IH still have an important role to play in the demonstration of MPO positivity in these cases.