[The sensitivity and specificity of various peroxidase stains--the difference between DAB and 3AC stainings]

FAB Cooperative Group has cited 3 or higher percentage in the peroxidase (PO) activity as one of criteria for the diagnosis of acute myeloid leukemia. We have previously reported in two cases of acute myelomonocytic leukemia (M4) that there was a great difference between 3,3'-diaminobenzidine (DAB) and 3-amino 9-ethyl carbazole (3AC) as substrates for PO reaction. In order to confirm the previous result, we extended the cases and compared the PO activities in blood films taken from several leukemic subjects, which were fixed with various kinds of fixatives, using DAB and 3AC as substrates. Their peroxidase activities were also determined through electron microscopy (EM-PO). There were no differences among fixatives and substrates in staining normal granulocytic cells (neutrophils and monocytes) and cells in 13 cases of leukemia, except for two cases, one with M4 and the other with relapse of M2. These showed the highest PO activity with 3AC staining and 10% formaldehyde acetone buffer for fixation. Besides, all types manifested the highest positivity in EM-PO, except for the relapse of M2 that showed a higher positivity in the peroxidase stain by 3AC staining, 10% formaldehyde acetone buffer fixation than the EM-PO. Unlike other leukemic blasts PO reaction products of blast cells of the two cases showed a scattered distribution of microscopic granules. These findings suggested the difference, which was seen in the previous reports, of the PO stain between two substrates might be due not only to sensitivity for staining but to the presence of some heterogeneity such as isoenzyme in the myeloperoxidase.