Evaluation of peroxidase activity by alpha-naphthol/pyronine staining compared with benzidine staining in 101 acute leukemia cases.

Cytochemical detection of myeloperoxidase (MPO) activity, a strong marker for myeloid differentiation, is usually performed by benzidine dihydrochloride staining, with the threshold at 3%. Several reports have demonstrated the potential toxicity of benzidine, and bans have been issued, under French law, prohibiting female technicians from being exposed to the aromatic hydrocarbon group, including benzidine. The aim of this study was to test an alpha-naphthol and pyronine-based substitute using a standardized kit (MYELOPEROXIDASE KIT, RAL [Réactifs RAL, Martillac, France]) to measure MPO activity in blast cells. This prospective, multicenter study made it possible to analyze 101 acute leukemia (AL) cases; it has also demonstrated both the 96% specificity and the 99% sensitivity of the method, with a threshold for positive staining of 3%, as well as good correlation (r = 0.95) between the staining method tested and the benzidine staining method. When using the alpha-naphthol/pyronine-based staining for MPO, the mean number of positive blast cells is statistically lower than that obtained using benzidine, but without incidence on AL classification. These results allow us to conclude that this method makes it possible to classify acute blood diseases by measuring MPO activity using reagents permitted by law, according to a standardized and reproducible protocol.