PRINCIPLE:

• At pH 7.6 myeloperoxidase enzyme (MPO) in myeloid cells and eosinophils reacts with 3, 3? diaminobenzidine (3, 3? DAB) to produce a golden-brown reaction product.

This stain is the ICSH recommended procedure and is a more reliable indicator of lineage specificity than the Sudan Black stain.

SPECIMEN:

• Peripheral Blood or Bone Marrow Aspirate smear.
• Unfixed slides stored at room temperature show a significant decline in MPO staining.
• Carry out MPO cytochemistry within a few days of preparation.

INTERPRETATION:

1. MPO activity is golden-brown in the myeloid lineage showing strong, clear granular resolution.
2. Immature leukaemic myeloid blasts often show a localised MPO reaction. They may show weak intensity.
3. Eosinophil peroxidase shows positivity.
4. Auer rods are easily seen.
5. Cells of lymphoid differentiation are uniformly MPO negative.
6. Monocytes and precursors show a weak scattered reaction.
7. Erythrocytes show a strong homogeneous staining pattern.

LIMITATIONS:

• Unfixed slides at room temperature show a decline in MPO activity. Stain within a few days of collection.
• MPO deficient cells (either acquired or defective) may not stain positive.
  
  It should be noted that even in AML cases with high proportions of MPO positive blasts, it is not possible to completely exclude ?hybrid, mixed lineage or biphenotypic variants without reference to immunophenotyping procedures. In this context it is rare for all blasts to be MPO positive.