Diagnosis and Management of Acute Myeloid Leukemia (AML) with Emphasis on Blast Percentage

Diagnostic Considerations

The American Society of Hematology recommends that a diagnosis of AML requires ≥20% blasts in blood or bone marrow, and patients with 17% blast cells and promonocytes in the differential blood count would be classified as having high-grade myelodysplastic syndrome (MDS), specifically MDS with excess blasts-2 (MDS-EB2) 1, 2, 3
In AML with monocytic differentiation, both monoblasts and promonocytes are counted as blast equivalents, but abnormal monocytes are not included in the blast count, as per the guidelines from the National Comprehensive Cancer Network 2, 4
A minimum of 500 nucleated cells should be counted on bone marrow smears to ensure accurate blast percentage determination, with a 95% confidence interval for a 5% blast count narrowing from 1.6-11.3% with a 100-cell count to 3.3-7.3% with a 500-cell differential, as recommended by the European Hematology Association 2, 5, 6

Bone marrow aspirate and biopsy with accurate 500-cell differential count is necessary to confirm blast percentage and assess for dysplasia, as recommended by the American Society of Clinical Oncology 2, 5
Conventional cytogenetic analysis is mandatory, as certain recurrent genetic abnormalities allow AML diagnosis even with <20% blasts, according to the National Comprehensive Cancer Network 1, 2
Immunophenotyping by multiparameter flow cytometry (at least 3-4 colors) is necessary to confirm monocytic lineage and detect aberrant antigen expression, as recommended by the European Hematology Association 2, 8
Molecular testing for NPM1, CEBPA, and FLT3 mutations is necessary for prognostic stratification, as recommended by the American Society of Hematology 1, 8

Patients with 10-19% blasts (MDS-EB2) may be candidates for hypomethylating agents (azacitidine or decitabine) rather than intensive AML-type induction chemotherapy, as recommended by the National Comprehensive Cancer Network 3, 9
Cytogenetic risk stratification, molecular mutations, and patient age are more important prognostic factors than the precise blast percentage near the 20% threshold, according to the European Hematology Association 1