What are the diagnostic criteria and differential diagnoses for a patient suspected of having acute myelogenous leukemia (AML)?

Acute Myelogenous Leukemia: Diagnostic Criteria and Differential Diagnoses

Diagnostic Criteria

The diagnosis of AML requires ≥20% myeloblasts in bone marrow or peripheral blood, confirmed through a comprehensive multimodal workup including morphology, immunophenotyping, cytogenetics, and molecular genetics. 1, 2

Core Blast Count Requirements

• Count at least 500 nucleated cells on bone marrow aspirate smears containing spicules 1, 2
• Include myeloblasts, monoblasts, and megakaryoblasts in the blast count 1
• In AML with monocytic differentiation, count monoblasts and promonocytes (but NOT abnormal monocytes) as blast equivalents 1, 2
• Erythroblasts are excluded from blast counts except in pure erythroid leukemia 1

Mandatory Initial Workup

Specimen Collection:

• Obtain fresh bone marrow aspirate as the primary specimen 3
• Collect peripheral blood for morphologic assessment and flow cytometry 1, 3
• Perform bone marrow trephine core biopsy if dry tap occurs or to supplement aspirate 1, 3

Essential Laboratory Studies:

• Morphology: Examine blood and marrow smears with May-Grünwald-Giemsa or Wright-Giemsa stain, counting at least 200 leukocytes on blood smears 1, 2

• Cytochemistry: Perform myeloperoxidase (MPO) and nonspecific esterase (NSE) stains for diagnosis and subclassification 1. MPO positivity in ≥3% of blasts indicates myeloid differentiation 1

• Immunophenotyping: Conduct multiparameter flow cytometry (minimum 3-4 color) with panels sufficient to distinguish AML from B-ALL, T-ALL, and acute leukemia of ambiguous lineage 1, 3, 2

• Conventional cytogenetics: Analyze minimum 20 metaphase cells from bone marrow to establish normal or abnormal karyotype 1, 3. This is mandatory and cannot be replaced by FISH or molecular testing alone 3, 2

Required Molecular Testing

Perform these molecular studies on all newly diagnosed AML patients:

• FLT3-ITD testing in all suspected or confirmed AML cases 1, 3
• NPM1, CEBPA, and RUNX1 mutation analysis for non-CBF, non-APL cases 1, 3
• PML-RARA rapid detection if acute promyelocytic leukemia (APL) is suspected based on morphology 1, 3
• KIT mutation analysis when core-binding factor (CBF) AML with t(8;21) or inv(16)/t(16;16) is diagnosed 1, 3
• Additional prognostic markers: IDH1, IDH2, TET2, WT1, DNMT3A, TP53 1

Special Diagnostic Considerations

For APL suspicion:

• Perform DIC profile immediately, as APL frequently presents with coagulopathy 1, 2
• Initiate rapid FISH for PML-RARA without waiting for conventional cytogenetics 1

For extramedullary disease:

• Obtain tissue biopsy with fresh tissue for flow cytometry, cytogenetics (karyotyping or FISH), and molecular studies 1, 3
• Perform PET/CT imaging if extramedullary involvement is suspected 3, 2

For CNS evaluation:

• Perform lumbar puncture with CSF cell count, cytology, and blast enumeration for patients receiving intrathecal therapy or AML without contraindication 1
• Use CT or MRI for patients with significant CNS signs or symptoms 3, 2

Differential Diagnoses

Primary Hematologic Differentials

Acute Lymphoblastic Leukemia (ALL):

• Distinguished by immunophenotyping showing lymphoid markers (CD19, CD10 for B-ALL; CD3, CD7 for T-ALL) 1, 2
• Terminal deoxynucleotidyl transferase (TdT) typically positive in ALL, negative in AML 1
• Lacks MPO expression (though rare MPO+ ALL exists) 1

Mixed Phenotypic Acute Leukemia (MPAL):

• Expresses both myeloid and lymphoid markers meeting WHO criteria 1
• Requires FISH for t(9;22)/BCR-ABL1 and KMT2A(MLL) gene translocations 1
• More common in adults than children 4

Myelodysplastic Syndrome (MDS):

• Blast count <20% distinguishes MDS from AML 1, 2
• However, certain cytogenetic abnormalities (e.g., t(8;21), inv(16), t(15;17)) establish AML diagnosis regardless of blast percentage 1

Secondary Considerations

Blastic Plasmacytoid Dendritic Cell Neoplasm:

• CD4+, CD56+, CD123+ immunophenotype 4
• Lacks MPO and other myeloid markers 4

Acute Leukemia with Myelodysplasia-Related Changes:

• Requires specific cytogenetic abnormalities or multilineage dysplasia 1
• History of prior MDS or MDS/MPN 1

Therapy-Related AML:

• History of prior chemotherapy or radiation exposure 2, 5
• Associated with complex karyotype and poor prognosis 5

Critical Diagnostic Pitfalls to Avoid

Never start chemotherapy before obtaining adequate diagnostic material for all required testing 3, 2. This includes sufficient cells for cytogenetics, flow cytometry, and molecular studies.

Do not rely solely on FISH or molecular testing without conventional cytogenetics 3, 2. Conventional karyotyping detects complex abnormalities and novel translocations that targeted testing misses.

Recognize hyperleukocytosis (WBC >100,000/μL) as a medical emergency 2. These patients require immediate leukapheresis or hydroxyurea before definitive chemotherapy to prevent leukostasis-related mortality.

Avoid excessive red blood cell transfusions in hyperleukocytosis 2, as this increases blood viscosity and worsens leukostasis symptoms.

Do not miscount blasts in monocytic AML 1, 2. Only monoblasts and promonocytes count as blasts; mature abnormal monocytes do not.

Recent growth factor therapy, transfusions, or certain medications can obscure morphologic features 3. Document this history before interpreting results.

Risk Stratification Requirements

Once AML is diagnosed, immediately stratify patients by:

• Cytogenetic risk: Favorable (t(15;17), t(8;21), inv(16)), intermediate, or adverse (complex karyotype, monosomy 5/7) 3, 2, 5
• Molecular mutations: NPM1-mutated without FLT3-ITD is favorable; FLT3-ITD high allelic ratio is adverse 3, 2
• Patient age: >60 years associated with significantly worse outcomes 2, 5
• Disease origin: Therapy-related or MDS-related AML carries poor prognosis 2, 5

Store extracted DNA, RNA, and viable cells for future testing 1, 3, as additional molecular characterization may be needed for treatment decisions or clinical trials.