What is the role of bone marrow aspiration tests in diagnosing and guiding treatment for a patient suspected of having Acute Myeloid Leukemia (AML)?

Bone Marrow Aspiration Tests and Findings for AML

Essential Role of Bone Marrow Aspiration

Bone marrow aspiration is mandatory for all patients suspected of having AML and serves as the cornerstone for diagnosis, classification, and treatment planning. 1 The aspirate must be fresh and processed immediately to prepare smears for morphologic evaluation, with at least 500 nucleated cells counted on marrow smears containing spicules. 1

Required Diagnostic Components from Bone Marrow Aspirate

Morphologic Assessment

• Blast count threshold: A marrow or blood blast count of ≥20% is required for AML diagnosis (except for AML with minimal differentiation, acute megakaryoblastic leukemia, and acute leukemias of ambiguous lineage). 1
• Cell types included in blast count: Myeloblasts, monoblasts, and megakaryoblasts are counted as blasts; in AML with monocytic differentiation, monoblasts and promonocytes (but not abnormal monocytes) count as blast equivalents. 1
• Erythroblasts are excluded from blast counts except in rare pure erythroid leukemia. 1
• The pathologist should evaluate the aspirate smears in conjunction with bone marrow trephine core biopsy and touch preparations. 1

Immunophenotyping by Flow Cytometry

• Multiparameter flow cytometry (at least 3-4 color) is mandatory and must be performed on the bone marrow aspirate. 1
• The panel must be comprehensive enough to distinguish AML from early T-ALL, B-ALL, and acute leukemia of ambiguous lineage. 1
• Key markers for AML diagnosis include: 1
  • Precursor markers: CD34, CD38, CD117, CD133, HLA-DR
  • Granulocytic markers: CD13, CD15, CD16, CD33, CD65, cytoplasmic myeloperoxidase (cMPO)
  • Monocytic markers: CD11c, CD14, CD64, lysozyme, nonspecific esterase (NSE)
  • Megakaryocytic markers: CD41, CD61, CD42
  • Erythroid marker: CD235a (glycophorin A)
• Cutoff for positivity: Most markers use ≥20% of leukemic cells as the criterion for positivity. 1

Conventional Cytogenetic Analysis (Karyotyping)

• Conventional karyotyping is mandatory and cannot be replaced by molecular genetic testing or FISH alone. 1
• Chromosome abnormalities are detected in the majority of AML cases and provide critical prognostic information. 1
• Favorable cytogenetics: t(15;17) (APL), t(8;21), inv(16)/t(16;16). 1
• Adverse cytogenetics: Complex aberrant karyotype, antecedent myelodysplastic syndrome-related changes. 1

Molecular Genetic Testing

• FLT3-ITD mutation testing is required for all patients with suspected or confirmed AML for prognostic stratification and targeted therapy decisions. 2
• For CBF AML (with t(8;21) or inv(16)/t(16;16)): KIT mutation analysis is essential. 2
• For suspected APL: Rapid detection of PML-RARA fusion is critical and should be performed immediately. 1, 2
• For non-CBF, non-APL cases: Mutational analysis for NPM1, CEBPA, and RUNX1 is required. 2
• Additional prognostic markers include: IDH1, IDH2, TET2, WT1, DNMT3A, TP53. 1

Cytochemical Studies (Optional but Helpful)

• Myeloperoxidase (MPO) or Sudan Black B (SBB): Detection of MPO in ≥3% of blasts indicates myeloid differentiation, though absence does not exclude myeloid lineage. 1
• Nonspecific esterase (NSE): Shows diffuse cytoplasmic activity in monoblasts (usually 80% positive) and monocytes (usually 20% positive). 1
• PAS stain: May show large globules in acute erythroid leukemia. 1

Critical Timing Considerations

Chemotherapy must be postponed until satisfactory material for all diagnostic tests has been obtained. 1, 3 This is a strong recommendation to avoid compromising the ability to perform comprehensive diagnostic workup. 1

When Bone Marrow Aspirate is Inadequate or Unobtainable

Alternative Approaches

• If dry tap (punctio sicca): Prepare touch imprint preparations from the core biopsy and evaluate them; submit an additional core biopsy unfixed in tissue culture medium for disaggregation for flow cytometry and genetic studies. 1
• If peripheral blood has sufficient blasts: All diagnostic studies (flow cytometry, cytogenetics, molecular testing) can be performed on peripheral blood specimens. 1
• If bone marrow biopsy is contraindicated: Use peripheral blood for diagnosis and ancillary studies if adequate blast numbers are present. 1
• Trephine biopsy is superior to aspiration for evaluating cellularity, pattern of blast infiltration, residual hematopoietic activity, and presence of inflammatory cells. 4

Additional Prognostic Information from Bone Marrow

Risk Stratification Factors

• Age >60 years: Associated with adverse prognosis and increased susceptibility to treatment complications. 1, 3
• Initial leukocyte count: High counts indicate adverse risk. 1, 3
• Presence of increased plasma cells and myelodysplasia: May indicate unfavorable prognosis. 4
• Antecedent or concomitant myelodysplastic syndrome: Adverse prognostic factor. 1

Sample Handling and Quality Control

• The same physician should ideally interpret both bone marrow aspirate smears and core biopsy specimens, or interpretations should be correlated if performed by different physicians. 1
• Cryopreserved cells or nucleic acid should be stored properly for potential future molecular or genetic studies in laboratories compliant with regulatory requirements. 1
• All laboratory testing must be performed in laboratories meeting regulatory and accreditation requirements. 1, 3

Role in Treatment Monitoring

• Day 14 bone marrow: Documents hypocellularity in response to induction chemotherapy, though its necessity is debated as it often doesn't affect immediate management. 5
• Response assessment at count recovery: Bone marrow sampling remains the standard for evaluating complete remission and measurable residual disease (MRD). 5, 6
• MRD assessment: Flow cytometry-based MRD detection on bone marrow correlates with relapse risk and survival, making it a robust biomarker for therapeutic decision-making. 6

Common Pitfalls to Avoid

• Never start chemotherapy before obtaining adequate diagnostic material for morphology, immunophenotyping, cytogenetics, and molecular studies. 1, 3
• Do not rely solely on FISH or molecular testing without conventional karyotyping, as this provides incomplete cytogenetic information. 1
• Ensure adequate sample volume: At least 200 leukocytes on blood smears and 500 nucleated cells on marrow smears should be counted. 1
• Avoid inadequate aspiration: If the first aspirate is inadequate, touch preparations from core biopsy can salvage the diagnostic workup. 1, 4