Why should patients with Acute Myeloid Leukemia (AML) undergo karyotyping as part of their initial diagnostic workup?

Why Karyotyping is Mandatory in AML

Chromosome analysis by conventional karyotyping is mandatory at diagnosis for all patients with AML because it is essential for risk stratification, defines specific disease entities, guides treatment decisions, and cannot be replaced by molecular genetic testing or FISH alone. 1

Critical Role in Risk Stratification and Prognosis

Karyotyping provides comprehensive genomic information that directly determines treatment intensity and predicts outcomes:

• Favorable-risk cytogenetics include t(15;17) (acute promyelocytic leukemia), t(8;21), and inv(16)/t(16;16), which are associated with better response to standard chemotherapy and improved survival 2
• Adverse-risk cytogenetics include complex karyotype (≥3 abnormalities), monosomy 5/del(5q), monosomy 7/del(7q), and abnormalities of 3q, which predict poor outcomes and may warrant more aggressive therapy or early transplant consideration 1, 2
• Intermediate-risk includes normal karyotype and other non-favorable, non-adverse abnormalities, where molecular mutations (NPM1, FLT3, CEBPA) become critical for further risk refinement 1

The prognostic impact of karyotype has been validated across multiple large studies and is incorporated into all major risk stratification systems, including the European LeukemiaNet classification 1, 3

Disease Entity Definition

Several AML subtypes are specifically defined by their cytogenetic abnormalities, making karyotyping diagnostically essential:

• AML with recurrent genetic abnormalities (t(8;21), inv(16), t(15;17), t(9;11), and others) are distinct WHO classification entities that require cytogenetic confirmation 1
• These entity-defining abnormalities dictate specific treatment approaches—for example, t(15;17) mandates ATRA and arsenic trioxide therapy for APL 1
• Certain cryptic translocations may be missed by molecular testing alone but detected through karyotype analysis combined with FISH 1

Why Karyotyping Cannot Be Replaced

Molecular genetic testing and FISH do not substitute for conventional karyotyping because:

• Karyotyping provides a comprehensive genome-wide view that can detect unexpected abnormalities, complex karyotypes, and novel translocations not targeted by specific FISH probes 1, 4
• FISH only detects pre-specified abnormalities and will miss other prognostically important findings 1
• In one study of 220 adult AML cases with adequate karyotyping, FISH provided additional prognostic information in only 0.5% of cases, but karyotyping identified important abnormalities that FISH would have missed 4
• Complex karyotypes (10-12% of AML cases) require comprehensive chromosome analysis to identify the full spectrum of abnormalities, which has critical prognostic implications 5

Practical Implementation

The diagnostic workup should proceed as follows:

• Obtain fresh bone marrow aspirate at diagnosis with sufficient material for karyotyping (minimum 20 metaphases analyzed for adequate study) 1, 2
• Perform conventional karyotyping first as the primary cytogenetic test 1
• Add targeted FISH or molecular testing when:
  • Karyotype fails or yields insufficient metaphases 1
  • Specific abnormalities are suspected clinically but not confirmed by karyotype 1
  • Screening for cryptic abnormalities like KMT2A or MECOM rearrangements when no other entity-defining abnormality is present 1
  • Rapid results are needed (e.g., PML-RARA for suspected APL) 1

Common Pitfalls to Avoid

• Do not rely solely on FISH panels as they will miss complex karyotypes, novel translocations, and unexpected abnormalities that alter prognosis 1, 4
• Do not skip karyotyping in elderly patients despite higher failure rates—these patients often have adverse cytogenetics that critically impact treatment decisions 5
• Ensure adequate specimen quality by obtaining fresh aspirate material and processing promptly, as karyotype failure eliminates the most comprehensive prognostic information 1
• Recognize that normal karyotype requires molecular testing (NPM1, FLT3, CEBPA) for complete risk stratification, as these mutations define prognostic subgroups within cytogenetically normal AML 1

The combination of karyotyping with molecular testing provides the most complete prognostic assessment and guides critical decisions about chemotherapy intensity, stem cell transplantation timing, and targeted therapy eligibility 1, 2