Which leukemia is associated with Auer rods?

Auer Rods are Primarily Associated with Acute Myeloid Leukemia (AML)

Auer rods are characteristically found in acute myeloid leukemia (AML) and are considered a diagnostic hallmark of myeloid differentiation. They can also be seen in certain subtypes of myelodysplastic syndromes (MDS), particularly those that are progressing toward AML.

Specific Leukemia Types Associated with Auer Rods

Acute Myeloid Leukemia (AML)

• Auer rods are most commonly found in AML subtypes M1 and M2, with highest incidence in M2 (78%) followed by M1 (63%) 1
• Less frequently found in M4 (47%) and rarely in M5 (5%) 1
• Their presence in AML is considered a marker of myeloid differentiation

Myelodysplastic Syndromes (MDS)

• According to WHO classification, the presence of Auer rods in MDS automatically classifies the disease as Refractory Anemia with Excess Blasts-2 (RAEB-2), even if blast percentage is low 2
• The finding of Auer rods in MDS with <5% blasts is rare but significant, as it indicates high risk of progression to AML 3

Prognostic Significance

• In AML, particularly in the M1 subtype, the presence of Auer rods is associated with:

  • Higher remission rates (statistically significant, p=0.01) 1
  • Longer remission duration (p=0.007) 1
  • Better overall prognosis as they indicate some degree of myeloid differentiation 4

• In MDS:

  • The presence of Auer rods upstages the disease to RAEB-2 regardless of blast percentage 2
  • MDS with Auer rods typically shows rapid progression to AML (within 5-24 months) or death 3

Morphology and Composition

• Auer rods are needle-shaped crystalline structures found in the cytoplasm of myeloid blasts 4
• They are composed primarily of myeloperoxidase in crystalline form 5
• When multiple Auer rods are clustered together in a cell, they form what is known as "faggot cells" 4

Diagnostic Implications

• The presence of Auer rods is considered definitive evidence of myeloid differentiation
• According to the WHO classification, cases with Auer rods and ≥5% myeloblasts in the blood and <10% in the marrow should be classified as RAEB-2 2
• Extremely rarely, Auer rod-like structures can be found in acute lymphoblastic leukemia (ALL) or leukemias of ambiguous lineage, but this is exceptionally uncommon 6

Clinical Relevance

• The finding of Auer rods during morphologic examination is critical for proper classification and risk stratification
• Their presence in residual blasts after treatment indicates lack of complete remission 4
• In MDS with <5% blasts, the presence of Auer rods signifies an aggressive biology with high likelihood of progression to AML 3

Auer rods remain an important morphological finding in the era of molecular diagnostics, providing valuable diagnostic and prognostic information at the initial microscopic examination of blood and bone marrow.