Regarding acute myeloid leukemia (AML), all of the following are characteristic features EXCEPT:

Question

Accepted Answer

A. Auer rods are pathognomonic for AML and present in all cases. ## Acute Myeloid Leukemia: Diagnostic Features

**Key Point:** Auer rods are highly specific for AML when present, but they are NOT pathognomonic and are absent in the majority of AML cases (~70% of cases lack Auer rods). Their absence does not exclude AML.

### Diagnostic Criteria for AML

| Feature | Details |
|---------|----------|
| **Blast percentage** | ≥20% blasts in bone marrow or peripheral blood (WHO 2016 criteria) |
| **Auer rods** | Highly specific when present (abnormal azurophilic granules) but NOT required for diagnosis |
| **Cytochemistry** | MPO+, Sudan black+, PAS variable |
| **Cytogenetics** | t(15;17), t(8;21), inv(16) are recurrent translocations |

### Auer Rods: Why NOT Pathognomonic

1. Present in only ~30% of AML cases
2. When present, highly suggestive of AML (not seen in ALL)
3. Absence does NOT exclude AML diagnosis
4. Found in AML-M3 (APL) most commonly

**High-Yield:** The diagnostic threshold of ≥20% blasts is the gold standard for AML diagnosis, regardless of Auer rod presence.

**Clinical Pearl:** In acute promyelocytic leukemia (APL), Auer rods are often abundant ("bundles of sticks"), but even in APL, diagnosis relies on the 20% blast criterion and t(15;17) cytogenetics, not Auer rods alone.

### Why Other Options Are Correct

- **≥20% blasts:** WHO diagnostic criterion for AML [cite:WHO Classification 2016]
- **t(15;17) in APL:** Pathognomonic translocation for AML-M3 [cite:Robbins 10e Ch 13]
- **MPO positivity:** Hallmark of myeloid differentiation; AML is MPO+ by definition [cite:Robbins 10e Ch 13]

Answer

B. More than 20% blasts in bone marrow or peripheral blood is diagnostic

Answer

C. MPO (myeloperoxidase) positivity on cytochemical staining is typical

Answer

D. t(15;17) translocation is associated with acute promyelocytic leukemia (APL)

Regarding acute myeloid leukemia (AML), all of the following are characteristic features EXCEPT:

Explanation

Acute Myeloid Leukemia: Diagnostic Features

Diagnostic Criteria for AML

Auer Rods: Why NOT Pathognomonic

Why Other Options Are Correct

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Feature	Details
Blast percentage	≥20% blasts in bone marrow or peripheral blood (WHO 2016 criteria)
Auer rods	Highly specific when present (abnormal azurophilic granules) but NOT required for diagnosis
Cytochemistry	MPO+, Sudan black+, PAS variable
Cytogenetics	t(15;17), t(8;21), inv(16) are recurrent translocations