Leukemoid Reaction vs. CML: How to Differentiate and When to Suspect Each Condition?

What is a Leukemoid Reaction?

A leukemoid reaction is a benign, exaggerated white blood cell (leukocyte) response to an underlying condition, such as infection or stress. It mimics leukemia but is not cancer. Key characteristics include:

• Benign Condition: Not a malignant process.
• Exaggerated Leukocyte Response: Typically, white blood cell counts exceed 50,000/µL.
• Causes: Infections (e.g., tuberculosis, pertussis), hemorrhage, drugs (e.g., dapsone, sulfa drugs, steroids), and stress.
• High LAP Score: Elevated leukocyte alkaline phosphatase (LAP) score, indicating mature and functional white blood cells.

Differentiating Leukemoid Reaction from CML

The primary differential diagnosis for leukemoid reaction is CML. Here’s how to distinguish between the two:

Key Differences:

• CML: A chronic myeloproliferative neoplasm characterized by the Philadelphia chromosome (t(9;22) translocation).
• Leukemoid Reaction: A reactive process without the Philadelphia chromosome.
• LAP Score: CML typically presents with a low LAP score, while leukemoid reactions have a high LAP score.

Diagnostic Approach:

1. Bone Marrow Biopsy and Cytogenetics: Essential for detecting the Philadelphia chromosome in CML.
2. LAP Score: Helps differentiate between the two conditions.
3. Clinical Presentation: Consider patient history, symptoms (e.g., splenomegaly), and other lab findings.

Clinical Case Example

Consider a 63-year-old female with a history of chronic neutrophilia and splenomegaly. A bone marrow biopsy reveals the t(9;22) translocation (Philadelphia chromosome). This confirms a diagnosis of CML, as the presence of the Philadelphia chromosome is a hallmark of this condition.

Lab Findings in CML:

• Low LAP score.
• Presence of the Philadelphia chromosome.
• No Auer rods (typically seen in acute myeloid leukemia).
• Blast cells are not significantly elevated, as in acute leukemia.

Conclusion

Accurate differentiation between leukemoid reaction and CML is critical for appropriate management. A high LAP score, absence of the Philadelphia chromosome, and resolution with treatment of the underlying cause suggest a leukemoid reaction. In contrast, the presence of the Philadelphia chromosome and a low LAP score indicate CML. Always consider a thorough clinical evaluation and appropriate diagnostic tests to ensure accurate diagnosis.