Harvey's Garden

Leukemoid reaction

2 min read

  • Pathophysiology/Etiology
    • An exaggerated, benign inflammatory response leading to a massive increase in the white blood cell (WBC) count, typically >50,000/µL. It is a physiological reaction to significant stress or underlying disease, not a primary bone marrow malignancy.
    • Common Causes: Severe infections (e.g., C. difficile, tuberculosis), malignancies (as a paraneoplastic syndrome), severe hemorrhage or hemolysis, certain drugs (e.g., corticosteroids, G-CSF), and intoxication.
  • Clinical Presentation
    • Patients typically present with signs and symptoms of the underlying condition (e.g., fever and cough in pneumonia; diarrhea in C. diff infection).
    • The leukemoid reaction itself is a laboratory finding and does not usually cause specific symptoms, unlike leukemia which can present with bone pain, weight loss, and night sweats.
  • Diagnosis
    • CBC: Markedly elevated WBC count (>50,000/µL), with a predominance of mature neutrophils and earlier precursors (a “left shift,” including bands, metamyelocytes, myelocytes). Blasts are typically absent or rare.
    • Leukocyte Alkaline Phosphatase (LAP) Score: High. This is a key finding that differentiates it from Chronic Myeloid Leukemia (CML). The normal range for a LAP score is 20-100; a high score suggests a reactive process.
    • Peripheral Smear: Shows mature neutrophils. Toxic granulations and Döhle bodies (light blue cytoplasmic inclusions) are often present, indicating a reactive state.
  • DDx (Differential Diagnosis)
    • The most critical differential diagnosis is Chronic Myeloid Leukemia (CML).
    • Leukemoid Reaction vs. CML:
      • LAP Score: High in leukemoid reaction vs. Low in CML.
      • Basophilia/Eosinophilia: Typically absent or mild in a leukemoid reaction, but characteristically prominent in CML.
      • Genetic Markers: Negative for Philadelphia chromosome (t(9;22), BCR-ABL) in leukemoid reaction vs. Positive in CML.
      • WBC Count: While both are high, counts >100,000/µL are more suggestive of CML.
      • Clinical Context: Presence of a clear underlying cause (like a severe infection) points toward a leukemoid reaction.
  • Management/Treatment
    • Treat the underlying cause. For example, administer antibiotics for a severe bacterial infection or manage the underlying malignancy.
    • The WBC count will normalize as the primary condition resolves. Direct treatment to lower the WBC count is generally not required.
  • Key Associations/Complications
    • While the reaction itself is benign, its presence can indicate a severe, life-threatening underlying illness and may be associated with a poor prognosis depending on the cause.
    • Buzzwords: High LAP score, toxic granulations, Döhle bodies, recent severe infection.

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