Which condition is most likely to present with a greatly elevated WBC count due to a reactive process rather than leukemia?

Which condition is most likely to present with a greatly elevated WBC count due to a reactive process rather than leukemia?

• A. Leukemoid reaction
• B. CML
• C. Acute lymphoblastic leukemia
• D. Hairy cell leukemia

Answer: (A)

A key idea here is telling apart a reactive rise in white blood cells from a malignant leukemia when the count is very high. A leukemoid reaction is a strong reactive neutrophilia caused by infection, inflammation, or severe stress. On a blood smear you often see a left shift with mature and immature neutrophils (bands, metamyelocytes) but no malignant clone, and the leukocyte alkaline phosphatase (LAP) score is usually elevated, supporting a reactive process rather than cancer. In contrast, chronic myeloid leukemia is a clonal myeloproliferative disorder with very high WBC, basophilia, and a low LAP score, plus the Philadelphia chromosome. Acute lymphoblastic leukemia involves malignant lymphoblasts, and hairy cell leukemia typically presents with cytopenias and splenomegaly rather than a marked reactive rise in WBC. Because leukemoid reaction represents a large WBC increase driven by a non-neoplastic, reactive process, it’s the best fit for this question.

A key idea here is telling apart a reactive rise in white blood cells from a malignant leukemia when the count is very high. A leukemoid reaction is a strong reactive neutrophilia caused by infection, inflammation, or severe stress. On a blood smear you often see a left shift with mature and immature neutrophils (bands, metamyelocytes) but no malignant clone, and the leukocyte alkaline phosphatase (LAP) score is usually elevated, supporting a reactive process rather than cancer.

In contrast, chronic myeloid leukemia is a clonal myeloproliferative disorder with very high WBC, basophilia, and a low LAP score, plus the Philadelphia chromosome. Acute lymphoblastic leukemia involves malignant lymphoblasts, and hairy cell leukemia typically presents with cytopenias and splenomegaly rather than a marked reactive rise in WBC. Because leukemoid reaction represents a large WBC increase driven by a non-neoplastic, reactive process, it’s the best fit for this question.