In adults presenting with incidental leukocytosis and no symptoms, which diagnosis is most likely?

• A. Acute myeloid leukemia
• B. Hairy cell leukemia
• C. Chronic Lymphocytic Leukemia
• D. Acute lymphoblastic leukemia

Answer: (C)

Incidental leukocytosis in an older adult without symptoms is most compatible with chronic lymphocytic leukemia. This disease often unfolds quietly, as a clonal proliferation of mature B lymphocytes that accumulates over time, so many patients are asymptomatic when first detected. The CBC typically shows a marked increase in lymphocytes (lymphocytosis) rather than a broad neutrophil-dominant elevation, which fits an incidental finding on routine testing. Visualized on a smear, these cells may appear mature with fragile “smudge” cells, and patients may later develop painless lymphadenopathy or splenomegaly; autoimmune phenomena can also occur. Confirmation comes from immunophenotyping showing a B-cell population expressing markers such as CD5 and CD23, consistent with CLL.

In contrast, acute myeloid leukemia and acute lymphoblastic leukemia usually present with abrupt, symptoms-driven illness—fatigue, fever, infections, easy bruising or bleeding—reflecting bone marrow failure, and are characterized by blasts on the smear. Hairy cell leukemia tends to cause cytopenias with splenomegaly and a low or normal WBC count rather than isolated leukocytosis, making incidental high WBC less likely.