Leukemoid reactions and acute leukemia can both present with fever and elevated white blood cell counts, but there are clear differences between the two.

Leukemoid reactions are often caused by severe bacterial or viral infections, with the main symptom being fever. Blood tests may show an increase in white blood cells, primarily neutrophils, and peripheral blood smear can reveal myelocytes and metamyelocytes.

Leukemia can also involve an increase in white blood cells and fever when there is a concurrent infection, but blood tests will also show reductions in hemoglobin and platelets. Neutrophil alkaline phosphatase score is significantly elevated in patients with leukemoid reactions, while it is generally normal in patients with leukemia. A bone marrow examination can easily differentiate between leukemoid reactions and leukemia.