What does Neutrophilic (neutrophil white blood cell) leukocytosis (elevated white blood cell count) and marked thrombocytosis (excessive platelet count) on a blood smear indicate?

From the Guidelines

Neutrophilic leukocytosis and marked thrombocytosis on a blood smear typically indicate a significant physiological response to an underlying condition, which could be an inflammatory or infectious process, a myeloproliferative disorder, or another malignancy. This combination suggests an active immune response, with neutrophils increasing to fight infection and platelets elevating as part of the acute phase reaction. Common causes include bacterial infections (particularly pneumonia, urinary tract infections, or abscesses), tissue damage from surgery or trauma, inflammatory conditions like rheumatoid arthritis or inflammatory bowel disease, and certain malignancies, especially myeloproliferative disorders such as essential thrombocythemia (ET) or polycythemia vera (PV) 1. In some cases, this pattern may represent a leukemoid reaction, which mimics leukemia but is triggered by severe infection or inflammation.

Key Considerations

• The finding warrants further investigation including complete blood count with differential, inflammatory markers like C-reactive protein and erythrocyte sedimentation rate, blood cultures if infection is suspected, and possibly bone marrow examination if a hematologic malignancy is considered 1.
• The presence of a condition associated with reactive thrombocytosis does not exclude the possibility of a myeloproliferative disorder if other diagnostic criteria are met 1.
• Clinical correlation is required to determine the underlying cause of neutrophilic leukocytosis and marked thrombocytosis, as this blood picture represents a significant physiological response to an underlying condition rather than a diagnosis itself.

Diagnostic Approach

• Evaluate for signs and symptoms of infection, inflammation, or malignancy.
• Perform a complete blood count with differential to assess the degree of leukocytosis and thrombocytosis.
• Consider bone marrow examination if a hematologic malignancy is suspected.
• Assess for the presence of other diagnostic criteria for myeloproliferative disorders, such as the JAK2 V617F mutation or other clonal markers 1.

From the Research

Neutrophilic Leukocytosis and Marked Thrombocytosis

• Neutrophilic leukocytosis refers to an elevated count of neutrophil white blood cells, which can be a reactive response to infection, inflammation, or other stressors 2.
• Marked thrombocytosis refers to an excessive count of platelets in the blood, which can be associated with myeloproliferative neoplasms (MPN) or other conditions 3.
• The combination of neutrophilic leukocytosis and marked thrombocytosis on a blood smear may indicate a myeloproliferative process, but clinical correlation is required to determine the underlying cause 4, 3.

Diagnostic Considerations

• A thorough diagnostic workup is necessary to distinguish between reactive and neoplastic causes of leukocytosis and thrombocytosis 3, 5.
• Examination of the peripheral blood smear, bone marrow biopsy, and genetic testing may be necessary to confirm a diagnosis of MPN or other conditions 3, 5.
• The presence of other abnormalities, such as anemia or splenomegaly, may also be relevant in determining the underlying cause of leukocytosis and thrombocytosis 2.

Clinical Implications

• Elevated white blood cell counts and platelet counts can increase the risk of thrombosis, particularly in patients with MPN 4, 6.
• However, the presence of leukocytosis and thrombocytosis does not necessarily indicate a malignant process, and reactive causes shouldn't be ruled out 2.
• Clinical correlation and further testing are necessary to determine the underlying cause of leukocytosis and thrombocytosis and to guide treatment decisions 3, 5.