CNL is a rare myeloproliferative neoplasm characterized by a consistently elevated neutrophil count, usually over 25 × 109/L with a slight increase in immature cells. The average age of patients diagnosed with CNL is 67. Patients may experience the general MPN symptoms discussed earlier and also often present with hepatosplenomegaly.
Neutrophils account for 80% or more of the WBCs with less than 10% immature granulocytes and less than 1% blasts. Basophils and monocytes are not typically increased. The bone marrow is hypercellular with CNL. However, dysplasia and fibrosis are usually not present.
CSF3RT618I or other activating CSF3R mutation is a major diagnostic criterion; however, diagnosis is still permitted in the absence of this mutation if neutrophilia is present for three months with no identifiable cause or if another clonal finding is identified. CSF3R encodes the receptor for colony-stimulating factor 3, a cytokine that controls the production, differentiation, and function of granulocytes.