Chronic Neutrophilic Leukemia with Monocytosis.

Chronic neutrophilic leukaemia is a very rare disease with diagnosis based on persistent leucocytosis >25×103/μl and monocytes <1×103/μl. The revised WHO criteria 2016 included CSF3R gene mutations as a diagnostic finding. We report the case of a 77-year-old man who was found to have asymptomatic persistent mature neutrophilic leucocytosis with monocytosis. Molecular study confirmed the presence of a CSF3R gene mutation in the absence of morphological or genetic features of myelodysplasia or other forms of myelodysplastic syndrome. The patient's medical history was significant for coronary artery disease, hypertension, chronic obstructive pulmonary disease, bilateral cystic bronchiectasis, moderate pulmonary hypertension, tuberculosis treated 27 years previously, hypothyroidism, and a thyroid nodule. He had hepatosplenomegaly but no lymphadenopathy, and no other malignancy was seen on computed tomography (CT) scanning. At the time of evaluation, he was free of symptoms and had no evidence of infection or drug-induced leucocytosis. The patient was referred to an oncology centre and treated with hydroxyurea and subsequently azacitidine. However, he developed pancytopenia with bone marrow aplasia. He died with neutropenia sepsis. The presence of persistent monocytosis in this case created a diagnostic dilemma as to whether the disease was a variant of chronic neutrophilic leukaemia or was reactive monocytosis. LEARNING POINTS: The presence of a CSF3R gene mutation is diagnostic for chronic neutrophilic leukaemia (CNL).The monocytosis in this patient might have been a new variant of CNL. © EFIM 2021.