Reversible Neutrophilia Driven by Monoclonal Gammopathy of Clinical Significance Simulating Chronic Neutrophilic Leukemia: A Case Report.

We encountered a 51-year-old male with a family history of acute myeloid leukemia who presented with persistent neutrophilia and splenomegaly progressing over eight years. Despite extensive workup, no myeloid driver mutation was identified and repeated bone marrow biopsies revealed no clonal myeloid disorder. However, an IgA kappa plasma cell clone was detected, consistent with monoclonal gammopathy of clinical significance (MGCS). Reactive neutrophilia secondary to the abnormal plasma cell clone was suspected, hypothesized to be mediated by paraneoplastic granulocyte colony-stimulating factor (G-CSF) secretion. G-CSF levels were elevated before treatment, supporting the hypothesis that it might induce neutrophilia. The patient received antimyeloma therapy and exhibited significant hematologic improvement: reduction in neutrophil count, normalization of G-CSF levels, and improvement in splenomegaly. This is the first report to describe improvement of neutrophilia following antimyeloma therapy, providing novel evidence that MGCS could simulate chronic neutrophilic leukemia (CNL).