Introduction

Multiple myeloma (MM) is a hematological malignancy that occurs due to the uncontrolled proliferation of plasma cells in the bones. High-dose chemotherapy with subsequent autologous stem cell transplant (ASCT) is the standard treatment for patients with transplant-eligible MM [1]. Hematopoietic stem cell (HSC) transplantation, with both allogeneic stem cell transplantation, introduces strong proliferation stimuli for the rapid expansion of the HSCs in the recipient's bone marrow. Additionally, the engraftment of the bone marrow by the HSCs creates a proinflammatory state. The combination of proliferation and hyperinflammation can trigger the amplification of cells with neoplastic abilities, causing clonal hematopoiesis (CH). According to Wilk et al., the frequency of CH in MM could be 21.6% [2].

The two most important causes of neutrophilia with WBC >50 K/uL are leukemoid reaction and hematologic neoplasms. Leukemoid reaction is marked by leucocytosis with "left" shift without any blasts, monocytosis, or basophilia and the absence of breakpoint cluster region-Abelson (BCR-ABL) translocation. It is diagnosed after one has excluded myeloproliferative neoplasms (MPNs) [3]. While leukemoid reaction secondary to infections can occur in MM, ruling out neoplastic causes is important. This is necessary to provide the indicated treatment (i.e., antimicrobials for infection versus oncologic treatment for neoplasms). A neoplastic cause for neutrophilia could be chronic neutrophilic leukemia (CNL). CNL has been defined as a BCR::ABL1-negative MPN characterized by the sustained presence of mature neutrophils in the peripheral smear in the setting of bone marrow granulocyte hyperplasia. As per the World Health Organization (WHO) in 2022, CNL can be diagnosed in a patient exhibiting leukocytosis ≥25 × 109/L with ≥80% neutrophils and <10% circulating precursors, no evidence of dysplasia, and concurrence of CSF3R mutation. Patients with CNL are commonly asymptomatic but could present with constitutional symptoms, such as fatigue, easy bruising, itching, or painful joint swelling secondary to gout. Physical examination may reveal splenomegaly in ~67% of cases, hepatomegaly, or lymphadenopathy [4].

As per our literature review, there have been a few case reports discussing the co-existence of CNL and MM. We will present a case of CNL following ASCT in MM and in remission for ~18 years.