A Rare Case of Chronic Myelomonocytic Leukemia in a Patient With Sarcoidosis: Diagnostic and Immunologic Challenges.

The coexistence of sarcoidosis and chronic myelomonocytic leukemia (CMML) is exceedingly rare and poses significant diagnostic challenges due to overlapping clinical, radiologic, and immunologic features. Sarcoidosis, a systemic granulomatous disorder driven by immune dysregulation, often manifests with lymphadenopathy, organomegaly, and elevated inflammatory biomarkers, while CMML is a clonal myeloid neoplasm characterized by monocytic proliferation and cytopenias. We report a case of CMML-1 in a 48-year-old man with a 14-year history of sarcoidosis, presenting with progressive dyspnea, and pancytopenia. Laboratory tests revealed unusual elevations in key immune markers, prompting further investigation. This case highlights the diagnostic pitfalls in distinguishing between granulomatous disease and clonal hematologic malignancy in patients with longstanding sarcoidosis. It underscores the importance of maintaining a high index of suspicion for myeloid neoplasms when unexplained cytopenias occur and emphasizes the emerging diagnostic utility of biomarkers such as sIL-2R in differentiating chronic immune activation from underlying clonal hematopoiesis.