Characteristics of 21 Patients with Secondary Hemophagocytic Lymphohistiocytosis—Insights from a Single-Center Retrospective Study

<i>Background and Objectives</i>: Hemophagocytic lymphohistiocytosis (HLH) is a rare hyperinflammatory condition characterized by excessive activation of cytotoxic lymphocytes and macrophages, resulting in a cytokine storm, multiorgan damage, and high mortality. HLH is classified into primary (genetic) and secondary (acquired) forms, with diagnosis often challenging due to nonspecific symptoms. Macrophage activation syndrome (MAS) refers to the secondary HLH triggered by rheumatic diseases. In this study, we retrospectively analyzed the clinical and laboratory features of patients with secondary HLH to enhance understanding of this life-threatening condition and summarize emerging management strategies. <i>Materials and Methods:</i> This single-center retrospective study analyzed medical records of patients hospitalized with HLH at the University Hospital in Kraków, Poland, from 2013 to 2024, based on HLH-2009 criteria and HScore > 169 points. Diagnostic criteria included clinical, laboratory, and histological findings, e.g., hemophagocytosis in bone marrow, circulating cytopenia, and elevated ferritin levels. <i>Results</i>: A total of 21 patients met the criteria for HLH diagnosis, with a median age of 35 (range: 19-67) years, including 12 women (57.1%). The median HScore among the patients was 244 (range: 208-304) points. Fever was the most common presenting symptom, occurring in all cases. High ferritin, hypertriglyceridemia, and hypofibrinogenemia in peripheral blood were also prevalent. Bone marrow hemophagocytosis was confirmed in 66.7% of cases (<i>n</i> = 12/18 of available data). Regarding immunosuppressive therapy, glucocorticosteroids were the most frequently used (used in all cases). Four (19.0%) patients died during HLH (cases triggered by lymphoma [twice], Epstein-Barr virus infection, unknown reason). Compared to survivors, these patients had lower counts of white blood cells, neutrophils, and lymphocytes at diagnosis (<i>p</i> < 0.05 for all). <i>Conclusions</i>: Secondary HLH is a severe syndrome requiring rapid diagnosis and timely intervention to improve patient outcomes. Lower white blood cell, neutrophil, and lymphocyte counts present worse prognostic factors.