- Patients must meet the diagnostic criteria for HPS (at least 5 of the following): fever, splenomegaly, cytopenia involving ≥2 cell lines (Hemoglobin <9 g/dL; platelets <100,000/μL; absolute neutrophil count <1000/μL), hypertriglyceridemia or hypofibrinogenemia, tissue demonstration of hemophagocytosis, low or absent NK (Natural Killer) cell activity, serum ferritin ≥3000 ug/L, soluble IL-2 receptor (CD25) >2400 U/mL.