Table 1: The diagnosis of HLH is established by fulfilling one of the following two criteria.

1. A molecular diagnosis consistent with HLH (e.g., PRF mutations, SAP mutations)
Or

2. Having 5 of the following 8 signs or symptoms:

a) Fever

b) Splenomegaly

c) Cytopenia (affecting ≥ 2 cell lineages; hemoglobin ≤ 9 g/dL [or ≤ 10 g/dL for infants < 4 wk of age], platelets < 100,000/μL, neutrophils < 1,000/μL)

d) Hypertriglyceridemia (≥ 265 mg/dL) and/or hypofibrinogenemia (≤ 150 mg/dL)

e) Hemophagocytosis in the bone marrow, spleen, or lymph nodes without evidence of malignancy

f) Low or absent natural killer cell cytotoxicity

g) Hyperferritinemia (≥ 500 ng/mL)

h) Elevated soluble CD25 (interleukin-2Rα chain; ≥ 2,400 U/mL)