The diagnosis FHL can be established if at least 1 of either 1 or 2 below is fulfilled

A molecular diagnosis consistent with HLH

Diagnostic criteria for HLH fulfilled (5 of the 8 criteria below):

• fever

• splenomegaly

• cytopenias (affecting ≥2/3 lineages in the peripheral blood:

• hemoglobin <90 g/L; platelets <100 × 10⁹/L; neutrophils <1.0 × 10⁹/L [in infants aged <4 wk: hemoglobin <100 g/L])

• hypertriglyceridemia and/or hypofibrinogenemia: fasting triglycerides ≥3.0 mmol/L; fibrinogen ≤1.5 g/L

• hemophagocytosis

• low or absent NK-cell activity (according to local laboratory reference)

• ferritin ≥500 μg/L

• sCD25 (ie, soluble interleukin-2 receptor) ≥2400 U/mL