For patients with fever of unknown etiology and lymphadenopathy, after excluding HLH or MAS, serum IFN-γ > 8.56 pg/mL and IFN-γ/IL-6 ratio > 0.45 may highly suggest the diagnosis of KFD; serum IL-6 > 50.45 pg/mL indicates that the probability of KFD may be small, and sJIA, KD, and acute infection should be excluded first. This evidence concerns the gene IL6 and macrophage activation syndrome.