ESR1 and macrophage activation syndrome: In the present study, we included ferritin level in the primary laboratory test to differentiate between KD with and without MAS early, but considering low incidence of KD-MAS, we recommend the ferritin test in KD patients with hepatosplenomegaly, abnormal results of biomarkers (low ESR/fibrinogen, high total and bilirubin/triglyceride/LDH), or refractoriness of initial IVIG treatment.