Table 1. Diagnostic criteria for MAS, principal features of severe KD, and clinical and laboratory data of our patient

Diagnostic criteria for MAS [5,18]	Severe KD (i.e., KDSS or refractory KD) [6,13]	13-year-old boy in this study
Persistent fever	Fever unresponsive to IVIG treatment	Persistent fever despite 2nd IVIG treatment
Splenomegaly	Splenomegaly	Hepatosplenomegaly
Cytopenia in ≥ 2 cell lines1)	Platelets ≤ 300,000/μL2)	Leukocytes, 3,930 /μL; platelets, 59,000 /μL
(Elevated AST or ALT as supportive evidence)	ALT ≥ 80 U/L2)	ALT, 97 U/L
	CRP ≥ 8 mg/dL2)	CRP, 247 mg/dL
Triglyceride > 265 mg/dL or fibrinogen < 150 mg/dL1)	Dyslipidemia	Triglyceride, 338 mg/dL
Ferritin > 500 ng/mL1)	Ferritin > 144 ng/mL	Ferritin, 790 ng/mL
Soluble IL-2 receptor (sCD25) >2,400 U/mL	High IL-1β and IL-18	INR, 1.37 (normal 0.85–1.13)
Decrease or absent natural killer cell activity	NA	NA
Hemophagocytosis in BM, liver, or lymph nodes (Organ dysfunction as a principal feature)	4.8% in KDSS and 6.3% in refractory KD [00]	NA
Hematologic, hepatic, and neurologic symptoms: commonCardiac, respiratory, and renal dysfunctions in severe MAS	In KDSS [3]: cardiac (100.0%), respiratory (50.0%), hematologic (54.8%), renal or neurologic (18.4%), gastrointestinal (74.6%), and dermatologic (76.9%)	Shock requiring dopamine infusion, coagulopathy (elevated INR), hepatic (elevated ALT), and ICU admission

The 2016 consensus criteria for MAS complicating sJIA [18] have different cut-off values: hyperferritinemia (ferritin > 684 ng/mL) plus any two of four criteria (Platelets ≤ 181,000 /μL, AST > 48 IU/L, triglycerides > 156 mg/dL or fibrinogen ≤ 360 mg/dL).
Egami’s scoring system for refractory KD [6].
MAS: macrophage activation syndrome; KD: Kawasaki disease; KDSS: Kawasaki disease shock syndrome; IVIG: intravenous immunoglobulin; AST: aspartate transaminase; ALT: alanine transaminase; CRP: C-reactive protein; IL: interleukin; BM: bone marrow; INR: international normalized ratio; NA: not available; ICU: intensive care unit; sJIA: systemic juvenile idiopathic arthritis.