The clinical features and laboratory findings in some patients with incomplete KD and other systemic inflammatory diseases such as macrophage activation syndrome or sJIA overlap. No cytopenia, hypertriglyceridemia and hypofibrinogenemia were found in his clinical course (serum triglyceride level of 217 mg/dL and plasma fibrinogen level of 323 mg/dL). There was no elevation of serum soluble interleukin-2 receptor level (344 U/mL). Based on these findings, because hemophagocytic lymphohistiocytosis was not suspected, a bone marrow examination was not performed for him. However, it has been reported that the patients who were considered as having refractory KD were finally diagnosed with sJIA [
13]. Therefore, some serum markers have been proposed to distinguish KD from sJIA. Mizuta et al. reported that the serum ferritin level was significantly higher in sJIA patients than in KD patients, for which the cutoff value was 368.6 ng/mL [
11]. In cytokines, the characteristics of serum markers in sJIA patients include significantly higher levels of IL-18 and lower levels of IL-6 than those in KD patients [
6,
7,
9,
10]. In our case, because extremely high serum ferritin and IL-18 levels, unelevated IL-6 levels, and arthritic symptoms were observed during the clinical course (Table
1), sJIA was suspected. However, we finally diagnosed the patient with incomplete KD accompanied with arthritis, because of the following reasons. First, perivascular echo brightness of the coronary arteries was found. Second, a periungual desquamation was observed during the recovery phase (Fig.
1). Third, his arthritis improved within 6 weeks, and the sJIA criteria were not completely fulfilled [
14]. Fourth, the IVIG treatment was effective. Fifth, the disease improved during PSL treatment, and no relapse was found after the dose of PSL was tapered. Finally, his clinical symptoms were different from those of sJIA, such as flares of clinical signs and intermittent fever with rash. He looked so sick and agitated during the disease period, although patients with sJIA look almost normal during the intermittent afebrile period.
In conclusion, patients with sJIA generally have high serum IL-18 and ferritin levels [
6‐
11]. This was a case of incomplete KD with extremely high serum levels of IL-18 and ferritin, although KD patients with coronary artery lesions have been reported to have mild IL-18 elevations [
15]. Serum cytokines and ferritin are often used for the differential diagnosis of KD and sJIA. However, we should need to recognize the existence of KD in patients with high serum IL-18 and ferritin levels. Further studies are needed about the novel biomarkers to clearly distinguish sJIA from KD at an early phase of the disease progression.