Incomplete Kawasaki Disease Versus Non-severe Multisystem Inflammatory Syndrome in Children: Distinguishing Features from Contemporaneous Patients.

Kawasaki disease (KD) and multisystem inflammatory syndrome in children (MIS-C) share overlapping clinical features, making diagnosis challenging. We compared patients with incomplete KD to those with non-severe MIS-C to identify distinguishing characteristics. Patients were enrolled in the International KD Registry (01/2020-10/2023) from 40 centers across eight countries. Among 2146 MIS-C patients, 769 met criteria for non-severe confirmed MIS-C (no shock or ICU admission). Among 1358 KD patients, 146 met American Heart Association (AHA) criteria for confirmed incomplete KD and 372 were classified as unconfirmed incomplete KD. Demographics, clinical and laboratory features, treatment, and cardiac outcomes were compared. Non-severe MIS-C patients were significantly older (median 7.4 years) than both confirmed (2.3 years) and unconfirmed (2.5 years; p < 0.001) incomplete KD patients, and had different ethnic distributions with more Black and fewer East or South Asian patients. MIS-C patients had shorter fever duration, fewer classic KD features, and more gastrointestinal and respiratory symptoms. They demonstrated lower white blood cell counts but higher inflammatory markers and markers of organ dysfunction. Cardiac involvement differed: MIS-C patients had more myocardial dysfunction, including lower ejection fraction and higher cardiac biomarkers. Coronary artery involvement in MIS-C (median maximum z-score 1.3) was similar to unconfirmed incomplete KD (median z-score 1.3) but less than confirmed incomplete KD (median z-score 2.0; p < 0.01). Older age, distinct ethnicity, prominent gastrointestinal and respiratory symptoms, and multisystem inflammation with ventricular dysfunction but relatively less coronary artery involvement help distinguish non-severe MIS-C from incomplete KD.