Distinguishing Incomplete Kawasaki and Nonsevere Multisystem Inflammatory Syndrome in Children

OBJECTIVES Characterizing inflammatory syndromes during the coronavirus disease 2019 pandemic was complicated by recognition of multisystem inflammatory syndrome in children (MIS-C), contemporaneous with episodes of Kawasaki disease. We hypothesized a substantial overlap between the 2 and assessed the performance of an MIS-C likelihood score in differentiating inpatients with nonsevere MIS-C from prepandemic incomplete Kawasaki disease (iKD) without coronary involvement. METHODS A retrospective review of inpatient records was conducted; the nonsevere MIS-C cohort (March 2020–February 2021) met the 2023 definition for MIS-C; the iKD cohort (January 2018–January 2019) met the American Heart Association criteria for iKD without coronary involvement. We applied the likelihood score to both cohorts. We estimated the percent of children with iKD who could have met the clinical criteria of the MIS-C, had they presented in 2023. RESULTS The 68 children in the nonsevere MIS-C cohort were older (8 vs 4 years, P < .001) than the 28 children in the iKD cohort. Those in the nonsevere MIS-C cohort had higher rates of thrombocytopenia (P < .001) and lymphopenia (P = .021); those in the iKD cohort had higher rates of pyuria (P < .001). Twenty-four (86%) children in the iKD cohort met the 2023 MIS-C definition. The scoring system correctly predicted 71% to 74% children with their respective clinical diagnoses. CONCLUSIONS Though there was considerable clinical overlap, thrombocytopenia, lymphopenia, and the absence of pyuria were the most helpful parameters to distinguish children with nonsevere MIS-C from those with iKD.