Abstract 4143723: Thrombocytosis is Prevalent and Associated with Greater Inflammation and Coronary Artery Involvement in Both Kawasaki Disease and Multisystem Inflammatory Syndrome in Children Associated with COVID-19

Introduction: Thrombocytosis, sometimes extreme, after acute Kawasaki disease (KD) is common and felt to be pathognomonic of this diagnosis, although has also been reported after multisystem inflammatory syndrome in children (MIS-C), a clinically similar condition. We sought to determine differences in factors associated with thrombocytosis for each condition. Methods: From 01/2020 to 10/2023 across 41 sites in 8 countries from the International KD Registry, 1674 MIS-C and 1290 contemporaneous KD patients with adequate laboratory data were included in the analysis. Age-related cutpoints (derived from the CALIPER Study of normal children/adolescents; AJCP 2020; 154:342) were applied to peak platelet counts to define thrombocytosis (age <1 year, >647 x10 9 /L; age 1 to <12 years, >434; age 12 to <21 years, >371). Associations of demographic, clinical, laboratory and outcome factors with thrombocytosis were determined for each diagnosis group. Results: Thrombocytosis was more prevalent after KD (57%) than MIS-C (49%; p<0.001), with higher median peak platelet count (492 vs. 413 x10 9 /L; p<0.001). The difference remained significant after adjustment for age. Thrombocytosis was significantly associated with greater total days of fever for both KD (median 8 vs. 7 days; p=0.005) and MIS-C (median 7 vs. 6 days; p<0.001), but not with any symptoms/signs or presentation with shock in either group. For both groups, thrombocytosis was associated with significantly greater laboratory abnormalities, specifically higher markers of inflammation, lower hemoglobin and lower albumin. Cardiac biomarkers (peak NTproBNP, peak troponin I) were significantly lower for KD patients with thrombocytosis. While there was no significant association of thrombocytosis with ventricular function in either group (FIGURE A), coronary artery involvement was worse for both groups if thrombocytosis occurred (FIGURE B), with an overall lower prevalence and severity for patients with MIS-C, despite more severe clinical presentation and greater laboratory abnormalities. Conclusions: Thrombocytosis has a high prevalence for both KD and MIS-C, and is associated with both more severe inflammation and greater coronary artery involvement for both diagnoses.