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Objectives: Severe obesity (SO) prevalence varies between reference curve-based definitions (WHO: =99th percentile, Centers for Disease Control and Prevention (CDC): >1.2×95th percentile). Whether SO definitions differentially predict cardiometabolic disease risk is critical for proper clinical care and management but is unknown.

Design: Prospective cohort study SETTING: SO definitions were applied at baseline (2005-2008, M_age=9.6 years, n=548), and outcomes (fasting lipids, glucose, homoeostatic model assessment (HOMA-IR) and blood pressure) were assessed at first follow-up (F1: 2008-2011, M_age=11.6 years) and second follow-up (2015-2017, M_age=16.8 years) of the Quebec Adipose and Lifestyle Investigation in Youth cohort in Montreal, Quebec.

Participants: Respondents were youth who had at least one biological parent with obesity.

Primary outcome measures: Unfavourable cardiometabolic levels of fasting blood glucose (=6.1 mmol/L), insulin resistance (HOMA-IR index =2.0), high-density lipoprotein <1.03 mmol/L, low-density lipoprotein =2.6 mmol/L and triglycerides <underline>></underline>1.24 mmol/L. Unfavourable blood pressure was defined as =90th percentile for age-adjusted, sex-adjusted and height-adjusted systolic or diastolic blood pressure.

Analysis: Area under the receiver operating characteristic curve (AUC) and McFadden psuedo R² for predicting F1 or F2 unfavourable cardiometabolic levels from baseline SO definitions were calculated. Agreement was assessed with kappas.

Results: Baseline SO prevalence differed (WHO: 18%, CDC: 6.7%). AUCs ranged from 0.52 to 0.77, with fair agreement (kappa=37%-55%). WHO-SO AUCs for detecting unfavourable HOMA-IR (AUC>0.67) and high-density lipoprotein (AUC>0.59) at F1 were statistically superior than CDC-SO (AUC>0.59 and 0.53, respectively; p<0.05). Only HOMA-IR and the presence of more than three risk factors had acceptable model fit. WHO-SO was not more predictive than WHO-obesity, but CDC-SO was statistically inferior to CDC-obesity.

Conclusion: WHO-SO is statistically superior at predicting cardiometabolic risk than CDC-SO. However, as most AUCs were generally uninformative, and obesity definitions were the same if not better than SO, the improvement may not be clinically meaningful.