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Background: Confusion exists when dosing heparin using a weight-based nomogram in the obese population. At 2 affiliated community teaching hospitals, we compared the activated partial thromboplastin time (aPTT) values in morbidly obese and nonmorbidly obese patients using a standardized nomogram and determined factors associated with achieving a supratherapeutic aPTT value., Methods: This was a retrospective study that included patients who had received intravenous heparin according to a standardized weight-based nomogram for >or=12 hours. The exclusion criteria were age <18 years, pregnancy, and insufficient data. Patients were stratified into morbidly obese (body mass index [BMI] >or=40 kg/m(2)) and nonmorbidly obese (BMI <40 kg/m(2)) groups. The aPTT values were compared and predictors for a supratherapeutic aPTT values were identified., Results: A total of 101 patients were included in the study. Greater aPTT values were noted at 6 hours (155 +/- 37 versus 135 +/- 44, P = .020) and 12 hours (141 +/- 45 versus 117 +/- 45, P = .012) for patients with morbid obesity than for those without it, respectively. Increasing BMI (odds ratio = 1.06, 95% confidence interval 1.02-1.1; P = .003) and age (odds ratio 1.05, 95% confidence interval 1.02-.09; P = .001] were independent predictors of supratherapeutic aPTT values., Conclusion: Heparin dosing with a weight-based nomogram will yield greater aPTT values in morbidly obese patients. Consideration of BMI and age can help identify those patients at risk of supratherapeutic aPTTs. Alternative strategies, such as a dose cap should be considered in patients with morbid obesity.