Limitations of a standardized weight-based nomogram for heparin dosing in patients with morbid obesity

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 >= 12 hours. The exclusion criteria were age <18 years, pregnancy, and insufficient data. Patients were stratified into morbidly obese (body mass index [BMI] >= 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. (Surg Obes Relat Dis 2008;4:748-753.) (C) 2008 American Society for Metabolic and Bariatric Surgery. All rights reserved.