Using the Mercy Method to Estimate Ideal Body Weight in Children: A Description and Validation of a New Methodology

Background In obese children, lipophilic medications should be dosed to total body weight (TBW) and hydrophilic medications to ideal body weight (IBW). During emergencies, these weights need to be estimated to ensure that urgent drug therapy is accurate and safe. The Mercy method is a well-established weight estimation system that has recently been adapted to provide estimations of body length in children. It was therefore conceivable that this could be further modified to provide estimations of IBW. Methods A model was developed a priori using the Mercy method’s humeral length (HL) segments to predict IBW. The accuracy of this model was then tested in a sample of 13,134 children from the National Health and Nutrition Examination Survey (NHANES) datasets. The accuracy of IBW estimation was determined from the percentage of estimations falling within 10% (p10) and 20% (p20) of true IBW. The model was also tested to see the accuracy of the detection of obesity in the study sample. Results From the sample of 13,134 children, a subset of 1,318 obese children were identified. In this subset, the new Mercy method model achieved an IBW estimation accuracy p10 of 66.9% and a p20 of 95.1%. For the detection of the obese child, the model had a sensitivity of 88.6% and a specificity of 75.8%. Conclusions This study established that the Mercy method can be modified to provide a reasonably accurate estimation of IBW in obese children, with very few critical errors. The ability of the model to identify the obese child was also reasonably accurate, on a par with other such predictive methods. While other accurate methods of estimating both TBW and IBW exist, such as the Paediatric Advanced Weight Prediction in the Emergency Room eXtra-Long tape (PAWPER XL tape), the modified Mercy method is an acceptable alternative if such other devices are not available.