Bedside estimation of lean body weight for regional anaesthesia for children

We reported a simplified formula, based on BMI, for estimating lean body weight (LBW) in adults: [(100 - BMI)/100] × total body weight (TBW) for men and [(90 - BMI)/100] × TBW for women [1]. Obesity now affects children as well as adults and LBW is required increasingly for precise dosing of anaesthetics, antimicrobials and other drugs. Using a similar methodology, we examined whether this formula could be applied to children, comparing it with three established paediatric equations [2-4]. We generated a synthetic dataset of simulated individuals, constructed from the means, standard deviations and percentile distributions reported by Fryar et al. [5], comprising 9000 boys and 9000 girls (18,000 simulated individuals in total) aged 2–19 y. The simplified formula showed excellent agreement with the established equations, with the coefficient of determination (R2) ≥ 0.97 for both boys and girls (n = 9000 per sex). In boys, it achieved the highest R2 (0.995) and the lowest mean absolute error (MAE 0.59 kg) and mean squared error (MSE 1.70 kg2), outperforming the Foster formula [4] (R2 0.970, MAE 3.01 kg, MSE 17.39 kg2) and the Al-Sallami formula [2] (R2 0.991, MAE 1.40 kg, MSE 3.27 kg2). In girls, the simplified formula again performed best (R2 0.998, MAE 0.56 kg, MSE 0.57 kg2), with the Al-Sallami formula also performing well (R2 0.995, MAE 1.11 kg, MSE 1.99 kg2), whereas the Peters formula [3] showed notably larger errors (R2 0.993, MAE 4.55 kg, MSE 27.24 kg2). Overall, the simplified formula was the most accurate and consistent, particularly in reducing the large errors seen with the other methods. To illustrate agreement further, we compared LBW estimates from the simplified formula with those from the widely used Al-Sallami equation (Fig. 1). Most data points lay within ± 10% of the mean difference, indicating close agreement between the two approaches. These findings suggest that the simplified formula may be applied to children with performance comparable to that of the Al-Sallami equation, while avoiding more complex calculations. Because these results derive from simulated rather than measured data, prospective validation against a reference method for body composition is required before clinical use.