Practical Estimation of Ideal Body Weight and Normalized Lean Weight
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Practical Estimation of Ideal Body Weight and Normalized Lean Weight John H P Friesen 1
# Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Simple formulas for estimating ideal body weight (IBW) and normalized lean weight (NLW) are proposed here. The equations are as follows: IBW ¼ W 22=BMI pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi NLW ¼ W 22=BMI where W is equal to total patient weight in kilograms and BMI is the body mass index in units of kg/m2. The calculated values are equivalent for practical purposes to previously published formulas. Keywords Ideal body weight . Normalized lean weight . Obesity . Drug dosage Safe and effective doses for most drugs are larger for patients who weigh more. Nonetheless, simply increasing the dose for patients with obesity in proportion to total body weight (TBW) is risk to overdose [1]. The IBW is a function of height alone and does not reflect the changes in body composition that occur in individuals with obesity. It is not itself a good candidate weight scalar [2], but it is useful in order to calculate the adjusted body weight (ABW)—the IBW plus a fraction of the difference between the IBW and the TBW [3]. Historically, several complicated formulas have been used to estimate the IBW, but a much simpler equation proposed by Lemmens et al. [4] closely approximates the mean of their calculated values. It is derived by substituting a nominal value of 22 kg/m2 into the BMI Equation (BMI = W/H2), resulting in a value for IBW of 22 × H2 where H is patient height in meters. The formula presented here is mathematically identical: it is the result of solving the BMI equation for H and substituting into the Lemmens IBW formula. Theoretical arguments and experimental data both suggest that for some drugs, doses tend to scale in proportion to lean
* John H P Friesen [email protected] 1
Department of Anesthesiology, Perioperative and Pain Medicine, University of Manitoba, 2nd Floor, Harry Medovy House, 671 William Avenue, Winnipeg, MB R3E 0Z2, Canada
body weight (LBW) [5]. LBW cannot function as a weight scalar because it does not correspond to TBW for patients of normal weight [6] but it can be normalized to IBW, resulting in the normalized lean weight (NLW), a weight scalar that is proportional to LBW and so is appropriate for those drug doses or pharmacokinetic parameters that are expected to scale in proportion to LBW [7]. It is possible to measure LBW, but for clinical purposes, it is almost always estimated by means of the formula published by Janmahasatian et al. [8]: LBW = a × TBW/(b + c × BMI) where the empirically determined parameters differ for male and female patients. To obtain the NLW, the LBW is multiplied by a normalization factor (1.23 for males or 1.53 for females) which is determined by substituting a nominal BMI of 22 kg/m 2 into the Janmahasatian LBW equation [9]. The NLW calculated in this way is a clinically useful weight scalar, but it is mathematically too complicated to be used routinely when estimating doses of drugs. It
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