Clinical background
In weight-based drug dosing and in setting up a ventilator, the choice of which body weight to base the calculation on is not trivial. Actual body weight includes fat mass, which in patients with obesity may make up a substantial proportion of the total without corresponding to any increase in the pharmacokinetically relevant tissue. Dosing by actual body weight then risks overdosing, particularly for drugs with a narrow therapeutic window. At the same time, dosing by ideal body weight alone can produce subtherapeutic levels, since it underestimates the actual volume of distribution in the obese patient [2].
The instrument in this calculator serves two purposes: to calculate the ideal body weight by the Devine formula for use where lung size or renal function is to be estimated from height rather than weight, and to calculate an adjusted body weight that compromises between ideal and actual body weight for dosing in obesity.
Calculating ideal and adjusted body weight
The ideal body weight by the Devine formula is calculated from sex and height as:
For patients shorter than 60 inches (152 cm), the bracket is set to zero, that is, the ideal body weight becomes the base weight of 50 kg or 45.5 kg respectively.
The adjusted body weight is calculated as:
The factor 0.4 means that 40 per cent of the difference between actual weight and ideal body weight is added to the ideal body weight. This is an empirically chosen correction factor intended to take partial account of the increased volume of distribution in obese patients without dosing fully by total body weight.
The Devine formula was published in 1974 and derived for aminoglycoside dosing. It rests on observations of the relationship between height and ideal body weight in adults and has since become the most widely used method for estimating ideal body weight in clinical practice, even though it never underwent formal validation against measured body composition in a modern cohort.
Interpretation in practice
| Situation | Weight to use | Comment |
|---|---|---|
| Lung-protective ventilation in ARDS | Ideal body weight | The tidal volume is set at 6 mL/kg ideal body weight. Lung size follows height, not fat mass [3]. |
| Estimating renal function (Cockcroft-Gault) | Ideal body weight at normal weight, adjusted body weight in obesity | For patients with a BMI above 30, adjusted body weight is often recommended to avoid overestimating creatinine clearance. |
| Dosing drugs with a narrow therapeutic window | Adjusted body weight in obesity | Aminoglycosides, vancomycin and cytotoxic drugs are typical examples where dosing by actual body weight risks toxicity. |
| Induction of anaesthesia | Ideal body weight or lean body weight | Propofol and opioids are best dosed by lean body weight, which is approximated by ideal body weight in patients without extreme obesity [2]. |
When ideal body weight is used for ventilator settings, the patient's height must be measured, not estimated. An incorrect estimate of height is one of the commonest sources of error in clinical practice and leads directly to an incorrect tidal volume. If the patient cannot stand, the height should be measured with a tape measure in the supine position (heel to crown) or derived from the knee height.
Validation and performance
The Devine formula has not been validated in the modern sense against reference methods for body composition in large cohorts. It rests on an assumption of a linear relationship between height and ideal body weight that does not necessarily hold for extremely short or extremely tall individuals. Nor does the formula give any information about the distribution of body composition between fat-free mass and fat mass, which is the pharmacokinetically decisive difference in obesity.
In a review of size descriptors for pharmacokinetic studies in obese patients, Green and Duffull found that total body weight was the best descriptor of volume of distribution, while fat-free (lean) body weight was the best descriptor of clearance [1]. No single size descriptor described both clearance and volume of distribution satisfactorily. For chronic drug dosing, where clearance is of primary importance, the authors recommended that dosing in obese patients should not be based on total body weight but rather on fat-free mass [1]. Adjusted body weight is a pragmatic approximation of that principle, but it has not been validated against measured fat-free mass in controlled studies.
For lung-protective ventilation, the recommendation of 6 mL/kg ideal body weight rests on the large randomised ARDS trial (ARMA, 2000), which showed reduced mortality with a low tidal volume compared with a traditional volume. In a 2017 review, Sahetya et al. point out that the ideal body weight approach is imperfect, since the aerated lung volume varies considerably between patients of the same height because of differences in inflammation, consolidation, oedema and atelectasis [3]. Despite this, 6 mL/kg ideal body weight remains the recommended strategy in current guidelines, and better methods for individual titration of tidal volume, such as driving pressure and transpulmonary pressure, have not yet replaced it in broad clinical practice [3].
Limitations
The Devine formula applies to adults. It is not validated for children and should not be used in patients under 18 years, in whom the height-weight relationship is different and age-dependent.
The formula presupposes that the patient's height is known and correct. In emergencies, particularly in intensive care, a measured height is often unavailable and is replaced by estimates that may be off by 5 to 10 cm, giving an incorrect ideal body weight and hence an incorrect dose or tidal volume.
Adjusted body weight with a correction factor of 0.4 is a compromise that lacks strong empirical support. The factor has not been validated in large prospective studies and may lead to both over- and underdosing depending on the drug's volume of distribution, lipophilicity and mechanism of clearance. For lipophilic drugs, where fat mass constitutes a substantial volume of distribution, adjusted body weight may underestimate the actual volume of distribution. For hydrophilic drugs, where fat mass is pharmacokinetically less relevant, adjusted body weight may still overestimate the relevant volume of distribution [1].
For patients with extreme obesity (BMI above 50), the difference between ideal and actual body weight becomes so large that even adjusted body weight can give unreasonable doses. In these cases, dosing should be individualised on the basis of therapeutic drug monitoring where available, rather than relying on a formula-based weight.
The instrument should not be used in patients with significant amputations, since height does not reflect the remaining body mass. In patients with muscular dystrophy or other conditions with markedly reduced muscle mass, the ideal body weight overestimates the pharmacokinetically relevant tissue.
References
- Green B, Duffull SB. What is the best size descriptor to use for pharmacokinetic studies in the obese? Br J Clin Pharmacol 2004;58(2):119–133. PMID: 15255794
- Ingrande J, Lemmens HJ. Dose adjustment of anaesthetics in the morbidly obese. Br J Anaesth 2010;105 Suppl 1:i16–23. PMID: 21148651
- Sahetya SK, Mancebo J, Brower RG. Fifty Years of Research in ARDS. Vt Selection in Acute Respiratory Distress Syndrome. Am J Respir Crit Care Med 2017;196(12):1519–1525. PMID: 28930639
