Adjusted body weight (AdjBW₀.₄)

Calculate adjusted body weight, the weight used for hydrophilic drugs such as aminoglycosides in obese patients.

What is this for?

Adjusted body weight sits between ideal and total body weight. It reflects the observation that adipose tissue is not pharmacokinetically inert — hydrophilic drugs distribute into roughly 40% of the excess weight above ideal. Adding that fraction back to IBW gives a dosing weight that avoids both the underdosing of pure IBW and the overdosing of pure TBW.

How to use it

Enter sex and height so that ideal body weight can be derived.
Enter the actual total body weight.
Keep the correction factor at 0.4 unless your institution specifies otherwise.

Worked example

A 165 cm female weighs 120 kg (BMI 44). Calculate the adjusted body weight for gentamicin dosing.

Answer: 165 cm = 64.96 in, 4.96 in over 5 ft. IBW = 45.5 + (2.3 × 4.96) = 56.9 kg. AdjBW = 56.9 + 0.4 × (120 − 56.9) = 82.1 kg.

Clinical pearls & pitfalls

Adjusted body weight applies to aminoglycosides in obesity. Do not apply it reflexively to every drug in every obese patient — most drugs still use total body weight.
Vancomycin is the classic counterexample: it is dosed on total body weight even in obesity, not on adjusted body weight.
AdjBW should always fall between IBW and TBW. If your answer lies outside that range, you have made an arithmetic error.
The 0.4 factor is an empirical estimate of the fraction of excess weight into which hydrophilic drugs distribute. It is a convention, not a measured constant, and different references cite 0.3 or 0.5 for particular agents.

Assumptions & limitations

Inherits every limitation of the Devine IBW equation, including being undefined below 5 feet.
The correction factor is drug-class specific and empirically derived. Applying the aminoglycoside factor to an unrelated drug has no evidentiary basis.
Not validated in extreme obesity (BMI > 50), where pharmacokinetic behaviour departs further from the underlying assumptions.

References

Bauer LA, et al. Influence of weight on aminoglycoside pharmacokinetics in normal weight and morbidly obese patients. Eur J Clin Pharmacol. 1983;24(5):643-647.
Pai MP, Bearden DT. Antimicrobial dosing considerations in obese adult patients. Pharmacotherapy. 2007;27(8):1081-1091.
Rybak MJ, et al. Therapeutic monitoring of vancomycin for serious MRSA infections: a revised consensus guideline. Am J Health Syst Pharm. 2020;77(11):835-864.