General
Adjusted Body Weight
Dosing weight in obesity.
Education and reference only. Not a substitute for clinical judgement, local policy or product labelling. Always verify before clinical use. Values are calculated in your browser and never stored.
Background
Adjusted body weight (AdjBW) is a dosing weight used in obesity for certain weight-based medicines, where neither actual body weight nor ideal body weight (IBW) gives the most appropriate dose. It adds a fraction of the excess weight above ideal to the ideal body weight, recognising that some, but not all, of the extra tissue contributes to drug distribution. The standard formula is AdjBW = IBW + 0.4 × (actual weight − IBW), with IBW most often calculated using the Devine equation from height and sex. The result is reported in kilograms.
Interpreting the result
The adjusted weight lies between the ideal and actual body weights and is intended for use as a dosing weight where guidance specifies it, for example in some aminoglycoside regimens. Using actual body weight in obesity risks overdosing, while ideal body weight may underdose; the adjusted value is a compromise. Whether to use actual, ideal or adjusted body weight is drug-specific, so the calculator output must always be checked against the individual medicine's recommendation.
Worked example
A man who is 175 cm tall has a Devine IBW of roughly 70 kg. If his actual weight is 110 kg, his adjusted body weight is 70 + 0.4 × (110 − 70) = 70 + 16 = 86 kg, which would be used as the dosing weight where adjusted body weight is recommended.
Critical actions
AdjBW = IBW + 0.4 × (actual − IBW), using Devine IBW. Always check the specific drug's recommended dosing weight.
Pearls / pitfalls
- Always check which dosing weight a specific drug requires — actual, ideal or adjusted — rather than applying adjusted body weight by default.
- The correction factor is conventionally 0.4 but a few protocols use a different value, so follow local guidance.
- It depends on an ideal-body-weight estimate (usually Devine), which itself has limitations at extremes of height.
- Adjusted body weight is generally only relevant when the actual weight substantially exceeds the ideal weight.
Evidence & validation
The approach builds on the Devine ideal-body-weight equation and a correction factor (commonly 0.4) derived from pharmacokinetic studies in obesity. It is embedded in clinical pharmacy and antimicrobial-dosing guidance for weight-based drugs in obese patients.
Frequently asked questions
When should adjusted body weight be used?
It is used in obesity for specific weight-based drugs where guidance recommends it, such as some aminoglycoside regimens. Whether actual, ideal or adjusted weight applies is drug-specific, so always check the individual medicine's recommendation.
How is adjusted body weight calculated?
A common formula is ideal body weight plus 0.4 times the difference between actual and ideal body weight. The ideal body weight is usually derived from height and sex using the Devine equation.
Why not just use actual body weight?
In obesity, dosing by actual body weight can lead to overdosing for drugs that do not distribute fully into excess adipose tissue, while ideal body weight may underdose. Adjusted body weight is a compromise between the two.
Is the correction factor always 0.4?
The factor is most commonly 0.4, but some protocols use a slightly different value. Follow your local or drug-specific guidance, as the appropriate dosing weight and factor can vary.
References
- Devine BJ. Gentamicin therapy. Drug Intell Clin Pharm. 1974;8:650–655.
- Pai MP, Paloucek FP. The origin of the "ideal" body weight equations. Ann Pharmacother. 2000;34(9):1066–1069.
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