Adjusted weight sits between ideal and actual
For patients with obesity, dosing some drugs on full body weight overshoots, because the medicine distributes through lean tissue rather than fat. Adjusted body weight starts from ideal weight and adds back only 40 percent of the excess, giving a fairer dosing weight.
IBW is ideal body weight from height and sex, ABW is the patient's actual weight, and (ABW − IBW) is the excess over ideal. The 0.4 factor is the standard for aminoglycosides and vancomycin; check the specific drug for exceptions.
A worked example
- Patient. Actual weight 100 kg, ideal weight 70 kg, so the excess is 30 kg.
- Adjusted weight. 70 + 0.4 × 30 = 82 kg, the weight you dose on.
- Why it matters. Gentamicin at 5 mg/kg on actual weight is 500 mg, a dose that risks kidney injury. On the 82 kg adjusted weight it is 410 mg — effective without the overshoot.
Which weight goes with which drug
| Dosing weight | Typical use |
|---|---|
| Actual (ABW) | Normal or underweight patients; fat-soluble drugs; most psychiatric drugs |
| Ideal (IBW) | Very conservative dosing; some agents in severe obesity |
| Adjusted (AjBW) | Aminoglycosides, vancomycin, heparin in obesity — the standard 0.4 case |
The correction factor is not fixed. Narrow-margin drugs may use 0.25 or 0.3, and fat-soluble drugs use actual weight (a factor of 1.0). The right choice is always the drug-specific guideline, not the formula alone.
Common questions
What is adjusted body weight used for?
It sets a dosing weight for certain drugs in patients with obesity. Some medicines spread mainly through lean tissue, not fat, so dosing on full actual weight would overshoot. Adjusted body weight counts only part of the excess weight to land between too little and too much.
Why the factor of 0.4?
It reflects that roughly 40 percent of the weight above ideal is metabolically active for these drugs, and 60 percent is largely inert fat. The 0.4 factor was calibrated against blood levels of aminoglycosides. Some drugs use 0.25 or 0.3 instead, so always check the drug-specific guidance.
What if the patient weighs less than their ideal weight?
Then you do not use adjusted body weight at all. When actual weight is below ideal, the formula would give a figure lower than the real weight, which is wrong. Use the actual body weight instead.
