Adjusted Body Weight Calculator — Free Online Calculator
Input
Adjusted Body Weight
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About This Calculator
The Adjusted Body Weight (AjBW) is used for drug dosing in obese patients (those with actual body weight > 120% of ideal body weight). Many drugs distribute into lean tissue, so using actual body weight would lead to overdosing, while using ideal body weight would lead to underdosing. The AjBW provides a compromise using a correction factor of 0.4. It is commonly used for aminoglycoside, vancomycin, and some chemotherapy dosing.
Formula
Interpretation
| ABW / IBW Ratio | Dosing Recommendation |
|---|---|
| ≤ 120% | Use actual body weight |
| > 120% | Use adjusted body weight |
References
Frequently Asked Questions
Why is the correction factor 0.4?
The 0.4 factor is based on studies showing that approximately 40% of excess body weight (above IBW) in obese patients consists of lean tissue that contributes to drug distribution volume. This means 40% of excess weight is metabolically active for drug distribution purposes.
Which drugs require adjusted body weight?
Common drugs that use AjBW for dosing include aminoglycosides (gentamicin, tobramycin), vancomycin, acyclovir, and some chemotherapy agents. The specific weight to use (ABW, IBW, or AjBW) depends on the drug's volume of distribution and is specified in pharmacokinetic references.
What if the patient weighs less than their IBW?
If ABW < IBW (underweight patient), use actual body weight for drug dosing. The AjBW formula is only intended for patients whose actual weight exceeds 120% of ideal body weight. Using AjBW in underweight patients would paradoxically increase the calculated weight.
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⚠ Medical Disclaimer
This tool is for educational and informational purposes only. It is not intended to replace professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional for clinical decisions.