';resultHtml+='Ideal Body Weight (IBW): '+ibwDisplay+'
';resultHtml+='Actual Body Weight: '+actualDisplay+'
';resultHtml+='1. Base Height: '+heightInches.toFixed(2)+' inches
';resultHtml+='2. IBW Formula (Devine): '+(gender==='male'?'50 + 2.3 × (Inches – 60)':'45.5 + 2.3 × (Inches – 60)')+'
';resultHtml+='3. Calculated IBW: '+ibw.toFixed(2)+' kg
';resultHtml+='4. AjBW Formula: IBW + '+factor+' × (Actual Weight – IBW)
';resultHtml+='5. Calculation: '+ibw.toFixed(2)+' + '+factor+' × ('+actualKg.toFixed(2)+' – '+ibw.toFixed(2)+') = '+ajbw.toFixed(2)+' kg
Calculator Use
The adjusted body weight calculator is a specialized tool used primarily by healthcare professionals to calculate dosages for specific medications in patients who are overweight or obese. While Ideal Body Weight (IBW) is often used for drug clearance estimations, it can lead to under-dosing in larger patients. Conversely, using Actual Body Weight (ABW) can result in toxicity. This calculator provides a middle ground, often referred to as "dosing weight."
By entering biological gender, height, and current weight, this tool calculates the Ideal Body Weight first and then applies a clinical correction factor (usually 0.4) to determine the adjusted value.
- Biological Gender
- Used to select the correct constant for the Devine formula (50kg for males, 45.5kg for females).
- Actual Body Weight
- Your current weight in pounds or kilograms.
- Adjustment Factor
- The clinical constant used to account for the volume of distribution in excess adipose tissue. 0.4 is the standard for most aminoglycoside dosing.
How It Works
Calculating Adjusted Body Weight (AjBW) requires a two-step mathematical process. First, the Ideal Body Weight must be established using the Devine Formula, which is based on height over 5 feet (60 inches).
AjBW = IBW + CF × (Actual Weight – IBW)
- IBW: Ideal Body Weight (kg)
- CF: Correction Factor (standardized at 0.4)
- Actual Weight: The patient's measured weight (kg)
The logic behind the 0.4 factor is that approximately 40% of the excess body weight (fat tissue) is composed of water, which affects the distribution of hydrophilic drugs. If a patient's actual weight is less than their ideal weight, clinical guidelines generally recommend using the Actual Body Weight instead of the adjusted value.
Calculation Example
Example: A male patient who is 6 feet tall (72 inches) and weighs 250 lbs (113.4 kg).
Step-by-step solution:
- Calculate IBW: 50kg + 2.3 × (72″ – 60″) = 50 + 27.6 = 77.6 kg
- Calculate Excess Weight: 113.4 kg – 77.6 kg = 35.8 kg
- Apply Adjustment Factor: 35.8 kg × 0.4 = 14.32 kg
- Final AjBW: 77.6 kg + 14.32 kg = 91.92 kg
- Result: The dosing weight for this patient is 91.9 kg (202.6 lbs).
Common Questions
When should I use an adjusted body weight calculator?
It is most commonly used in clinical settings for patients whose actual body weight is more than 20-30% above their ideal body weight. It is vital for calculating the volume of distribution for drugs like Amikacin, Gentamicin, and sometimes Vancomycin.
Is AjBW the same as Lean Body Mass?
No. Lean Body Mass (LBM) subtracts all fat weight from the total. Adjusted Body Weight purposefully includes a fraction (40%) of the excess weight because that tissue still influences how certain medications are processed by the body.
Why is 0.4 the default factor?
Research into aminoglycoside pharmacokinetics found that a 0.4 factor most accurately predicts the volume of distribution in obese populations. However, some clinicians may use 0.3 or 0.25 for specific protocols or different classes of medication.