These are different dosing weights you should know as a pharmacist.
Know the different type of body weights used for dosing drugs.
Total Body Weight (TBW)
TBW (total body weight) is the actual body weight. Some sources use the abbreviation ABW (actual body weight). ABW is the same as TBW.
Ideal Body Weight (IBW)
IBW (ideal body weight) this is the healthy weight of an individual. IBW is based on weight and height.
For drugs with narrow therapeutic index, IBW is used to avoid toxicity.
Men (IBW) = 50 kg + (2.3kg)( number of inches over 5 feet)
Women (IBW) = 45 kg + (2.3kg)( number of inches over 5 feet)
1 foot= 12 inches 5 feet= 60 inches
***If patient is less than 5 feet, use their actual weight (TBW) in dose calculations. There is no universal consensus or literature supporting any method to calculate IBW for people less than 5 feet.
Adjusted Body Weight (AdjBW)
AdjBW is the body weight used for people who are overweight or obese.
AdjBW is defined as weight ≥ 120-130 % of IBW.
AdjBW = IBW + 0.4 (TBW -IBW)
TBW, IBW and AdjBW are used clinically for:
- dose adjustments
- to calculate renal function
- dosing in obese patient
Always check the drug resource information to see which body weight is applicable to the drug being dosed.
Scenario #1: If weight < IBW, use TBW to dose (less likely to overdose).
Scenario #2: If weight roughly the same as IBW, use TBW in most cases. Roughly the same weight is where the weight is < 120% of IBW. Some drugs here may require IBW instead (. e.g. drugs like acyclovir, digoxin, gentamicin, levothyroxine). Always check your drug dosing information for the weight specifics.
Scenario #3: If overweight or obese, use TBW, IBW or AdjBW according to the dosing source. Examples: (TBW=vancomycin, IBW= theophylline, AdjBW=gentamicin). Again, always check your drug dosing information for the weight specifics.