Table 2.

Measures used for calculating drug doses and associated formulae

Term	Derivation	Explanation	Use	Advantages	Disadvantages
TBW (kg)	LBW+FW	TBW, as measured, includes both fat and lean weight	•Dosing of many drugs; most appropriate in the non-obese (where TBW tends to LBW). •Succinyl choline. •TIVA maintenance (accounts for larger compartment size).	•Easy to measure and is widely used. •Validated for normal-weight population.	•May lead to overdose if used in the obese population. •Does not account for pharmacogenetics or sex differences in metabolism. •TIVA boluses by TBW may exaggerate haemodynamic effects.
LBW (kg)7	1.1×TBW–0.0128×BMI×TBW (male); 1.07×TBW–0.0148×BMI×TBW (female)	LBW comprised of the non-adipose tissues	•Useful for polar drugs with a small VD, such as NMBAs •TCIs (bolus induction: avoids overdosing and instability)	•Correlates with metabolism and clearance •Accounts for sex-related differences in metabolism •Less prone to overdosing in obese populations	•Several formulae for calculation, and may be complex •Error prone, particularly at extremes of weight, depending on formula used •Does not account for pharmacogenetic differences in metabolism •Not possible to measure directly in clinical practice
IBW (kg)8	22×height2 (m)	The weight of an individual based on height and assuming a normal BMI of 22	•Recommended for calculation of local anaesthetic maximal doses •Used for selection of tracheal tube size •Originally shown to be consistent with the pharmacokinetics of gentamicin	•Easily calculated (although measures of height in critically ill patients can be challenging)	•Does not account for pharmacogenetics or age-related differences in metabolism •Assumes 15% body fat, which is not ‘normal’ across all age ranges
ABW (kg)	LBW+C×(TBW–LBW) (where C is a drug-specific correction based on the solubility of the drug)	Assumes drug distribution to lean tissues and a proportion of the FW depending on the physiochemical properties of the drug	•Dosing of drugs with a narrow therapeutic index (e.g. gentamicin)	•Drug specific	•Requires calculation using a correction factor specific to every drug •Uses LBW (which may be error prone, depending on the formula used) •Does not account for pharmacogenetic differences in metabolism
Body surface area9 (m2)	Weight0.425×height0.725×0.007184	Based on the surface area given height and weight, both of which are easily measured	•Mostly only used for dosing chemotherapy agents	•Easily calculated	•Requires calculation •Does not account for pharmacogenetic differences in metabolism •Not validated; poor correlation with clearance
PNWT10 (kg)	1.57×weight−0.0183×BMI×WT−10.5 (male) 1.75×Weight−0.0242×BMI×WT−12.6 (female)	Consists of lean and fatty weight, corrected for the non-obese individual, an extension of IBW	•Developed to overcome limitations of scaling to LBW or TBW •Not validated or in widespread clinical use currently	•Specifically derived for drug dosing (rather than classification of obesity) •Includes sex, weight and height	•Complicated to calculate •Does not account for pharmacogenetic differences in metabolism •Not validated
PBW (kg)	50+[0.91×(height in cm–152.4)] (males); 45.5+[0.91×(height in cm–152.4)] (females)	Based on the original ARDSNet study, normalises to lung size; comparable to IBW	•Used for calculating ventilatory requirements •Does not have a significant role in drug dosing	•Considers height, weight, and sex	•Requires calculation (both height and weight can be difficult to measure in critical illness)