Weight-based and body surface area-based dosing are two methods of individualizing drug therapy to account for patient size and body composition. While fixed dosing assumes that interpatient variability is negligible, size-based dosing recognizes that drug disposition is correlated with body size and adjusts the dose accordingly. The choice between weight-based and BSA-based dosing depends on the drug’s pharmacokinetic properties, the patient population, and the clinical context.
When Weight-Based Dosing Is Appropriate
Weight-based dosing, expressed as milligrams per kilogram of body weight, is the most commonly used size-based dosing approach. It is standard for pediatric patients, where weight varies enormously across age groups, and for many intravenously administered drugs. Weight-based dosing is appropriate when there is a reasonably linear relationship between weight and drug clearance or volume of distribution, and when the therapeutic index is wide enough that small errors in dosing do not produce toxicity.
Heparin is a classic example of weight-based dosing. The initial bolus dose and infusion rate for unfractionated heparin are calculated based on the patient’s weight in kilograms, and dose adjustments are guided by activated partial thromboplastin time measurements. Many antibiotics, including aminoglycosides and vancomycin, are dosed by weight. Neuromuscular blocking agents, thrombolytics, and many chemotherapeutic agents also follow weight-based protocols.
Body Surface Area Calculation
BSA is calculated from height and weight using formulas such as the Mosteller formula: BSA in square meters equals the square root of height in centimeters multiplied by weight in kilograms divided by 3600. The DuBois formula, BSA equals 0.007184 multiplied by weight in kilograms raised to 0.425 multiplied by height in centimeters raised to 0.725, is an alternative that predates the Mosteller formula. The Mosteller formula is simpler and has comparable accuracy, making it the preferred method in most clinical settings.
Average adult BSA is approximately 1.73 square meters, though individual values range from approximately 1.4 to 2.2 square meters depending on height and weight. Doses expressed as milligrams per square meter are multiplied by the patient’s calculated BSA to determine the individual dose.
Drugs Requiring BSA Dosing
Chemotherapy agents are the most prominent class of drugs requiring BSA-based dosing. Most cytotoxic anticancer drugs have narrow therapeutic indices, and the relationship between BSA and drug clearance is better established than the relationship with weight alone. Carboplatin dosing uses the Calvert formula, which incorporates GFR and target AUC in addition to BSA. Doxorubicin, cyclophosphamide, paclitaxel, and many other antineoplastic agents are dosed by BSA.
Biologics and monoclonal antibodies are increasingly dosed by BSA or by weight. The pharmacokinetics of large molecules often correlates with body size, and dosing based on size metrics reduces interpatient variability in exposure. Rituximab, trastuzumab, and bevacizumab are examples of biologics that use BSA-based or weight-based dosing.
Ideal Body Weight versus Total Body Weight
A critical decision in weight-based dosing is whether to use total body weight (TBW) , ideal body weight (IBW) , or an adjusted body weight. IBW for males is 50 kilograms plus 2.3 kilograms per inch over 5 feet. For females, IBW is 45.5 kilograms plus 2.3 kilograms per inch over 5 feet. For hydrophilic drugs, using TBW in obese patients can result in overdose because the drug does not distribute extensively into adipose tissue. For lipophilic drugs, TBW may be appropriate because the drug does distribute into fat.
An adjusted body weight (ABW) is sometimes used: ABW equals IBW plus 0.4 multiplied by (TBW minus IBW). This formula is commonly applied when dosing aminoglycosides and other hydrophilic drugs in obese patients. The correction factor of 0.4 accounts for the distribution of these drugs into some, but not all, of the excess weight.
Obese Patient Considerations
Obesity presents particular challenges for size-based dosing. Body composition changes in obesity include increased adipose tissue, increased lean body mass, and increased total body water, but the proportional increases differ. Drug clearance and volume of distribution may not increase linearly with weight, and the pharmacokinetics of many drugs in obese patients is incompletely characterized.
For highly lipophilic drugs such as propofol and fentanyl, weight-based dosing using TBW is generally appropriate because the large volume of distribution requires higher doses for pharmacological effect. For hydrophilic drugs such as neuromuscular blocking agents, IBW or ABW is more appropriate. When uncertainty exists, therapeutic drug monitoring provides the most reliable guide to dose individualization in obese patients, particularly for drugs with narrow therapeutic indices.
The selection of weight-based versus BSA-based dosing, and the choice of weight metric, should be guided by drug-specific pharmacokinetic data and clinical evidence rather than by general preference. Proper application of size-based dosing principles reduces interpatient variability and improves the likelihood of achieving therapeutic drug concentrations across diverse patient populations.
