Drug Dosing in Obese Patients: IBW vs TBW
For most medications in obese patients, use lean body weight (LBW) or adjusted body weight (ABW) rather than total body weight (TBW), as dosing based on TBW increases the risk of relative overdose and adverse effects. 1
General Principle
Dosing to total body weight is rarely appropriate for most drugs in obese patients because excess weight is primarily fat tissue with relatively low blood flow, and changes in volume of distribution are drug-specific rather than predictable. 1
Weight Descriptors and Calculations
Key Definitions
Ideal Body Weight (IBW): IBW (kg) = height (cm) - 105 (females) or height (cm) - 100 (males) 1
Lean Body Weight (LBW): Calculated using the Janmahasatian formula; rarely exceeds 100 kg in men and 70 kg in women regardless of total body weight 1
Adjusted Body Weight (ABW): ABW (kg) = IBW (kg) + 0.4 × (TBW (kg) - IBW (kg)) 1, 2
Drug-Specific Dosing Algorithm
1. Hydrophilic Drugs or Those Restricted to Lean Tissues
Use IBW for dosing because these drugs distribute minimally in adipose tissue. 2, 3, 4
Examples include:
- Aminoglycosides 3, 4
- Neuromuscular blocking agents (vecuronium, atracurium, rocuronium) 2, 5, 4
- H2-blockers 4
Critical caveat: For rocuronium specifically, use LBW (not IBW) as it is higher than IBW and accounts for increased lean mass in obesity. 5
2. Moderately Lipophilic Drugs
Use ABW for dosing as these drugs partially distribute into adipose tissue. 2, 3
Examples include:
- Antibacterials (many require IBW plus a percentage of excess bodyweight) 3
- Acyclovir (IBW dosing leads to subtherapeutic levels; ABW more appropriate) 6
3. Highly Lipophilic Drugs
Consider TBW or ABW depending on the specific drug, as distribution patterns are not predictable based solely on lipophilicity. 2, 3, 7
Examples with variable recommendations:
- Benzodiazepines: Use IBW to avoid prolonged sedation and recovery times 2, 4
- Some beta-blockers: Distribution varies despite high lipophilicity 3, 4
4. Anesthetic Induction Agents
Use LBW or ABW for initial bolus dosing, as induction dose correlates well with lean body weight. 1
Critical safety concern: Dosing based on TBW causes significant hypotension, while underdosing with IBW followed by delayed maintenance increases risk of accidental awareness under anesthesia. 1
5. Chemotherapeutic Agents
Use TBW regardless of obesity status - this is the major exception to the general rule. 2
- Studies show no increase in hematologic or non-hematologic toxicity when dosing cytotoxic chemotherapy by TBW in obese patients 2
- Probability of febrile neutropenia actually decreases as BMI increases with TBW-based dosing 2
Pharmacokinetic Considerations
Volume of Distribution
- Loading doses should be adjusted based on whether the drug distributes into adipose tissue 3, 4, 7
- TBW is generally the best descriptor for volume of distribution, particularly for lipophilic drugs 7
Clearance
- Maintenance doses depend on clearance, which is best described by lean body mass 7, 8
- Clearance via oxidation, conjugation, or reduction is generally not decreased in obesity 1, 4, 8
- Renal clearance may be increased in obese patients 4, 8
Common Pitfalls to Avoid
Never assume all lipophilic drugs behave the same way - distribution in obesity is drug-specific, not class-specific 1, 3, 7
Avoid using TBW for chronic dosing of most medications, as this leads to accumulation and toxicity 7
Do not use IBW for acyclovir - this leads to substantially lower systemic exposure and subtherapeutic levels 6
Maintain consistency in which weight descriptor you use for a given patient throughout their treatment course 2
For narrow therapeutic index drugs, consider therapeutic drug monitoring rather than relying solely on weight-based calculations 2, 3