Special Populations and Antimicrobial Dosing Considerations
Core concepts
Dosing must adapt to altered physiology. In renal impairment, adjust renally cleared drugs (aminoglycosides, vancomycin, most beta-lactams) by extending interval, reducing dose, or both. In augmented renal clearance (common in young, critically ill, or burn patients), standard doses may be subtherapeutic. In hepatic impairment, consider hepatically metabolized drugs (e.g., metronidazole, some azoles) and monitor. Obesity increases volume of distribution for lipophilic drugs and may require weight-based dosing using adjusted body weight for certain agents. Pregnancy alters volume of distribution and clearance; choose agents with safety data (e.g., penicillins, cephalosporins) and avoid teratogens. Pediatrics requires weight-based dosing and attention to maturation of renal/hepatic function. Older adults have reduced renal function and polypharmacy risk.
Traps
- Using actual body weight for all drugs in obesity.
- Forgetting that serum creatinine may underestimate renal function in low-muscle-mass patients.
- Applying adult doses to neonates without maturation adjustments.
- Missing drug interactions in polypharmacy.
Rule of thumb
If the drug is renally cleared, estimate renal function and adjust; if the patient is critically ill or obese, reassess volume of distribution and exposure.
Worked example: A patient with CrCl 25 mL/min on vancomycin: extend the interval and monitor AUC or trough per institutional protocol rather than using standard dosing.