Dosing in obese patients

Obesity is defined as patients with a BMI over 30kg/m².
Obesity is associated with changes in volume of distribution and clearance of some antibiotics.

Advice about the dosing of antibiotics in obese patients is given below. Early discussion with a pharmacist or infection specialist is advised.

Patients with renal and/or hepatic impairment are outside the scope of this document. Discussion with an antimicrobial pharmacist or infection specialist is strongly recommended.

Recommended changes to antibiotic dosing in obese patients

Drug class	Antibiotic	Dosing modification
Beta lactam	Penicillins	Use higher end of standard dose
	Cephalosporins	Standard dosing
	Carbapenems	Standard dosing Meropenem preferred over ertapenem; in ITU patients consider prolonged (3hour) infusion
Quinolones	Ciprofloxacin	Consider using higher end of standard dose range
Quinolones	Levofloxacin / Moxifloxacin	Standard dosing
Lincosamides	Clindamycin	Standard dosing Consider using higher dosages, especially for serious/life threatening infections
Glycopeptides	Vancomycin Teicoplanin	Use Total body weight (TBW) to calculate dose
Aminoglycoside	Amikacin Gentamicin Tobramycin	Use Adjusted body weight (AdjBW) to calculate dose (This is standard in OUH calculator)
Other	Daptomycin	Use Total body weight (TBW) to calculate dose
	Linezolid	Standard dosing
	Colistimethate sodium	Use (Ideal body weight) IBW to calculate dose
	Metronidazole	Standard dosing

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For obese patients, alternative measures of weight are sometimes used to modify antibiotic dosing.

Weight type	Definition	Used for
TBW: Total body weight	Patient’s true weight in kg	Vancomycin/Teicoplanin Daptomycin
IBW: Ideal body weight	50kg (male) or 45kg (female) + 2.3kg for each inch over 5 feet	Colistimethate sodium
AdjBW: Adjusted body weight	IBW + 0.4(TBW – IBW)	Aminoglycosides

Monitor Vancomycin, Teicoplanin, Gentamicin and Amikacin levels as per standard OUH protocol. See vancomycin, teicoplanin, gentamicin or amikacin monographs.
Monitoring of other antibiotics can be considered but are send-away tests (to Bristol Antimicrobial Reference Laboratory). Discuss with Infection Team before doing this.

Recommended changes to antibiotic dosing in obese patients

Drug class

Antibiotic

Dosing modification

Common measures of weight

Therapeutic drug monitoring

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