Dosing of antimicrobials in obesity

Calculating Body Weight

Statistic	Equation (Male)	Equation (Female)	Notes
Body mass index (BMI) (kg/m2)	weight, kg / (height, m)2
Body surface area (BSA) (m2)	(height, cm × weight, kg / 3600)1/2
Ideal body weight (IBW) (kg)	50 + 2.3 × (inches over 5 ft)	45.5 + 2.3 × (inches over 5 ft)
Adjusted body weight (ABW) (kg)	IBW + C × (TBW - IBW)		C = 0.3 to 0.4
Lean body weight (LBW) (kg)	9270 × TBW / 6680 + 216 x BMI	9270 × TBW / 8780 + 244 x BMI
Fat-free weight (FFW) (kg)	0.00139 × (height, cm)2 - 0.0801 × R + 0.187 × TBW + 39.83	0.00151 × (height, cm)2 - 0.0344 × R + 0.14 × TBW - (0.158 × age) + 20.387	R = resistance

Antibiotics

Penicillins

Drug	Usual Dose	Obesity	Max dose
Amoxicillin		1 g PO q8h	No data
Ampicillin		2 g IV q4h
Nafcillin		2 g IV q4h	No data
Piperacillin-tazobactam	4.5 g IV q6-8h	Prolonged infusion	18 g daily

Cephalosporins

Drug	Usual Dose	Obesity	Max dose
Cefazolin	1-2 g IV q8h	2-3 g IV q6-8h	No data; 12 g daily
Cephalexin	500-1000 g PO qid	1000 g PO qid	No data
Cefipime	2 g IV q8h	2 g IV q8h prolonged infusion	2 g IV q8h prolonged infusion
Ceftazidime	2 g IV q8h	2 g IV q8h prolonged infusion	2 g IV q8h prolonged infusion
Ceftazidime-avibactam		Usual dose
Ceftolozane-tazobactam		Usual dose

Carbapenems

Drug	Usual Dose	Obesity	Max dose
Doripenem		Consider extended infusion
Ertapenem	1 g IV q24h	Usual dose
Imipenem
Meropenem		Consider prolonged infusion

Quinolones

Drug	Usual Dose	Obesity	Max dose

Macrolides

Drug	Usual Dose	Obesity	Max dose

Tetracyclines

Drug	Usual Dose	Obesity	Max dose

Others

Drug	Usual Dose	Obesity	Max dose
Aztreonam		2 g IV q6-8h
Colistin		Use IBW	360 mg daily (nephrotoxicity)
Polymyxin B		Use ABW0/4	200 mg or 2 MU daily (toxicity)

Further Reading

• Comprehensive Guidance for Antibiotic Dosing in Obese Adults. Pharmacotherapy. 2017;37(11):1415-1431. doi: 10.1002/phar.2023