Cefazolin 3g Dosing Based on Body Weight
For surgical prophylaxis, administer cefazolin 3g for patients weighing ≥120 kg (approximately BMI ≥40 kg/m²), while 2g is appropriate for patients <120 kg.
Weight-Based Dosing Thresholds
The 120 kg cutoff represents the established threshold where standard 2g dosing becomes inadequate:
- Patients ≥120 kg: Require 3g cefazolin for surgical prophylaxis to achieve adequate tissue concentrations 1, 2, 3
- Patients <120 kg: Standard 2g dosing is generally sufficient 2, 4
This recommendation applies specifically to surgical prophylaxis rather than treatment of active infections, where higher doses up to 9g/day may be used 1.
Pharmacokinetic Rationale
The increased dose requirement in high body weight patients is driven by altered drug distribution:
- Volume of distribution increases proportionally with total body weight, resulting in lower peak concentrations with standard dosing 3
- Protein binding is affected by BMI and albumin levels, further reducing free drug availability in obese patients 3
- Pharmacokinetic studies demonstrate that 2g cefazolin provides protective duration of only 5.1 hours in morbidly obese patients (BMI 40-50 kg/m²), while 3g extends this to 6.8 hours in super-morbidly obese patients (BMI >50 kg/m²) 2
Clinical Evidence Supporting 3g Dosing
Cesarean delivery data provides the strongest clinical evidence for weight-based dosing:
- In women with BMI 30-40 kg/m², only 20% achieved target minimum inhibitory concentration (MIC ≥8 μg/mL) with 2g, compared to 100% with 3g 5
- In women with BMI >40 kg/m², 0% reached target MIC with 2g, while 71% achieved it with 3g 5
General surgical prophylaxis studies show trends toward increased surgical site infections with 2g dosing in patients ≥120 kg (9.8% vs 5.0% in patients <120 kg), though not statistically significant in smaller studies 4.
Practical Dosing Algorithm
Step 1: Weigh the patient
- If ≥120 kg → Give 3g cefazolin
- If <120 kg → Give 2g cefazolin
Step 2: Consider timing
- Administer 30-60 minutes before incision 5
- For procedures >4 hours, consider redosing (2g for <120 kg, 3g for ≥120 kg) 2
Step 3: Administration method
- Both IV push over 5 minutes and 30-minute infusion achieve similar concentrations 2
- Prolonged infusion may improve target attainment for less-susceptible organisms 3
Important Caveats
- The 3g recommendation is based primarily on pharmacokinetic studies rather than large randomized controlled trials demonstrating improved clinical outcomes 4
- For treatment of active infections (not prophylaxis) in patients ≥120 kg, doses up to 9g/day (3g every 8 hours) have been used safely 1
- Patients with renal impairment require dose adjustment regardless of weight 3
- The protective effect of 2g cefazolin appears adequate for procedures <5 hours duration even in morbidly obese patients, but 3g provides a safety margin 2