Cefazolin Dosing for Cesarean Delivery Prophylaxis in a 139 kg Patient
For a 139 kg woman undergoing cesarean delivery, administer 3 grams of cefazolin, not 2 grams.
Weight-Based Dosing Algorithm
The critical threshold is 120 kg body weight:
Since this patient weighs 139 kg, she clearly exceeds the 120 kg threshold and requires the 3 g dose 2.
Pharmacokinetic Evidence Supporting Higher Dosing
The evidence strongly demonstrates that obesity significantly reduces cefazolin tissue concentrations, creating a dose-dependent relationship:
Body mass index has a proportionally inverse relationship with antibiotic concentrations 2. For every 1 kg/m² increase in BMI, there is an associated 13.77 μg/mL lower plasma concentration of cefazolin 3.
With 2 g dosing in obese women (BMI >40 kg/m²), none achieved adequate tissue concentrations at the minimal inhibitory concentration (MIC) of ≥8 μg/mL 2. The median adipose concentration was only 4.7 μg/g with 2 g dosing 2.
With 3 g dosing, 71% of women with BMI >40 kg/m² reached target MIC values 2, and all women with BMI 30-40 kg/m² achieved adequate concentrations 2.
The area under the plasma concentration curve was significantly greater with 3 g versus 2 g (27,204 μg/mL per minute vs 14,058 μg/mL per minute; P = 0.001) 3.
Timing and Administration
- Administer 30-60 minutes before skin incision 1, 2
- Redose if surgery exceeds 4 hours: Give an additional 1 g cefazolin 1
- Do not extend prophylaxis beyond 24 hours postoperatively 1
Guideline Context
While the 2018 ERAS Society guidelines note that two randomized trials showed no difference in infectious morbidity between 2 g and 3 g dosing 1, these trials did not specifically examine patients at the extreme of obesity (≥120 kg). The pharmacokinetic studies consistently demonstrate inadequate tissue penetration with 2 g dosing in this weight range 2, 4, 5.
Common Pitfall to Avoid
Do not use standard 2 g dosing in patients ≥120 kg simply because it is the "usual" dose. The pharmacokinetic data clearly shows tissue concentrations fall below therapeutic thresholds in heavier patients 2, 4. At 139 kg, this patient requires the higher dose to achieve adequate surgical site tissue concentrations and prevent surgical site infection 2, 5.