Why is cefazolin used as a prophylactic antibiotic for obstetric (OB) procedures and what specific pathogens does it cover?

From the Guidelines

Cefazolin is the preferred prophylactic antibiotic for obstetric procedures due to its effective coverage of common pathogens and favorable safety profile. It is a first-generation cephalosporin that provides excellent coverage against gram-positive organisms, particularly Staphylococcus aureus and Streptococcus species, which are common skin flora that can contaminate surgical sites 1. Cefazolin also covers some gram-negative bacteria including Escherichia coli, which is particularly important in obstetric procedures as E. coli is a common cause of urinary and genital tract infections.

• The most common pathogens that cause surgical site infections in obstetric procedures include:
  • Gram-positive organisms such as Staphylococcus aureus and Streptococcus species
  • Gram-negative bacteria including Escherichia coli
• For standard prophylaxis, cefazolin is typically administered at a dose of 2 grams IV (increased to 3 grams for patients weighing >120 kg) within 60 minutes before surgical incision, as recommended by recent guidelines 1.
• Cefazolin should be given prior to skin incision rather than after cord clamping in cesarean deliveries, as this timing has been shown to reduce infection rates without harming the newborn 1.
• Cefazolin is preferred over broader-spectrum antibiotics because it has targeted activity against likely pathogens, minimizes antimicrobial resistance, has low rates of adverse reactions, crosses the placenta minimally, and is cost-effective 1.
• For patients with a documented beta-lactam allergy, alternatives such as clindamycin or gentamicin may be considered, though these have different coverage profiles.
• Recent studies have emphasized the importance of proper surgical antibiotic prophylaxis, including the administration of antibiotics within 120 min prior to the incision and the use of a single dose for most procedures 1.

From the FDA Drug Label

The prophylactic administration of Cefazolin for Injection, USP preoperatively, intraoperatively, and postoperatively may reduce the incidence of certain postoperative infections in patients undergoing surgical procedures which are classified as contaminated or potentially contaminated (e.g., vaginal hysterectomy and cholecystectomy in high-risk patients such as those older than 70 years, with acute cholecystitis, obstructive jaundice, or common duct bile stones).

Cefazolin is used as a prophylactic antibiotic for obstetric (OB) procedures because it covers a range of pathogens that could be a concern for these procedures, including:

• S. aureus (including beta-lactamase-producing strains)
• S. pyogenes
• E. coli
• P. mirabilis
• Streptococci (including other strains)

These pathogens can cause infections such as urinary tract infections, skin and skin structure infections, and septicemia, which can be devastating in the context of OB procedures. The use of cefazolin as prophylaxis can help reduce the incidence of these infections 2.

From the Research

Use of Cefazolin in OB Procedures

Cefazolin is commonly used as a prophylactic antibiotic for obstetric (OB) procedures, including cesarean sections, due to its broad spectrum of activity against common obstetric pathogens. The specific pathogens that cefazolin covers include:

• Bacteroides fragilis 3
• Other gram-positive and gram-negative bacteria

Efficacy of Cefazolin in Preventing Infections

Studies have shown that cefazolin is effective in reducing the incidence of postoperative infections, including endomyometritis 4 and surgical site infections 5. For example:

• A study published in 1978 found that cefazolin reduced the incidence of endomyometritis from 51% to 29% in patients undergoing cesarean section 4
• A study published in 2024 found that cefazolin was 1.43 and 1.77 times more effective in preventing postoperative infections when the operation lasted for 45 minutes or more 5

Safety of Cefazolin in Patients with Penicillin Allergy

Cefazolin is considered safe for use in patients with a penicillin allergy, except for those who have experienced a severe, life-threatening delayed hypersensitivity reaction to penicillin 6. In fact, studies have shown that more than 90% of patients with a documented penicillin allergy do not have a true allergy on skin testing 6.

Comparison with Other Antibiotics

Cefazolin has been compared to other antibiotics, such as cefoxitin 3 and ceftriaxone 7, in terms of its efficacy in preventing postoperative infections. While these studies have shown that cefazolin is effective, they have also highlighted the importance of considering factors such as the timing of administration, operation duration, and patient characteristics, such as body mass index (BMI) and wound type, when selecting an antibiotic for prophylaxis 5.