What antibiotic is given to a postpartum patient with a 3rd degree perineal tear who has undergone surgical repair?

Antibiotic Prophylaxis for 3rd Degree Perineal Tear Repair

A second- or third-generation cephalosporin should be administered as a single preoperative dose before surgical repair of a 3rd degree perineal tear, with metronidazole and consideration of adding gentamicin (or clindamycin plus gentamicin in penicillin-allergic patients) to provide adequate coverage for both vaginal and bowel flora. 1

Specific Antibiotic Regimens

First-Line Options (Non-Allergic Patients)

• Cefazolin 2g IV as a single dose is the preferred first-generation cephalosporin, administered 30 minutes to 1 hour before repair begins 1, 2

  • Re-inject 1g if procedure duration exceeds 4 hours 2

• Cefoxitin 2g IV or cefotetan 1g IV as second-generation cephalosporins provide broader anaerobic coverage and are specifically validated in randomized controlled trials for 3rd and 4th degree tears 3, 4

  • These agents reduced perineal wound complications from 24.1% to 8.2% (p=0.037) 4

• Cefamandole 1.5g IV or cefuroxime 1.5g IV are alternative second-generation options 1

  • Re-inject 0.75g if duration exceeds 2 hours 1

Penicillin Allergy Regimen

• Clindamycin 900mg IV slow PLUS gentamicin 5mg/kg IV as a single dose 1
• This combination provides coverage for gram-positive cocci, anaerobes, and gram-negative organisms from both vaginal and bowel flora 1

Additional Coverage Consideration

• Metronidazole 500mg IV should be added to the regimen or considered as part of the prophylactic coverage, particularly given the proximity to bowel flora 1

Evidence Supporting Antibiotic Prophylaxis

The recommendation for prophylactic antibiotics in obstetric anal sphincter injuries (OASIS) is based on strong evidence:

• A randomized controlled trial demonstrated that antibiotic prophylaxis reduced purulent discharge from 17.2% to 4.1% (p=0.04) and any wound complication from 24.1% to 8.2% (p=0.04) 1

• A prospective cohort study found antibiotic administration at primary OASIS repair decreased wound infection risk with an adjusted odds ratio of 0.50 (95% CI 0.27-0.94, p=0.03) 1

• Quality improvement initiatives implementing routine antibiotic prophylaxis achieved a 55% reduction in wound infection or dehiscence 1

Timing and Administration

• Administer antibiotics before initiating the repair, ideally 30 minutes to 1 hour before the first incision to ensure adequate tissue levels 1, 2

• Single-dose prophylaxis is sufficient—extending beyond 24 hours converts this to therapeutic rather than prophylactic treatment 1

• Re-dosing is only necessary if the procedure duration exceeds the specific time thresholds for each antibiotic (typically 2-4 hours depending on the agent) 1, 2

Target Organisms

The antibiotic regimen must cover both vaginal and bowel flora given the anatomic location of 3rd degree tears:

• Vaginal flora: Staphylococcus aureus, Streptococcus species, Escherichia coli, Klebsiella pneumoniae 1

• Bowel flora: Anaerobic bacteria (Bacteroides species), gram-negative enteric organisms 1

Critical Pitfalls to Avoid

• Never omit prophylactic antibiotics—this triples the risk of wound complications from 8% to 24% 5, 4

• Do not delay antibiotic administration until after the repair begins, as tissue levels must be therapeutic at the time of bacterial contamination 1, 2

• Avoid extending prophylaxis beyond 24 hours postoperatively—this increases antibiotic resistance without additional benefit and represents therapeutic rather than prophylactic use 1

• Do not use third-generation cephalosporins (ceftriaxone, ceftazidime) as first-line prophylaxis, as they offer no advantage over first- or second-generation agents and promote resistance 6, 7