Antibiotic prophylaxis means a short course of antibiotic — usually one dose — given before the incision to reduce surgical site infection. It is not treatment. Getting it right is about four things: whether to give it, which drug, when, and for how long.
1Whether: the wound classification
The risk of infection depends on how contaminated the operation is. Classify every operation.
| Class | Definition | Examples | Antibiotic |
|---|---|---|---|
| I Clean | No viscus entered; no inflammation | Thyroidectomy, hernia repair, breast lump excision | Not routinely; yes if a prosthesis or mesh is implanted, or infection would be catastrophic |
| II Clean-contaminated | GI, biliary, respiratory or urinary tract entered under controlled conditions | Cholecystectomy, elective colectomy, gastric bypass, sleeve gastrectomy | Yes, prophylaxis |
| III Contaminated | Spillage from the gut, acute non-purulent inflammation, open fresh trauma, a major break in sterile technique | Uncomplicated appendicitis, bowel spillage during surgery | Yes; may become treatment |
| IV Dirty | Established infection: pus, perforated viscus, old traumatic wound | Perforated appendix with pus, faecal peritonitis | Treatment, not prophylaxis: a course of antibiotics |
Laparoscopic cholecystectomy
For elective, low-risk laparoscopic cholecystectomy, routine prophylaxis is not required by several guidelines. Give it to higher-risk patients: age over 70, acute cholecystitis, jaundice or common bile duct stones, conversion to open, diabetes, immunosuppression, or pregnancy. Many units give a single dose to all — follow your local policy.
2Which drug
Choose an antibiotic that covers the organisms likely to contaminate the wound: skin organisms (Staphylococcus aureus, streptococci) for clean surgery, plus gut organisms (Gram-negative bacilli and anaerobes) when the bowel is opened.
| Operation | Common first choice | Severe penicillin allergy |
|---|---|---|
| Hernia repair with mesh, breast, vascular, implants | Cefazolin | Clindamycin or vancomycin |
| Gastric, duodenal and bariatric surgery | Cefazolin (weight-adjusted) | Clindamycin or vancomycin, plus gentamicin |
| Biliary surgery (high-risk patients) | Cefazolin | Clindamycin or vancomycin, plus gentamicin |
| Appendicectomy, small bowel, colorectal | Cefazolin plus metronidazole (or co-amoxiclav in many UK units) | Clindamycin plus gentamicin; or gentamicin plus metronidazole |
| Known MRSA carrier | Add vancomycin or teicoplanin, and decolonise before elective surgery | — |
Doses (adults)
| Drug | Dose | Repeat during surgery after |
|---|---|---|
| Cefazolin | 2 g IV; 3 g if 120 kg or more | 4 hours |
| Cefuroxime | 1.5 g IV | 4 hours |
| Metronidazole | 500 mg IV | Not usually needed |
| Co-amoxiclav | 1.2 g IV | About 2–4 hours (local policy) |
| Gentamicin | Single dose by weight (for example 5 mg/kg) | Not usually needed |
| Clindamycin | 900 mg IV | 6 hours |
| Vancomycin | 15 mg/kg IV, infused over at least 60 minutes | Not usually needed |
Penicillin allergy: ask what happened
Most patients labelled penicillin-allergic are not. A rash years ago is not anaphylaxis. With a non-severe history, cefazolin is usually safe, because cross-reactivity is very low. Keep the alternatives for anaphylaxis, angioedema or a severe skin reaction.
3When: timing is everything
- Give it within 60 minutes before the incision, so the drug is in the tissues when the skin is cut. Vancomycin and fluoroquinolones need longer to infuse: start them 60–120 minutes before.
- Give it before a tourniquet is inflated.
- Repeat the dose if the operation outlasts about two half-lives of the drug (cefazolin after 4 hours), or if blood loss exceeds 1500 mL in an adult.
- For caesarean section, give it before the skin incision, not after the cord is clamped.
The WHO surgical safety checklist asks this in the time-out: “Has antibiotic prophylaxis been given within the last 60 minutes?”
4How long: one dose is usually enough
For prophylaxis, a single dose is enough for most operations, and it should not go beyond 24 hours. Continuing antibiotics after the wound is closed does not reduce infection, but it does increase antibiotic resistance, Clostridioides difficile infection and kidney injury. The WHO recommends against prolonging prophylaxis after surgery.
Prophylaxis versus treatment
If infection is already present — a perforated appendix with pus, faecal peritonitis, an abscess — the antibiotic is treatment. After adequate source control, about 4 days of antibiotics is usually enough for complicated intra-abdominal infection (the STOP-IT trial).
Antibiotics are one part of preventing wound infection
Practise
Sources and local practice
Based on the ASHP/IDSA/SIS/SHEA clinical practice guidelines for antimicrobial prophylaxis in surgery (2013), the WHO global guidelines for the prevention of surgical site infection (2016, updated 2018), NICE NG125 (surgical site infections), and the STOP-IT trial (Sawyer et al., 2015). Antibiotic choice depends on local resistance patterns: always follow your hospital’s antibiotic policy.