Antibiotic Prophylaxis in Surgery
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Direct answer
Surgical antibiotic prophylaxis aims to achieve bactericidal tissue levels at the moment of incision, so the first dose must be given within 60 minutes before surgery (within 120 minutes for vancomycin or fluoroquinolones because of longer infusion times). A single preoperative dose suffices for most procedures, with redosing indicated if the operation runs beyond about 3–4 hours, blood loss exceeds 1500 mL, or excessive irrigation occurs; prophylaxis should stop within 24 hours in almost all cases. Prophylaxis supplements but never replaces meticulous sterile technique, and choice follows the likely contaminating flora of the operation — cefazolin for clean surgery, additional anaerobic cover such as metronidazole for colorectal procedures.
What you must remember
- Timing: intravenous administration within 60 minutes before incision; vancomycin and ciprofloxacin require a 120-minute window; a dose given after incision is too late for prophylaxis.
- Redose when the operation exceeds roughly twice the dosing interval (commonly quoted at about 4 hours for cefazolin), or with major blood loss over 1500 mL.
- Duration: discontinuation within 24 hours is standard for most surgeries; prolonged courses do not prevent surgical site infection and select resistant organisms.
- Standard agents: first-generation cephalosporin (cefazolin) for most clean and clean-contaminated cases; add metronidazole for colorectal and appendicular surgery; clindamycin or vancomycin for beta-lactam allergy or known MRSA colonisation.
- Prophylaxis for implant placement (mesh, prosthesis, cardiac valves) is accepted; it is not indicated for clean surgery without implants in most guidance.
- Mechanical bowel preparation combined with oral antibiotics is used selectively in elective colorectal surgery per current practice.
- Poor prophylaxis substitutes: shaving instead of clipping, using antibiotics to treat a collection that needs drainage, and treating wound classification as an indication for prolonged postoperative courses.
Common confusion
The recurring error is conflating prophylaxis with treatment: a dirty, already infected wound (perforated appendix with abscess) requires a therapeutic course, whereas an elective contaminated-but-uninfected field needs only appropriately timed perioperative cover. A second classic mistake is ordering the first dose 'on call to theatre' in a way that it lands hours before or after incision, missing the window of adequate tissue levels; anaesthetic induction is the practical trigger.
Exam-focused takeaway
NEET-PG tests this as precise one-best-answer facts: the 60-minute window, the 24-hour stop, the redosing triggers, and drug selection by operation — metronidazole with colorectal cases, vancomycin for MRSA-colonised or beta-lactam-allergic patients. Stems may show a chart with drug timings and ask which regimen is correct, or ask which factor does not increase surgical site infection risk. Remember also that prophylaxis has no role once an infection is established; source control comes first.
Frequently asked questions
Why must prophylaxis be given before incision rather than after?
Antibiotics need time to reach bactericidal levels in tissue; starting after contamination means the bacteria have already established during the window of inadequate levels.
When should intraoperative redosing occur?
Commonly when surgery extends beyond about 3–4 hours from the preoperative dose, after blood loss over 1500 mL, or per the agent's half-life, to maintain tissue levels.
Which antibiotic is standard for elective colorectal surgery?
A cephalosporin plus metronidazole for anaerobic cover, with selective addition of oral antibiotics to mechanical bowel preparation in current practice.
Is prolonged postoperative prophylaxis beneficial?
No; continuing beyond 24 hours does not reduce infection rates, increases adverse effects and Clostridioides difficile diarrhoea, and promotes resistance.
What if the patient is MRSA colonised?
Vancomycin (or teicoplanin where used) is substituted for or added to the standard agent, ideally with nasal decolonisation before elective surgery per guidance.