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Antibiotic Prophylaxis — Timing, Weight Dose, Redose, Allergy Decision
TEXTPre-anesthesia · 6 min read
Wrong drug, wrong dose, wrong time = preventable surgical site infection + a SCIP/SUS bundle failure. The penicillin-allergy decision tree has changed dramatically in the last decade.
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3 min read6 sectionsTiming — the 60-minute (or 120-minute) window
Vancomycin and fluoroquinolones require 120 minutes because of longer infusion times (vanc must be given over 60 min to avoid red-man syndrome).
Tourniquet inflation counts as incision for orthopedic cases — antibiotic must be fully infused before the tourniquet goes up.
Document the time the infusion completed, not started.
SCIP/SUS bundle counts this metric for joint replacement, CABG, and colorectal cases.
Cefazolin — the workhorse + weight-based dosing
Dose: 2 g IV for adults <120 kg; 3 g IV for ≥120 kg.
Peds: 30 mg/kg.
Redose intraop every 4 hr from the FIRST dose (not from incision) or after blood loss >1500 mL.
Half-life ~1.8 hr in normal renal function.
Add vancomycin (not replace cefazolin) for institutions with high MRSA prevalence or known carriers undergoing joint replacement or cardiac surgery — combo is now the standard at most academic centers.
Other agents + redose intervals
15 mg/kg (actual body weight, max 2 g per dose at most institutions) over 60 min; redose only after 8-12 hr (long half-life) — most cases don't need redose.Clindamycin 900 mg over 10-60 min, redose q6h.
Metronidazole 500 mg, redose q12h (long half-life).
Gentamicin 5 mg/kg, redose q24h typically.
Cefoxitin or ceftriaxone for colorectal/biliary; ampicillin-sulbactam alternative.
Piperacillin-tazobactam reserved for established infection, not prophylaxis.
Renal dose adjustment for vancomycin + aminoglycosides only — cefazolin and most beta-lactams don't need adjustment for a single dose.
Procedure-specific regimens
2-3 g + vancomycin 15 mg/kg if MRSA risk.Cardiac (CABG, valve): cefazolin + vancomycin standard; redose cefazolin after CPB initiation (hemodilution drops levels).
- cefazolin + metronidazole
- OR cefoxitin alone
- OR ertapenem (single-dose, long half-life)
Hysterectomy/C-section: cefazolin (given before incision for C-section per ACOG 2018 — earlier practice of after-cord-clamp is obsolete).
Neurosurgery (craniotomy, shunt): cefazolin; vancomycin if shunt or CSF leak.
Vascular (groin incisions): cefazolin + vancomycin if prosthetic.
Beta-lactam allergy — the modern decision tree
Modern evidence (Blumenthal et al., BMJ 2018): >90% of patients labeled penicillin-allergic do NOT have a true IgE-mediated allergy, and cross-reactivity between penicillin and cefazolin is <1%.
Modern decision tree: (1) Rash-only, GI upset, or unknown reaction history cefazolin is safe; give it.
Allergy delabeling pathways are becoming standard preop, with single-dose oral amoxicillin challenges for low-risk histories.
Common errors that fail the SCIP/SUS bundle
Tourniquet inflated before vancomycin done infusing.
Forgetting redose after long case (>4 hr cefazolin, >1500 mL blood loss).
Defaulting to clindamycin for a 'penicillin rash 30 years ago' when cefazolin is actually safe + more effective (clinda has higher SSI rates in joint replacement per recent data).
Wrong weight band (3 g cefazolin for 90 kg patient — unnecessary; 2 g for 130 kg patient — underdosed).
Documenting 'antibiotic given' without time-of-completion.
End of lecture
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