intra-abdominal infections
in review
5 min read
Updated 2026-09-29
intra-abdominal infections
Source control (drainage, debridement, surgery) is the primary intervention — antibiotics are adjunctive. Cover aerobic Gram-negatives + anaerobes. Community-acquired, mild–moderate: ceftriaxone + metronidazole. Shock/ICU or healthcare-associated severe: piperacillin-tazobactam (meropenem if prior ESBL). 4 days after adequate source control (STOP-IT (2015)); stop post-op if the infection was fully removed.
intra-abdominal infection empiric antibiotics
- uncomplicated (non-perforated appendicitis): cefazolin 2 g IV q8h + metronidazole 500 mg PO/IV q12h
- complicated, community-acquired, mild–moderate: ceftriaxone 1–2 g IV q24h + metronidazole 500 mg PO/IV q12h
- shock / new organ failure / ICU: piperacillin-tazobactam 4.5 g IV q6h (meropenem 1 g IV q8h if prior ESBL)
- healthcare-associated (≥5 days in hospital), mild–moderate: ceftriaxone + metronidazole
- if β-lactams must be avoided: ciprofloxacin 400 mg IV q12h + metronidazole — see penicillin allergy
- SOURCE CONTROL within 24 h — antibiotics do not substitute for drainage/surgery
- duration: 4 days after source control; no source control → 7–10 days with repeat imaging
local antibiogram
E. coli fluoroquinolone resistance is rising across Canadian centres — ciprofloxacin-based regimens need local susceptibility >90% to stay a reasonable alternative.
classification
| type | definition | examples |
|---|---|---|
| uncomplicated | confined to a single organ, no peritoneal extension | non-perforated appendicitis, cholecystitis |
| complicated | extends beyond organ of origin into peritoneum | perforated appendicitis, diverticular abscess, bowel perforation |
| primary peritonitis (SBP) | infection of ascitic fluid, no surgical source | Spontaneous Bacterial Peritonitis |
| secondary peritonitis | contamination from GI perforation/leak | perforated viscus, anastomotic leak |
| tertiary peritonitis | persistent/recurrent infection after adequate source control | post-operative, resistant organisms or Candida |
source control
source control is definitive
Antibiotics without source control → treatment failure. Percutaneous drainage or surgery within 24 h for drainable collections, perforated viscus, and ischaemic bowel.
- percutaneous drainage for a well-defined abscess with safe access — diverticular/pericolic abscess <3 cm often settles on antibiotics alone; ≥3 cm → drain
- surgery for diffuse peritonitis, ischaemic bowel, free perforation
- cholecystectomy within 72 h for acute cholecystitis
- appendicectomy for complicated appendicitis; antibiotics-first is an option for selected uncomplicated appendicitis
empiric therapy — detail
community-acquired
| scenario | regimen | β-lactams contraindicated |
|---|---|---|
| uncomplicated (non-perforated appendicitis) | cefazolin 2 g IV q8h + metronidazole 500 mg q12h → cephalexin + metronidazole PO | ciprofloxacin + metronidazole |
| perforation without established infection (stomach, duodenum, traumatic bowel to OR within 12–24 h) | cefazolin + metronidazole | ciprofloxacin + metronidazole |
| complicated, mild–moderate (perforated appendicitis/diverticulitis) | ceftriaxone 1–2 g IV q24h + metronidazole → amoxicillin-clavulanate 875/125 mg PO BID | ciprofloxacin + metronidazole |
| high severity (shock, new organ failure, ICU) | piperacillin-tazobactam 4.5 g IV q6h (meropenem if prior ESBL) | meropenem |
healthcare-associated (≥5 days in hospital, post-op)
- mild–moderate (anastomotic leak, post-op abscess): ceftriaxone + metronidazole; ertapenem 1 g IV q24h if β-lactam alternative needed
- high severity: piperacillin-tazobactam; meropenem if prior ESBL
- Enterococcus: pip-tazo and ampicillin cover E. faecalis; add vancomycin only for E. faecium risk (prior colonisation, transplant, prolonged cephalosporin exposure) — VRE colonisation → linezolid or daptomycin instead
- Candida cover (echinocandin) only if: tertiary peritonitis, upper GI perforation/leak after prior broad-spectrum antibiotics, or Candida on Gram stain
biliary
| scenario | regimen | duration |
|---|---|---|
| acute calculous cholecystitis, no shock | cefazolin 2 g IV q8h if local E. coli cefazolin susceptibility is high; otherwise ceftriaxone 1–2 g IV q24h | stop post-op if no perforation; 4 days post-op if perforated; 7 days if managed medically |
| acute cholangitis, no shock | ceftriaxone 1–2 g IV q24h | 4 days after decompression; 7 days if managed medically |
| either, with shock/ICU | piperacillin-tazobactam (meropenem if prior ESBL) | as above |
Anaerobic cover for biliary infection only with biliary-enteric anastomosis.
other intra-abdominal syndromes
- uncomplicated diverticulitis (immunocompetent, no systemic features, no abscess on CT): antibiotics can be omitted — observation is non-inferior
- acute pancreatitis: no prophylactic antibiotics, even for necrotising disease; suspect infected necrosis with gas on CT or deterioration after week 1 → carbapenem (or fluoroquinolone + metronidazole), delay intervention ≥4 weeks where possible (step-up approach)
- pyogenic liver abscess: drain if large or not responding; ceftriaxone + metronidazole; 4–6 weeks total; Klebsiella → screen for eye/CNS metastatic foci; send Entamoeba serology if travel exposure
penicillin allergy
- ceftriaxone and cefazolin share no side chains with penicillins → usable after most penicillin reactions, including anaphylaxis (except near-fatal within 10 years)
- avoid all β-lactams only after SCAR, serum sickness, AIN, or haemolysis → ciprofloxacin + metronidazole only if local E. coli fluoroquinolone susceptibility ≥90%; otherwise ID input (e.g. aminoglycoside-based regimen)
- carbapenems: negligible penicillin cross-reactivity
- full framework: penicillin allergy
duration
STOP-IT (2015): fixed ~4 days after adequate source control was non-inferior to treating until physiological resolution (median ~8 days), across community- and healthcare-associated complicated IAI.
| scenario | duration |
|---|---|
| complicated IAI + adequate source control | 4 days after source control |
| infected tissue fully removed (uncomplicated appendicitis/cholecystitis, early perforation repair) | stop post-op (≤24 h) |
| complicated IAI managed medically | 10 days (10–14 if healthcare-associated) — repeat imaging to guide |
| SBP | 5 days (see Spontaneous Bacterial Peritonitis) |
| tertiary peritonitis | individualised; guided by cultures and source control |
when to broaden or escalate
- no improvement at 48–72 h → repeat CT with contrast for undrained collection, leak, or new perforation
- Candida on sterile-site culture → echinocandin (anidulafungin 200 mg load → 100 mg daily or micafungin 100 mg daily)
- resistant Gram-negatives → tailor to susceptibilities; carbapenem for ESBL; ID for carbapenem-resistant organisms
traps
- antibiotics without source control is the most common error — always look for a drainable collection
- post-op antibiotics beyond 24 h after appendicectomy/cholecystectomy for uncomplicated disease add nothing
- Enterococcus in community-acquired polymicrobial IAI usually needs no specific cover
- avoid routine antifungal cover
- prolonged antibiotics for IAI → [[Clostridioides difficile infection|C. difficile]] risk without benefit
- SBP in cirrhosis is a distinct entity with different empirics — see Spontaneous Bacterial Peritonitis