intra-abdominal infections
in review
5 min read
Updated 2026-08-23
Contents
intra-abdominal infections
Source control (drainage, debridement, or surgery) is the primary intervention — antibiotics are adjunctive. Empiric cover must target aerobic gram-negatives + anaerobes. Community-acquired, mild-moderate: ceftriaxone + metronidazole or piperacillin-tazobactam. Post-operative or healthcare-associated: broader cover with piperacillin-tazobactam or carbapenem. Duration is 4 days after adequate source control (STOP-IT (2015)).
intra-abdominal infection empiric antibiotics
- first-line (community-acquired, mild-moderate): ceftriaxone 2 g IV daily + metronidazole 500 mg IV q8h
- alternative first-line: piperacillin-tazobactam 4.5 g IV q6h (monotherapy — covers gram-negatives + anaerobes)
- if penicillin/cephalosporin allergy: ciprofloxacin 400 mg IV q12h + metronidazole 500 mg IV q8h
- if healthcare-associated / post-op / resistant organisms: pip-taz 4.5 g IV q6h or meropenem 1 g IV q8h ± vancomycin
- if critically ill / septic shock: meropenem 1 g IV q8h ± vancomycin
- SOURCE CONTROL within 24 h — antibiotics do not substitute for drainage/surgery
- duration: 4 days after adequate source control (STOP-IT)
local antibiogram
E. coli fluoroquinolone resistance is rising across Canadian centres. Empiric ciprofloxacin-based regimens require local susceptibility rates >90% to remain first-line alternatives.
classification
| type | definition | examples |
|---|---|---|
| uncomplicated | infection confined to single organ, no peritoneal extension | uncomplicated appendicitis, cholecystitis |
| complicated | extends beyond organ of origin into peritoneum | perforated appendicitis, diverticular abscess, bowel perforation |
| primary peritonitis (SBP) | spontaneous infection of ascitic fluid, no surgical source | cirrhosis with Spontaneous Bacterial Peritonitis |
| secondary peritonitis | peritoneal contamination from GI perforation/leak | perforated viscus, anastomotic leak |
| tertiary peritonitis | persistent/recurrent infection after adequate source control | post-operative, often resistant organisms or Candida |
source control
source control is definitive
Antibiotics without source control → treatment failure. Percutaneous drainage or surgery within 24 h for all drainable collections, perforated viscus, and ischaemic bowel.
- percutaneous drainage preferred for well-defined abscess with safe access route
- surgery for diffuse peritonitis, ischaemic bowel, perforation without contained collection
- cholecystectomy within 72 h for acute cholecystitis
- appendicectomy remains standard for complicated appendicitis (antibiotics-first is an option for uncomplicated appendicitis in selected patients)
empiric therapy — detail
community-acquired, mild-moderate
| regimen | notes |
|---|---|
| ceftriaxone 2 g IV daily + metronidazole 500 mg IV q8h | most common Canadian choice — good gram-negative + anaerobic cover |
| piperacillin-tazobactam 4.5 g IV q6h | single-agent convenience; slightly broader spectrum |
| ertapenem 1 g IV daily | once-daily dosing; reserve for penicillin-tolerant patients with ESBL risk |
| ciprofloxacin 400 mg IV q12h + metronidazole 500 mg IV q8h | β-lactam allergy option — check local fluoroquinolone resistance |
healthcare-associated / high-risk
Risk factors: prior antibiotics, post-operative, healthcare facility resident, prior resistant organisms.
- piperacillin-tazobactam 4.5 g IV q6h — covers most healthcare-associated pathogens
- if ESBL risk or critically ill: meropenem 1 g IV q8h
- add vancomycin if Enterococcus concern (biliary source, post-operative, immunocompromised)
- Candida cover (anidulafungin or micafungin) if: tertiary peritonitis, post-operative leak with prior broad-spectrum antibiotics, Candida on gram stain
biliary-source infections
- uncomplicated cholecystitis (mild): ceftriaxone 2 g IV daily (anaerobic cover not needed — bile is normally sterile)
- complicated cholecystitis / cholangitis: piperacillin-tazobactam 4.5 g IV q6h
- post-ERCP cholangitis: broader cover including Enterococcus — pip-taz or ampicillin + gentamicin
penicillin allergy pathway
| allergy history | approach |
|---|---|
| non-immune side effects | cephalosporins safe — ceftriaxone + metronidazole |
| distant mild rash | ceftriaxone generally safe (<2% cross-reactivity) |
| confirmed anaphylaxis | ciprofloxacin + metronidazole; or aztreonam 2 g IV q8h + metronidazole |
| severe allergy + ESBL/critically ill | meropenem (negligible penicillin cross-reactivity with carbapenems) |
duration
STOP-IT (2015): 4 days of antibiotics after adequate source control is non-inferior to continuation until clinical and laboratory resolution (median 8 days). This applies to community-acquired complicated IAI with adequate source control.
| scenario | duration |
|---|---|
| complicated IAI + adequate source control | 4 days after source control |
| complicated IAI + inadequate source control | continue until source controlled, then 4 days |
| uncomplicated appendicitis (post-appendicectomy) | 24 h or less |
| uncomplicated cholecystitis (post-cholecystectomy) | 24 h or less |
| SBP | 5 days (see Spontaneous Bacterial Peritonitis) |
| tertiary peritonitis | individualised — often 2–4 weeks; guided by cultures and clinical response |
when to broaden or escalate
- no clinical improvement at 48–72 h → repeat imaging (CT with contrast) to assess for undrained collection, anastomotic leak, or new perforation
- persistent Candida on cultures → add echinocandin (anidulafungin 200 mg load → 100 mg daily or micafungin 100 mg daily)
- resistant gram-negatives on culture → tailor per susceptibilities; carbapenem for ESBL; consult ID for carbapenem-resistant organisms
traps
- antibiotics without source control is the most common error — always assess for drainable collection
- do not treat “peritoneal contamination” — brief intra-operative contamination from a clean perforation (e.g. traumatic bowel injury repaired within 12 h) requires only 24 h of antibiotics
- uncomplicated appendicitis and cholecystitis: post-operative antibiotics beyond 24 h provide no benefit
- Enterococcus in community-acquired polymicrobial IAI usually does not require specific cover — treat only if isolated from blood cultures, immunocompromised, or healthcare-associated
- avoid routine anti-fungal cover — reserve for tertiary peritonitis, immunocompromised, or Candida on gram stain/culture
- SBP in cirrhosis is a distinct entity with different empirics — see Spontaneous Bacterial Peritonitis