Rationale for empirical therapy for intra-abdominal infections
Intra-abdominal infections are generally polymicrobial and caused by intestinal flora. Empirical therapy for intra-abdominal infections should include antibiotics with activity against:
- Enterobacterales (enteric gram-negative bacilli)
- streptococci
- enteric anaerobic bacteria – only if the distal small bowel, appendix or colon is involved, or if obstruction or paralytic ileus is present in patients with more proximal gastrointestinal perforations.
An aminoglycoside (in combination with amoxicillin or ampicillin) is usually preferred to broad-spectrum penicillins or cephalosporins for empirical therapy because it is active against a greater percentage of Enterobacterales. Aminoglycosides are also less likely to contribute to the development of Clostridioides difficile (formerly known as Clostridium difficile) infection and the selection of antibiotic-resistant organisms. To avoid the need to switch between intravenous antibiotics at 72 hours, a non-aminoglycoside–containing regimen may be used if it is suspected that intravenous therapy will continue for at least 72 hours. If the likely duration of intravenous therapy is not known, start with the aminoglycoside-containing regimen; do not delay antibiotic administration to make this determination.
Intravenous amoxicillin+clavulanate is not recommended for first-line empirical therapy for intra-abdominal infections in these guidelines. In Australia, amoxicillin+clavulanate resistance among some gram-negative bacteria is higher than rates of resistance to other (first-line) antibiotics. Local guidelines should be informed by local susceptibility data. Oral amoxicillin+clavulanate is recommended for empirical therapy for nonsevere intra-abdominal infections and as oral continuation therapy because, in these situations, illness is not severe and the bacterial load is likely to be low.
If empirical therapy with activity against anaerobic bacteria is required, metronidazole may be included in the empirical regimen. If amoxicillin+clavulanate or piperacillin+tazobactam is used, additional anaerobic therapy with metronidazole is not required. If clindamycin is used for suspected or confirmed infection with gram-negative anaerobes (especially Bacteroides species) and prompt source control is unlikely to occur (eg within 24 hours), consider adding metronidazole because there is increasing resistance to clindamycin in these species.
Empirical therapy does not need to have activity against enterococci; therapy should be modified if enterococci are isolated from diagnostic samples.
In community-acquired infections, the antimicrobial susceptibility of intestinal flora is usually predictable. However, multidrug-resistant Enterobacterales can occur in patients with risk factors. These patients may also develop infection with other resistant pathogens (eg vancomycin-resistant enterococci) and Candida species1. Seek expert advice to guide antimicrobial choice.
Empirical antifungal therapy is not usually required, but should be considered in patients with:
- high-risk presentations (eg upper gastrointestinal perforation, recurrent bowel leak) if they are not improving with empirical antibiotic therapy or are rapidly deterioratingKeighley 2021
- sepsis or septic shock from a biliary or gastrointestinal tract source who are at increased risk of invasive candidiasis
- an isolated yeast identified in samples from deep surgical sites.
Recommendations for antifungal therapy for intra-abdominal infections are beyond the scope of these guidelines – seek expert advice and consult specialist guidelines (eg Consensus guidelines for the diagnosis and management of invasive candidiasis in haematology, oncology and intensive care settings).