Residency · Residency · Infectiousdisease

Intra-Abdominal Infections

Classification

Uncomplicated vs. Complicated

Intra-abdominal infections are classified along a spectrum of anatomic complexity and clinical severity. Uncomplicated intra-abdominal infections are confined to a single organ without peritoneal extension, exemplified by acute uncomplicated appendicitis and cholecystitis. Complicated intra-abdominal infections, in contrast, extend beyond the organ of origin into the peritoneal space, producing localized or diffuse peritonitis and abscess formation. Primary peritonitis, or spontaneous bacterial peritonitis, occurs in the setting of cirrhosis and ascites without a surgical source. Secondary peritonitis arises from perforation, anastomotic leak, or penetrating injury. Tertiary peritonitis describes persistent or recurrent peritonitis after initially adequate source control and is often caused by MDR or fungal organisms, reflecting the consequences of prolonged ICU stays and repeated antibiotic courses.

Anatomic Sources

The common anatomic sources of intra-abdominal infections include appendicitis, which is the most frequent source, diverticulitis, cholecystitis and cholangitis, perforated peptic ulcer, small bowel perforation, infected pancreatic necrosis, post-surgical anastomotic leak, and pelvic infections including tubo-ovarian abscess.

Microbiology

Polymicrobial Nature

Intra-abdominal infections are characteristically polymicrobial, involving aerobic gram-negatives, anaerobes, and enterococci. E. coli is the most common aerobic gram-negative pathogen, followed by Klebsiella, Proteus, and Pseudomonas in healthcare-associated settings. Among the anaerobes, Bacteroides fragilis is the most clinically important organism, possessing virulence factors including a polysaccharide capsule that promotes abscess formation, and producing beta-lactamase that confers resistance to most penicillins and cephalosporins while maintaining susceptibility to metronidazole, carbapenems, and piperacillin-tazobactam, though declining susceptibility to ampicillin-sulbactam has been reported. The pathogenic role of Enterococcus in community-acquired IAI is debated, but coverage is indicated in healthcare-associated or immunocompromised settings. Candida should be considered in healthcare-associated IAI, post-operative settings, immunocompromised patients, recurrent perforations, and upper gastrointestinal sources in patients on acid suppression.

Diagnosis

Clinical Presentation

The clinical presentation of complicated IAI includes abdominal pain that may be localized or diffuse, fever, and peritoneal signs including guarding, rebound tenderness, and rigidity. Sepsis and septic shock may develop in severe cases. Post-operative IAI may present more subtly with ileus, fever, leukocytosis, and wound drainage.

Imaging

CT of the abdomen and pelvis with intravenous contrast is the imaging modality of choice, achieving sensitivity exceeding 95 percent for abscess, free air, and inflammatory changes. Ultrasound is the first-line imaging study for cholecystitis, demonstrating gallstones, gallbladder wall thickening, pericholecystic fluid, and the sonographic Murphy sign. MRI and MRCP are useful for cholangitis evaluation when ERCP is not immediately available and offer the advantage of avoiding radiation. Plain abdominal radiography has limited utility but may demonstrate free air suggesting perforation or air-fluid levels suggesting obstruction.

Laboratory

Laboratory findings typically include leukocytosis with a left shift, elevated CRP and procalcitonin, and elevated lactate as a marker of disease severity. Blood cultures should be obtained when the patient is septic and are positive in 20 to 30 percent of complicated IAI cases. Peritoneal fluid and abscess fluid should be submitted for Gram stain and aerobic and anaerobic cultures.

<image>An anatomic illustration of the abdomen showing common sources of intra-abdominal infections. Use a transparent anterior view of the abdomen with major organs visible. Label and highlight with different colors: (1) Perforated peptic ulcer (stomach), (2) Cholecystitis/cholangitis (gallbladder and bile ducts), (3) Appendicitis (right lower quadrant), (4) Diverticulitis with perforation (sigmoid colon), (5) Small bowel perforation, (6) Tubo-ovarian abscess (pelvis), (7) Infected pancreatic necrosis. For each source, include a small text box listing the most common causative organisms. Include an inset showing the peritoneal cavity with fluid/abscess. Use medical illustration style with anatomic accuracy.</image>

Source Control

Principles

Source control is the cornerstone of IAI management, and antibiotics administered without adequate source control will fail. Delay in source control beyond 24 hours increases mortality, particularly in secondary peritonitis. The modalities of source control include percutaneous drainage, which is the preferred approach for accessible abscesses; surgical debridement or repair; and endoscopic intervention, most notably ERCP for cholangitis.

Specific Source Control Approaches

Appendicitis is traditionally managed with appendectomy, with laparoscopic technique preferred, though the CODA trial in 2020 demonstrated that antibiotics alone are non-inferior to surgery at 30 days for uncomplicated appendicitis, with the caveat that 29 percent of antibiotic-managed patients required surgery within 90 days. Complicated diverticulitis with abscess larger than 3 centimeters is managed with percutaneous drainage, while surgery is indicated for diffuse peritonitis or failed drainage. Cholecystitis requires cholecystectomy, ideally within 72 hours, with cholecystostomy tube placement as an alternative for patients too ill for surgery. Cholangitis demands emergent ERCP with biliary decompression for severe cases. Infected pancreatic necrosis benefits from delayed intervention, ideally beyond four weeks to allow walling off, following a step-up approach: percutaneous drainage first, escalating to endoscopic necrosectomy if unsuccessful, and open surgical necrosectomy as a last resort, as established by the PANTER and TENSION trials.

Antibiotic Therapy

Community-Acquired, Mild-Moderate

Single-agent options for community-acquired mild-to-moderate IAI include ertapenem 1 gram intravenously daily, which provides excellent coverage including anaerobes with convenient once-daily dosing but lacks Pseudomonas activity, and moxifloxacin 400 milligrams intravenously or orally daily, which covers gram-negatives and anaerobes but raises concerns for resistance. Combination regimens include ceftriaxone 2 grams intravenously daily plus metronidazole 500 milligrams intravenously every 8 hours, ciprofloxacin plus metronidazole, and ampicillin-sulbactam 3 grams intravenously every 6 hours, though the utility of ampicillin-sulbactam is limited by E. coli resistance exceeding 30 percent in many institutions.

Healthcare-Associated or Severe

For healthcare-associated or severe IAI, piperacillin-tazobactam 4.5 grams intravenously every 6 hours as an extended infusion provides coverage of gram-negatives including Pseudomonas, anaerobes, and enterococcus. Meropenem 1 gram every 8 hours or imipenem-cilastatin 500 milligrams every 6 hours offers the broadest coverage and should be reserved for patients with MDR risk or severe illness. Cefepime 2 grams every 8 hours plus metronidazole provides anti-pseudomonal plus anaerobic coverage. Vancomycin is added if MRSA is suspected. Antifungal therapy with fluconazole or an echinocandin is indicated when Candida risk factors are present.

Enterococcal Coverage

For community-acquired mild-to-moderate IAI, routine enterococcal coverage is not required, as supported by the SIS/IDSA guidelines. In healthcare-associated or severe infections, Enterococcus should be covered, and piperacillin-tazobactam and carbapenems other than ertapenem provide E. faecalis coverage. VRE coverage with linezolid or daptomycin is indicated in immunocompromised patients, liver transplant recipients, and patients with known VRE colonization.

Duration of Therapy

STOP-IT Trial (2015)

The STOP-IT trial was a landmark randomized controlled trial that compared four days of antibiotic therapy to continuation until clinical resolution, with a median of eight days, after adequate source control had been achieved. The trial demonstrated no difference in surgical site infection, recurrent IAI, or death, establishing four days of post-source-control antibiotic therapy as the standard for most complicated IAI. The critical prerequisite is that adequate source control must have been achieved.

Current Duration Recommendations

For complicated IAI with adequate source control, four days of antibiotic therapy is the standard based on the STOP-IT trial. Without adequate source control, antibiotics should be continued until source control is achieved plus four days afterward, or until clinical resolution. Uncomplicated appendicitis or cholecystitis post-operatively requires 24 hours or less of antibiotics, and a single preoperative dose may suffice for uncomplicated cases. Uncomplicated diverticulitis requires four to seven days of therapy, and notably, mild cases may not require antibiotics at all, as demonstrated by the AVOD and DIABOLO trials.

Special Situations

Spontaneous Bacterial Peritonitis (SBP)

SBP is diagnosed by an ascitic fluid polymorphonuclear cell count of 250 cells per cubic millimeter or above, with or without a positive culture. The microbiology is characteristically monomicrobial, with E. coli, Klebsiella, and S. pneumoniae being the most common organisms. Recovery of polymicrobial organisms from ascitic fluid culture should raise suspicion for secondary peritonitis from a surgical source, prompting CT imaging. Treatment consists of ceftriaxone 2 grams intravenously daily for five days, with narrowing based on culture results. Albumin infusion at 1.5 grams per kilogram on day 1 and 1 gram per kilogram on day 3 reduces renal failure and mortality, as demonstrated in the Sort 1999 trial, and is indicated when creatinine exceeds 1 milligram per deciliter, BUN exceeds 30, or bilirubin exceeds 4. Secondary prophylaxis with norfloxacin 400 milligrams daily or trimethoprim-sulfamethoxazole double-strength daily is recommended after a prior SBP episode, and primary prophylaxis for seven days is indicated during gastrointestinal bleeding in cirrhosis.

Infected Pancreatic Necrosis

Infected pancreatic necrosis develops in 20 to 40 percent of necrotizing pancreatitis cases, usually two to four weeks after onset. Diagnosis is suggested by gas within a necrotic collection on CT imaging, and CT-guided fine-needle aspiration achieves sensitivity of 80 to 90 percent. Carbapenems are the preferred antibiotic class given their superior pancreatic penetration, with fluoroquinolones plus metronidazole as an alternative. The step-up approach established by the PANTER trial favors percutaneous drainage as the initial intervention, followed by minimally invasive retroperitoneal necrosectomy if drainage is insufficient, with open necrosectomy reserved as the last resort. Prophylactic antibiotics for acute pancreatitis are not recommended, as they have not been shown to reduce the incidence of infected necrosis and risk selecting for resistant organisms and Candida.

<image>A step-by-step management algorithm for complicated intra-abdominal infections. Start with "Complicated IAI suspected (peritonitis, abscess)." Step 1: "Resuscitation (IV fluids, vasopressors if needed) + obtain cultures (blood, peritoneal fluid) + CT abdomen/pelvis with IV contrast." Step 2: "Start empiric antibiotics" with two branches: "Community-acquired, mild-moderate" (listing ceftriaxone + metronidazole or ertapenem) and "Healthcare-associated or severe" (listing pip-tazo or meropenem +/- antifungal). Step 3: "SOURCE CONTROL within 24 hours" in a prominent red box, with options: "Percutaneous drainage (abscess >3cm)," "Surgical intervention (perforation, diffuse peritonitis)," or "Endoscopic (cholangitis: ERCP)." Step 4: "48-72h reassessment: de-escalate based on cultures." Step 5: "Duration: 4 days after adequate source control (STOP-IT trial)." Use a professional clinical pathway format with timing markers.</image>

Key Clinical Pearls

• Source control is the most important intervention in complicated IAI -- antibiotics without source control will fail
• The STOP-IT trial established 4 days as adequate antibiotic duration after successful source control -- longer courses do not improve outcomes
• Routine enterococcal coverage is NOT needed for community-acquired mild-moderate IAI
• Prophylactic antibiotics for acute pancreatitis do NOT reduce infected necrosis and should be avoided
• For infected pancreatic necrosis, delay intervention to ≥4 weeks when possible and use a step-up approach (drain before you debride)
• SBP is a monomicrobial infection -- polymicrobial ascitic fluid culture should raise suspicion for secondary peritonitis (surgical source) requiring CT imaging
• Albumin infusion with SBP treatment reduces mortality in patients with renal dysfunction -- do not forget this critical adjunct

References

1. Solomkin JS, Mazuski JE, Bradley JS, et al. Diagnosis and management of complicated intra-abdominal infection in adults and children (SIS/IDSA 2010). Clin Infect Dis. 2010;50(2):133-164.
2. Sawyer RG, Claridge JA, Nathens AB, et al. Trial of short-course antimicrobial therapy for intraabdominal infection (STOP-IT). N Engl J Med. 2015;372(21):1996-2005.
3. van Santvoort HC, Besselink MG, Bakker OJ, et al. A step-up approach or open necrosectomy for necrotizing pancreatitis (PANTER). N Engl J Med. 2010;362(16):1491-1502.
4. Sort P, Navasa M, Arroyo V, et al. Effect of intravenous albumin on renal impairment and mortality in patients with cirrhosis and spontaneous bacterial peritonitis. N Engl J Med. 1999;341(6):403-409.
5. CODA Collaborative. A randomized trial comparing antibiotics with appendectomy for appendicitis. N Engl J Med. 2020;383(20):1907-1919.

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