How to classify intra-abdominal infections

Intra-abdominal infections (IAIs) include many pathological conditions, ranging from uncomplicated appendicitis to faecal peritonitis, presenting a wide variation in the severity of illness for the different forms. Designing a classification which is accepted worldwide stratifying patients according to the their risk for poor prognosis may be important to modulate the treatment. However the term risk is “vague” and since there is no typical risk profile, risk stratification remain difficult for patients with IAIs.

A traditional classification universally accepted divides intra-abdominal infections into complicated and uncomplicated. In the event of uncomplicated IAIs, the infection only involves a single organ and does not extend to the peritoneum and when the focus of infection is treated effectively by surgical excision, perioperative prophylaxis is sufficient. Patients with uncomplicated intra-abdominal infection, including acute diverticulitis and certain forms of acute appendicitis and acute cholecystitis, may be managed non-operatively. In the event of complicated IAIs (cIAIs) the infectious process proceeds beyond the organ, causing either localized or diffuse peritonitis. The treatment of patients with complicated intra-abdominal infections involves generally both source control and antibiotic therapy. Complicated IAIs can be further subcategorized into localized or diffuse cIAIs (peritonitis).

This classification has been questioned by some authors because it often may incite confusion by mixing elements of anatomical barrier disruption and severity of disease expression. To differentiate complicated Vs uncomplicated intra-abdominal infections is often difficult by clinical and laboratory diagnosis and radiology is necessary. When it is available, computerized tomography (CT) with intravenous contrast is the standard imaging modality of choice for most detecting intra-abdominal infections. CT can generally identify the source of infection and can value the extension of the infectious process distinguishing uncomplicated from complicated IAIs. In experienced hands, the ultrasound can reliably diagnose most acute abdominal conditions and differentiate complicated from uncomplicated in most patients. Abdominal ultrasound has the advantage of being portable and may be helpful in the evaluation of right upper quadrant (eg, complicated Vs uncomplicated cholecystitis), right lower quadrant (eg, complicated Vs uncomplicated appendicitis), but the examination is sometimes limited because of patient discomfort, abdominal distension, and bowel gas interference.

Peritonitis is an inflammation of the peritoneum. Depending on the underlying pathology, it can be infectious or sterile. Infectious peritonitis is classified into primary peritonitis, secondary peritonitis, and tertiary peritonitis. Primary peritonitis is a diffuse bacterial infection (usually caused by a single organism) without loss of integrity of the gastrointestinal tract, typically seen in cirrhotic patients with ascites or in patients with a peritoneal dialysis catheter. It has a low incidence on surgical wards and is usually managed without any surgical intervention. Secondary peritonitis, the most common form of peritonitis, is an acute peritoneal infection resulting from loss of integrity of the gastrointestinal tract. Tertiary peritonitis is a recurrent infection of the peritoneal cavity that occurs >48 h after apparently successful and adequate surgical source control of secondary peritonitis. It is more common among critically ill or immunocompromised patients and may be often associated with multidrug-resistant organisms (MDROs). It is typically associated with high morbidity and mortality. Tertiary peritonitis has been accepted as a distinct entity. However, it represents an evolution and complication of secondary peritonitis . The term “ongoing peritonitis”  or “persistent peritonitis” may better indicate that it is not a different disease to secondary peritonitis, but rather represents secondary peritonitis lasting longer and harboring other (selected and more resistant) pathogens.

Traditionally, infections have been classified into community or hospital-acquired, according to the place of acquisition. However, hospital-acquired infections are now incorporated into healthcare-associated infections (HCAIs), including all infections acquired during the course of receiving healthcare. HCIs encompass hospital-acquired (nosocomial), nursing home-acquired, long-term care-associated, outpatient care-associated (such as. dialysis, chemotherapy) and finally home care-associated infections (such as intravenous therapy). In the setting of IAIs, there is relatively little data regarding the concept of healthcare-associated infections as opposed to hospital-acquired or nosocomial infections.

Differentiating community-acquired intra-abdominal infections (CA-IAIs) from hospital-acquired intra-abdominal infections (HA-IAIs) is useful to define the presumed resistance patterns and identify patients with increased likelihood of infection caused by multidrug resistan organisms. Hospital-acquired IAIs include also post-operative peritonitis, that is a life-threatening IAI with high rates of mortality.

Infection severity may be determined using clinical predictors and may be predicted by the presence of risk factors for treatment failure. Many risk factors have been described such as severely affected general state, degree of peritoneal involvement or diffuse peritonitis, APACHE II score >15, comorbidity and degree of organic dysfunction, ipoalbuminemia, poor nutritional status, cancer, clinically significant immunosuppression, advanced age, inability to achieve adequate debridement or control of drainage, delay in the initial intervention (>24 hrs). In 2015 WSES has published a new practical Sepsis Severity Score for patients with cIAIs including the clinical conditions at the admission (severe sepsis/septic shock), the origin of the cIAIs, the delay in source control, the setting of acquisition and any risk factors such as age and immunosuppression.  The statistical analysis shows that the sepsis severity score has a very good ability of distinguishing those who survived from those who died. The overall mortality was 0.63 % for those who had a score of 0–3, 6.3 % for those who had a score of 4–6, 41.7 % for those who had a score of ≥ 7. In patients who had a score of ≥ 9 the mortality rate was 55.5 %, those who had a score of ≥ 11 the mortality rate was 68.2 % and those who had a score ≥ 13 the mortality rate was 80.9 %. Grading of clinical severity in patients with cIAIs may be also well represented by the sepsis definitions.  It has been demonstrated that mortality rates increase dramatically in the patients with organ failure.

Organ failure risk factors include the causative organism and the patient’s genetic composition, underlying health status, and pre-existing organ function Well known risk factors that most commonly may precipitate severe sepsis and septic shock, are advanced age, acquired immunodeficiency syndrome, and use of immunosuppressive agents.

Critically ill patients poses serious problems for the choice of the treatment strategy because need more aggressive management and it is well known that in patients with organ failure or septic shock an early correct empirical antimicrobial therapy has a significant impact on the outcome, independently by the site of infectionThe data from WISS study showed that mortality was significantly affected by sepsis status when divided into four categories. Mortality rates increase in patients developing organ dysfunction and septic shock. Mortality by sepsis status was as follows: no sepsis 1.2%, sepsis only 4.4%, severe sepsis 27.8%, and septic shock 67.8%.

A complete classification of intra-abdominal infections should always include the origin of source of infection, the anatomical extent of infection, the presumed pathogens involved and risk factors for major resistance patterns, and the patient’s clinical condition. Designing a classification which is accepted worldwide stratifying patients according to the their risk for poor prognosis may be important to modulate the treatment.


Sartelli M, Abu-Zidan FM, Catena F, Griffiths EA, Di Saverio S, Coimbra R, et al.  Global validation of the WSES Sepsis Severity Score for patients with complicated intra-abdominal infections: a prospective multicenter study (WISS Study). World J Emerg Surg. 2015 Dec 16;10:61.

Sartelli M, Catena F, Abu-Zidan FM, Ansaloni L, Biffl WL, Boermeester MA,et al. Management of intra-abdominal infections: recommendations by the WSES 2016 consensus conference. World J Emerg Surg. 2017 May 4;12:22.