Intra-abdominal infections are generally the result of invasion and multiplication of enteric bacteria in the wall of a hollow viscus or beyond. When the infection extends into the peritoneal cavity or another normally sterile region of the abdominal cavity, the infection is described as a ‘‘complicated’’ intra-abdominal infection. The term ‘‘uncomplicated’’ intra-abdominal infection is less well defined; usually, it refers to an inflammatory process or infection in the wall of an abdominal organ, which may result in the development of a complicated intra-abdominal infection if not treated expeditiously.


Initial Evaluation

Risk Categories

With different risk category, the approach is different. First we should categorize patients with intra-abdominal infection into two groups: community-acquired intra-abdominal infection or healthcare-associated intra-abdominal infection. For patients with community-acquired intra-abdominal infection, we can manage them as mild-to-moderate severity group or high severity group.

To divide patients with community-acquired intra-abdominal infection into the two groups above, we use the criteria of APACHE II score system. Patients with APACHE score ≥15 are regards as high severity of illness.

APACHE II 1

Three subgroups:

1.Community-Acquired Intra-abdominal Infection – Mild-To-Moderate Severity

2.Community-Acquired Intra-abdominal Infection – High Severity

3.Healthcare-Associated Intra-abdominal Infection

The criteria for healthcare-associated intra-abdominal infection is based on the definitions for healthcare-associated infection, including “community-onset” and “hospital-onset”. Community-onset healthcare-associated infection includes cases involving patients with at least 1 of the following health care risk factors: 1.presence of an invasive device at time of admission; 2.history of MRSA infection or colonization; 3.history of surgery, hospitalization, dialysis, or residence in a long-term care facility in the 12 months preceding the culture date. Hospital-onset infection includes cases involving patients with positive culture results from a normally sterile site obtained >48 h after hospital admission.

Microbiology – Sampling/Specimens

1.Community-acquired infection lower risk

There is no proven value in obtain a routine Gram stain of the infected material. Routine blood cultures are not recommended since it dose not provide additional clinically relevant information for patients with community-acquired intra-abdominal infection. However, if a patient appears clinically toxic or is immunocompromised, knowledge of bacteremia may be helpful in determining duration of antimicrobial therapy.

The routine cultures of infected material for lower risk cases are optional but may be of value in detecting epidemiological changes in the resistance patterns of pathogens and in guiding follow-up oral therapy.

If Pseudomonas, Proteus, Acinetobacter, Staphylococcus aureus, and/or predominant Enterobacteriaceae, as determined by moderate-to-heavy growth have been identified, routine susceptibility testing should be obtained as these species are more likely than others to yield resistant organisms.

2.Community-acquired infection higher risk

There is no proven value in obtain a routine Gram stain of the infected material. Routine blood cultures are not recommended since it dose not provide additional clinically relevant information for patients with community-acquired intra-abdominal infection. However, if a patient appears clinically toxic or is immunocompromised, knowledge of bacteremia may be helpful in determining duration of antimicrobial therapy.

Cultures of infected material should routinely be obtained, particularly in patients with prior antibiotic exposure, who are more likely than other patients to harbor resistant pathogens.

Susceptibility of infected material should be routinely obtained to guide and adjust antibiotic therapy.

3.Healthcare-associated infection

For healthcare-associated intra-abdominal infection Gram stains may help define the presence of gram-positive cocci and yeast.

Blood cultures are seldom useful adjuncts for diagnosing intra-abdominal infection, even in healthcare-associated or complicated cases. So routine blood cultures for healthcare-associated intra-abdominal infection is not recommended. However, if a patient appears clinically toxic or is immunocompromised, knowledge of bacteremia may be helpful in determining duration of antimicrobial therapy.

Cultures and susceptibility of infected material should be routinely obtained.

However, if follow conditions are met, we should routinely obtain cultures and susceptibility.

If there is significant resistance of a common community isolate to an antimicrobial regimen in widespread local use, routine culture and susceptibility studies should be obtained for all cases.

PS: Anaerobic cultures are not necessary for community-acquired intra-abdominal infection if empiric antimicrobial therapy active against common anerobic pathogens is provided.

Once the decision has been made to collect microbiological specimens, one should know how to collect them. Cultures should be performed from 1 specimen, provided it is of sufficient volume (at least 1 mL of fluid or tissue, preferably more) and is transported to the laboratory in an appropriate transport system. For optimal recovery of aerobic bacteria, 1–10 mL of fluid should be inoculated directly into an aerobic blood culture bottle. In addition, 0.5 mL of fluid should be sent to the laboratory for Gram stain and, if indicated, fungal cultures. If anaerobic cultures are requested, at least 0.5 mL of fluid or 0.5 g of tissue should be transported in an anaerobic transport tube. Alternately, for recovery of anaerobic bacteria, 1–10 mL of fluid can be inoculated directly into an anaerobic blood culture bottle.


The Initial Management of Complicated Intra-abdominal Infection

Mild-To-Moderate Intra-abdominal Infection

Antibiotics used for empiric treatment of community-acquired intra-abdominal infection should be active against enteric gram-negative aerobic and facultative bacilli and enteric gram-positive streptococci. Coverage for obligate anaerobic bacilli should be provided for distal small bowel, appendiceal, and colon-derived infection and for more proximal gastrointestinal perforations in the presence of obstruction or paralytic ileus. Empiric  coverage of enterococcus is not necessary in patients with mild-to-moderate community-acquired intra-abdominal infection. Empiric antifungal therapy for Candida is not recommended for patients with community-acquired intra-abdominal infection.

  • Ampicillin-sulbactam is not recommended for use because of high rates of resistance to this agent among community-acquired E. coli.
  • Cefotetan and clindamycin are not recommended for use because of increasing prevalence of resistance to these agents among the Bacteroides fragilis group.
  • Aminoglycosides are not recommended for routine use in adults with community-acquired intra-abdominal infection as availability of less toxic agents demonstrated to be at least equally effective.
  • Quinolone-resistant E. coli have become common in some communities, and quinolones should not be used unless hospital surveys indicate >90% susceptibility of E. coli to quinolones.

PS: The use of agents listed as appropriate for higher-severity community and healthcare-associated infection is not recommended for patients with mild-to-moderate community-acquired infection, because such regimens may carry a greater risk of toxicity and facilitate acquisition of more resistant organisms.

High Severity Intra-abdominal Infection

Antibiotics used for empiric treatment of community-acquired intra-abdominal infection should be active against enteric gram-negative aerobic and facultative bacilli and enteric gram-positive streptococci.

Coverage for obligate anaerobic bacilli should be provided for distal small bowel, appendiceal, and colon-derived infection and for more proximal gastrointestinal perforations in the presence of obstruction or paralytic ileus.

Empiric  coverage of enterococci is recommended in patients with high-severity community-acquired intra-abdominal infection. We can choose between ampicillin, piperacillin-tazobactam, and vancomycin.

Empiric antifungal therapy for yeast or MRSA is not recommended for patients with community-acquired intra-abdominal infection in the absence of evidence proving for this condition.

The empiric use of antimicrobial regimens with broad-spectrum activity against gram-negative organisms (See the table left). But attention must be paid that broad-spectrum antimicrobial therapy should be tailored when culture and susceptibility reports become available, to reduce the number and spectra of administered agents.

  • Ampicillin-sulbactam is not recommended for use because of high rates of resistance to this agent among community-acquired E. coli.
  • Cefotetan and clindamycin are not recommended for use because of increasing prevalence of resistance to these agents among the Bacteroides fragilis group.
  • Aminoglycosides are not recommended for routine use in adults with community-acquired intra-abdominal infection as availability of less toxic agents demonstrated to be at least equally effective unless in the presence of evidence that the patient is likely to harbor resistant organisms that require such therapy.
  • Quinolone-resistant E. coli have become common in some communities, and quinolones should not be used unless hospital surveys indicate >90% susceptibility of E. coli to quinolones.

Healthcare-associated Intra-abdominal infection

Empiric antibiotic therapy for healthcare-associated intra-abdominal infection should be driven by local microbiologic results. To achieve empiric coverage of likely pathogens, multi-drug regimens that include agents with expanded spectra of activity against gram-negative aerobic and facultative bacilli may be needed. But attention must be paid that broad-spectrum antimicrobial therapy should be tailored when culture and susceptibility reports become available, to reduce the number and spectra of administered agents.

Antifungal therapy is recommended and fluconazole is an appropriate choice for treatment if C. albicans is isolated but if the patient were critical ill echinocandin instead of triazole is recommend. For fluconazole-resistant Candida species, therapy with an echinocandin is appropriate.

Empiric anti-enterococcal therapy is recommended for patients with healthcare-associated intra-abdominal infection, particularly those with postoperative infection, those who have previously reveived cephalosporins or other antimicrobial agents selecting for Enterococcus species, immunocompromised patients, and those with valvular heart disease or prosthetic intravascular materials. Initial empiric anti-enterococcal therapy should target against Enterococcus faecium, in which we can use ampicillin, piperacillin-tazobactam, or vancomycin.

Empiric therapy directed against vancomycin-resistant Enterococcus faecium is not recommend unless strong evidence existed or the patient was at high risk for this.

Empiric antimicrobial coverage directed against MRSA should be provided to patients with health care–associated intra-abdominal infection who are known to be colonized with the organism or who are at risk of having an infection due to this organism because of prior treatment failure and significant antibiotic exposure. Vancomycin is recommend for antibiotic therapy for MRSA.

Biliary Infection

Patients with suspected biliary infection either acute cholecystitis or cholangitis should receive antimicrobial therapy without anaerobic antibiotics unless anastamosis is present. For coummunity-acquired biliary infection, antibiotics activity against enterococci is not required since the pathogenicity of enterococci has not been demonstrated. However selected patients with immunosuppression, particularly those with hepatic transplantation, antimicrobial treatment against enterococcal infection may be required.


Source Control Approaches

Source control is defined as any single procedure or series of procedures that eliminate infectious foci, control factors that promote ongoing infection, and correct or control anatomic derangements to restore normal physiologic function. For some patients (diffuse peritonitis) source control must be performed as soon as possible and resuscitation should be continuded intraoperatively. For some other patients (hemodynamically stable) without peritonitis, delay of up to 1 day may be appropriate, but with antimicrobial therapy and careful clinical monitoring first.

Source control is recommended for nearly all patients with intra-abdominal infection.

Source control failure is more likely to occur in patients with delayed (>24 h) procedural intervention, higher severity of illness (APACHE II score ≥15), advanced age (>70 years), preexisting chronic medical conditions, poor nutritional status, and a higher degree of peritoneal involvement and is heralded by persistent or recurrent intra-abdominal infection, anastomotic failure, or fistula formation.


Subsequent Antimicrobial Therapy

Culture Directed Antimicrobial Therapy

Lower-risk patients with community-acquired intra-abdominal infection do not require alteration of therapy if satisfactory clinical response to source control and initial therapy occurs, even if unsuspected and untreated pathogens are later reported. However, if resistant bacteria were identified at the time of initial intervention and there are persistent signs of infection, pathogen-directed therapy is recommended for these patients.

Use of culture and susceptibility results to determine antimicrobial therapy in high-severity community-acquired or healthcare-associated infection should be based on pathogenic potential and density of identified organisms.

Microbes recovered from blood cultures should be assumed to be significant if they have established pathogenic potential or are present in ≥2 blood cultures or if they are recovered in moderate or heavy concentrations from samples obtained from drainage.

Duration of Antibiotic Therapy

Antimicrobial therapy of established infection should be limited to 4-7 days, unless it is difficult to achieve adequate source control. Resolution of clinical signs of infection should be used to judge the termination point for antimicrobial therapy. For patients whose signs and symptoms of infection are resolved, no further antibiotic therapy is required.

For patients recovering from intra-abdominal infection, completion of the antimicrobial course with oral forms of moxifloxacin, ciprofloxacin plus metronidazole, levofloxacin plus metronidazole, an oral cephalosporin with metronidazole, or amoxicillin-clavulanic acid is acceptable while the patients are able to tolerate an oral diet and in whom susceptibility studies do not demonstrate resistance.

The Management of Suspected Treatment Failure

In patients who have persistent or recurrent clinical evidence of intra-abdominal infection after 4-7 days of therapy, appropriate diagnostic investigation (e.g., extra-abdominal sources of infection, noninfectious infalmmatory conditions, etc.) should be undertaken. This should include CT or ultrasound imaging. Antimicrobial therapy effective against the organisms initially identified should be continued.

For patients with persistent clinical symptoms an signs but in whom no evidence of a new or persistent infection is uncovered after a careful investigation, termination of antimicrobial therapy is warranted.


PK-PD

Pharmacokinetic-pharmacodynamic properties of specific antibiotics should be considered in selecting an adequate dosage regimen.

β-lactam antibiotics have time-dependent bactericidal activity and a minimal postantibiotic effect (exception of carbapenems against P. aeruginosa). The best predictor of microbiological and clinical response to β-lactam antibiotics is the duration during which unbound drug concentrations exceed the minimum inhibitory concentration (MIC) of the microorganism.

In contrast, agents such as fluoroquinolones, aminoglycosides, and metronidazole have concentration-dependent bactericidal activity and a moderate to prolonged postantibiotic effect. Parameters that incorporate the exposure of unbound drug concentrations, either the ratio of AUC or maximum drug concentration to MIC of the microorganism are the best predictors of microbiological and clinical response for these agents.