Peritonitis and Abdominal Sepsis Workup

Diagnostic paracentesis should be performed in all patients who do not have an indwelling peritoneal catheter and are suspected of having spontaneous bacterial peritonitis (SBP). In peritoneal dialysis patients with a peritoneal catheter, fluid should be withdrawn using sterile technique. Ultrasonography may aid paracentesis if ascites is minimally detectable or questionable.

The results of aerobic and anaerobic bacterial cultures, used in conjunction with the cell count, prove the most useful in guiding therapy for those with SBP. ^[17] With regard to ascitic fluid culture, direct inoculation of routine blood culture bottles at the bedside with 10 mL of ascitic fluid has been reported to significantly increase the sensitivity of microbiologic studies.

The diagnostic and therapeutic approach to peritonitis and peritoneal abscess is summarized in the algorithm below.

Peritonitis and abdominal sepsis. Diagnostic and therapeutic approach to peritonitis and peritoneal abscess.

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Complete blood cell count and other blood studies

Most patients will have leukocytosis (>11,000 cells/μ L), with a shift to the immature forms on the differential cell count. Patients who have severe sepsis, are immunocompromised, or have certain types of infections (eg, fungal, cytomegaloviral) may not have leukocytosis or leukopenia. In cases of suspected spontaneous bacterial peritonitis (SBP), hypersplenism may reduce the polymorphonuclear leukocyte count.

Blood chemistry findings may reveal dehydration and acidosis. Obtaining prothrombin time (PT), partial thromboplastin time (PTT), and international normalized ratio (INR) is indicated. Liver function tests may be indicated. Amylase and lipase levels should be obtained if pancreatitis is suspected. Blood culture results are positive for the offending agent in as many as 33% of patients with SBP and may help guide antibiotic therapy. Measurement of serum albumin allows calculation of the serum-to-ascites albumin gradient (SAAG). A SAAG of more than 1.1 is noted in SBP.

Procalcitonin level

Findings from a 2019 retrospective study (2016-2017) of 99 patients with abdominal sepsis suggests that serum procalcitonin level can be an indicator of severity and mortality in abdominal sepsis due to secondary peritonitis. ^[18] There appeared to be statistical significance with a procalcitonin level above 10.1 mcg/L, Mannheim peritonitis score over 26 points, controlling nutritional status (CONUT) score above 6 points, and the presence of organic faults, but not with APACHE (Acute Physiology and Chronic Health Evaluation) and SOFA (Sequential Organ Failure Assessment) scores and mortality. ^[18]

Urinalysis

A urinalysis is used to rule out urinary tract diseases (eg, pyelonephritis, renal stone disease); however, patients with lower abdominal and pelvic infections often demonstrate white blood cells (WBCs) in the urine and microhematuria.

Stool sample

In patients with diarrhea, evaluate a stool sample—employing a Clostridium difficile toxin assay, a WBC count, and a specific culture (ie, Salmonella, Shigella, cytomegalovirus [CMV])—if the patient's history suggests infectious enterocolitis.

The single best predictor of spontaneous bacterial peritonitis (SBP) is an ascitic fluid neutrophil count of greater than 500 cells/µL, which carries a sensitivity of 86% and a specificity of 98%. By lowering the ascitic fluid neutrophil count threshold to 250 cells/μ L, the sensitivity increases to 93% with only a minimal decrease in specificity to 94%.

The fluid should be evaluated for glucose, protein, lactate dehydrogenase (LDH), cell count, Gram stain, and aerobic and anaerobic cultures. If pancreatitis or a pancreatic leak is suspected, amylase analysis should be added to the panel. Bilirubin and creatinine levels can be analyzed as well, if a biliary or urinary leak is suspected as a possible etiology. The peritoneal/ascitic fluid characteristics or levels are then compared with their respective serum values.

The fluid in bacterial peritonitis generally demonstrates a low pH and low glucose levels with elevated protein and LDH levels. Traditionally, ascitic fluid pH of less that 7.34 was consistent with a diagnosis of SBP; however, ascitic pH is less commonly measured because it is unreliable and lacks specificity for the condition.

The diagnosis of SBP is established when the polymorphonuclear neutrophil (PMN) count is 250 cells/µL or greater in conjunction with a positive bacterial culture result. In most of these cases, as mentioned previously, cultures are positive for a single organism. Obviously, these patients should receive antibiotic therapy. Although up to 30% of cultures remain negative, most of these patients are presumed to have bacterial peritonitis; they should be treated. A significantly decreased peritoneal fluid glucose level (< 50 mg/dL), a peritoneal fluid LDH level much greater than the serum LDH, a peritoneal fluid white blood cell (WBC) count greater than 10,000 cells/µL, a pH lower than 7.0, high amylase levels, multiple organisms on Gram stain, or recovery of anaerobes from the culture raises the suspicion of SP in these patients. Some authors recommend repeating the paracentesis in 48-72 hours to monitor treatment success (decrease in neutrophil count to < 50% of the original value).

Culture-negative neutrocytic ascites (probable SBP) is established when the ascitic fluid culture results are negative but the polymorphonuclear neutrophil (PPMN) count is 250 cells/µL or greater. This may happen in as many as 50% of patients with SBP and may not actually represent a distinctly different disease entity. Rather, it may be the result of poor culturing techniques or late-stage resolving infection. Nonetheless, these patients should be treated just as aggressively as those with positive culture results.

Monomicrobial nonneutrocytic bacterascites exists when a positive culture result coexists with a PMN count of less than 250 cells/µL. Although this may often be the result of contamination of bacterial cultures, 38% of these patients develop SBP. Therefore, monomicrobial nonneutrocytic bacterascites may represent an early form of SBP. All study patients described who eventually developed SBP were symptomatic. For this reason, any patient suspected clinically of having SBP in this setting must be treated.

Tuberculous peritonitis (TP) is identified by ascites with high protein content, a low glucose and low SAAG, elevated ascitic fluid WBC count, and lymphocyte predominance. In TP, the fluid Gram stain and acid-fast stain results are rarely positive, and routine culture results are falsely negative in as many as 80% of cases. A peritoneal fluid protein level greater than 2.5 g/dL, LDH level greater than 90 U/mL, and predominantly mononuclear cell count of more than 500 cells/µL should raise the suspicion of TP, but specificity for the diagnosis is limited. Laparoscopy with visualization of granulomas on peritoneal biopsy and specific culture (which requires 4-6 wk) may be needed for definitive diagnosis.

Peritonitis in patients receiving continuous ambulatory peritoneal dialysis (CAPD) is indicated by contamination of the dialysis catheter; cloudy effluent, total fluid WBC count of greater than 100 neutrophils/µL, or presence of organisms on Gram stain.

Routine intraoperative peritoneal fluid cultures in defined acute disease entities (ie, gastric or duodenal ulcer perforation, appendicitis, diverticulitis or perforation of the colon caused by obstruction or ischemia) are controversial. Several studies found no significant difference in patients with appendicitis, diverticulitis, and other common etiologies for bacterial peritonitis with regard to postoperative complication rates or overall outcomes. The antibiotic regimen was altered only 8-10% of the time based on operative culture data. In patients who had previous abdominal operations or instrumentation (eg, peritoneal dialysis catheter, percutaneous stents) and patients with prolonged antibiotic therapy, critical illness, and/or hospitalization, these cultures may reveal resistant or unusual organisms that should prompt alteration of the antibiotic strategy.

For a summary of ascitic fluid analysis, see Table 4, below.

Table 4. Ascitic Fluid Analysis Summary ^[5] (Open Table in a new window)

A development in the rapid diagnosis of spontaneous bacterial peritonitis (SBP) has been the proposed use of bedside reagent strips read by a portable spectrophotometric device. In a pilot study, this combination achieved a 100% sensitivity in diagnosis of SBP. ^[19]  In a separate, small cohort, the average time saved from dipstick to laboratory results ranged from 2.73 hours (dipstick to validated result from automated counter) to 3 hours (dipstick to validated manual cell count of ascitic fluid). ^[20]  

More recently, a study that evaluated the sensitivity of a bedside leukocyte esterase reagent strip for the detection of SBP in 330 emergency department ascites patients undergoing paracentesis (635 fluid analyses) found a 95% sensitivity, 48% specificity, 11% positive predictive value, and 99% negative predictive value at 3 minutes at the trace threshold of SBP prediction. ^[21]  Given these results, the reagent strip is not recommended as a standalone test.

This diagnostic method holds promise in replacing the time-consuming process of manual cell counting, which is often unavailable in many laboratories "after hours." The decreased time to diagnosis may result in a significant reduction of the time from paracentesis to antibiotic treatment of presumptive SBP.

Plain films of the abdomen (eg, supine, upright, and lateral decubitus positions) are often the first imaging studies obtained in patients presenting with peritonitis. Their value in reaching a specific diagnosis is limited.

Free air is present in most cases of anterior gastric and duodenal perforation but is much less frequent with perforations of the small bowel and colon and is unusual with appendiceal perforation. Upright films are useful for identifying free air under the diaphragm (most often on the right) as an indication of a perforated viscus. Remember that the presence of free air is not mandatory with visceral perforation and that small amounts of free air are missed easily on plain films.

Abdominal ultrasonography may be helpful in the evaluation of pathology in the right upper quadrant (eg, perihepatic abscess, cholecystitis, biloma, pancreatitis, pancreatic pseudocyst), right lower quadrant, and pelvis (eg, appendicitis, tubo-ovarian abscess, Douglas pouch abscess). However, the examination is sometimes limited because of patient discomfort, abdominal distention, and bowel gas interference.

Ultrasonography may detect increased amounts of peritoneal fluid (ascites), but its ability to detect quantities of less than 100 mL is limited. The central (perimesenteric) peritoneal cavity is not visualized well with transabdominal ultrasonography. Examination from the flank or back may improve the diagnostic yield, and providing the ultrasonographer with specific information about the patient's condition and the suspected diagnosis before the examination is important. With an experienced ultrasonographer, a diagnostic accuracy of greater than 85% has been reported in several series.

Ultrasonographically guided aspiration and placement of drains has evolved into a valuable tool in the diagnosis and treatment of abdominal fluid collections.

Advantages of ultrasound include low cost, portability, and availability. Disadvantages are that the test is operator dependent, and there is reduced visualization in the presence of overlying bowel gas and abdominal dressings.

If the diagnosis of peritonitis is made clinically, a computed tomography (CT) scan is not necessary and generally delays surgical intervention without offering clinical advantage. However, CT scanning is indicated in all cases in which the diagnosis cannot be established on clinical grounds and the findings on abdominal plain films. CT scans of the abdomen and pelvis remain the diagnostic study of choice for peritoneal abscess and related visceral pathology.

Whenever possible, the CT scan should be performed with enteral and intravenous contrast. CT scans can detect small quantities of fluid, areas of inflammation, and other gastrointestinal tract pathology, with sensitivities that approach 100%. (See the image below.) CT scanning can be used to evaluate for ischemia, as well as to determine bowel obstruction. An abscess is suggested by the presence of fluid density that is not bound by the bowel or other known structures. Gas within an abdominal mass or the presence of an enhancing wall and adjacent inflammatory changes are also highly suggestive of an abscess. Ischemia can be demonstrated by a clot in a large vessel or by the absence of blood flow. Gas within the intestinal wall or in the portal vein may also suggest ischemia.

Peritonitis and abdominal sepsis. A 78-year-old man was admitted with a history of prior surgery for small bowel obstruction and worsening abdominal pain, distended abdomen, nausea, and obstipation. In figure A, a marked amount of portal venous gas within the liver, mesenteric venous gas, and pneumatosis intestinalis are consistent with ischemic small intestine. The superior mesenteric artery appears patent. The liver has a nodular contour consistent with cirrhosis. In figures B and C, markedly distended loops of small intestine containing fluid and air-fluid levels are consistent with a small bowel obstruction. No focal fluid collections are identified.

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In abscess formation subsequent to secondary peritonitis (SP), approximately half of patients have a simple abscess without loculation, and the other half have complex abscesses secondary to fibrinous septation and organization of the abscess material. Abscess formation occurs most frequently in the subhepatic area, the pelvis, and the paracolic gutters, but it may also occur in the perisplenic area, the lesser sac, and between small bowel loops and their mesentery.

Peritoneal abscesses and other fluid collections may be aspirated for diagnosis and drained under CT guidance; this technique has become a mainstay of therapy.

Magnetic resonance imaging (MRI) is a useful imaging modality for the diagnosis of suspected intra-abdominal abscesses. Abdominal abscesses demonstrate decreased signal intensity on T1-weighted images and homogeneous or heterogeneous increased signal intensity on T2-weighted images; abscesses are observed best on gadolinium-enhanced, T1-weighted, fat-suppressed images as well-defined fluid collections with rim enhancement.

Limited availability and high cost, as well as the need for MRI-compatible patient support equipment and the length of the examination, currently limit its usefulness as a diagnostic tool in acute peritoneal infections, particularly for patients who are critically ill.

Nuclear scanning

Nuclear imaging diagnostic studies have little use in the initial evaluation of patients with suspected peritonitis or intra-abdominal sepsis. They are most frequently used in the evaluation of fever of unknown origin or in patients with persistent fever despite adequate antibiotic treatment and negative computed tomography scan findings.

Contrast studies

Conventional contrast studies (ie, Gastrografin swallow, upper gastrointestinal tract study with follow-through, colorectal contrast enema, fistulogram, contrast studies of drains and stents) are reserved for specific indications in the setting of suspected peritonitis or peritoneal abscess.