Spontaneous Bacterial Peritonitis (SBP)

At a Glance

Spontaneous bacterial peritonitis (SBP; also known as primary peritonitis) is an infection of the peritoneal cavity without an evident intra-abdominal source. SBP must be distinguished from secondary peritonitis because of a surgically-treatable cause, such as a ruptured appendix.

Among adults, SBP usually occurs in patients with cirrhosis and ascites. Although relatively rare, SBP may occur in pediatric patients with nephrotic syndrome or cirrhosis. SBP develops in the setting of large-volume, clinically-evident ascites. The condition develops in pre-existing ascites and does not itself cause ascitic fluid to accumulate. Patients with SBP typically have nonspecific symptoms like nausea, fever, abdominal pain, malaise, and mild confusion. Occasionally, patients are asymptomatic. In children, SBP often presents as an acute febrile illness that may be confused clinically with acute appendicitis.

SBP should always be considered in the setting of decompensation of a patient with previously stable chronic liver disease. In addition, children with abdominal pain and gross hematuria should be evaluated for SBP. The risk of SBP increases with concurrent gastrointestinal hemorrhage, a previous episode of SBP, or a low ascitic fluid protein content (<1 gm/dL).

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

Ascitic fluid obtained by paracentesis should be submitted for cell count and differential, protein and albumin concentration, and culture. The diagnosis of SBP is established by a polymorphonuclear leukocyte (PMN) count in the peritoneal fluid greater than 250 cells/mm3, even if the culture of ascitic fluid is negative.

Approximately 1 mL of fluid should be injected into a purple-top EDTA tube for cell count. Cell counts of ascitic fluid are generally performed by laboratory technologists but are also reliable when performed by an automated system. The PMN count should be corrected if the fluid is bloody by subtracting one PMN from the count for every 250 red blood cells/mm3.

Several milliliters of fluid should be injected into a red-top tube for chemistries. Measurement of ascitic fluid total protein concentration is helpful in distinguishing SBP from secondary peritonitis, as the latter condition typically presents with protein concentrations greater than 1 g/dL. Albumin concentration aids in the calculation of the serum-ascites albumin gradient, which is an indirect measurement of portal pressure. The serum and ascitic albumin levels should be measured simultaneously. The ascitic fluid value is subtracted from the serum value, and a difference of greater than 1.1 g/dL correlates with portal hypertension.

Culture of ascitic fluid is positive in only 40-65% of SBP cases, even if the ascitic fluid PMN count is greater than 250 cells/mm3. The poor sensitivity of ascitic fluid cultures is due to SBP's being typically associated with low numbers of bacteria. The most frequently isolated organisms are enteric gram-negative rods, such as Escherichia coli and Klebsiella pneumoniae. In addition, Enterococcus spp., Streptococcus spp., and, rarely, Staphylococcus aureus have been associated with SBP.

SBP is characteristically monomicrobial in nature. However, cases of polymicrobial SBP are possible, especially cases that are due to anaerobes or following a bowel puncture by the paracentesis needle. In the latter case, various bacterial forms are seen on the Gram stain or they grow in culture, and the ascitic fluid count contains fewer than 250 PMNs/mm3. Routine blood culture bottles should be inoculated with 10 mL of ascitic fluid at the bedside. In general, bacteria grow better in liquid media than on solid media. Culturing the ascitic fluid in this manner, as if it were blood, increases the sensitivity of the culture as compared to inoculation of ascitic fluid onto agar culture plates (solid media).

At some institutions, cultures are performed using both traditional solid agar plates and blood culture bottles. If the bacteria are present in large numbers, they may grow as colonies on the solid agar plates sooner than in the blood culture bottles since the bottles must first flag positive for growth and then must be inoculated onto media. In such cases, the bacterial colonies on the solid media would be available earlier for presumptive identification and susceptibility testing. If traditional cultures are performed, ascitic fluid must be received in the laboratory in a separate syringe or tube.

Additional optional tests to perform on ascitic fluid include glucose, lactate dehydrogenase, amylase, and Gram stain. Ascitic fluid glucose concentration usually measures greater than 50 mg/dL in SBP. Glucose values in ascitic fluid correlate inversely with the number of PMNs in the fluid. Lactate dehydrogenase levels in ascitic fluid of patients with SBP are lower than the upper limit of normal for serum. An elevated amylase in ascitic fluid is suggestive of pancreatitis or upper intestinal perforation.

A Gram stain may be helpful if the PMN count is greater than 250 cells/mm3. If ascitic fluid is collected into blood culture bottles and a Gram stain is requested, a separate syringe or tube of fluid must be sent to the microbiology laboratory for Gram stain. Gram stain is usually negative in SBP, but it may be positive if the organism is present in high numbers. Occasionally, Gram stains are falsely positive because of likely contaminants.

Follow-up tests for SBP are usually not necessary if the patient is improving on antibiotics. If the clinical response is poor or if the organism recovered is unusual, a second culture with cell count and differential should be considered. In such cases, sterility of the follow-up culture and a marked decrease in the PMN count should be seen. A decline in the PMN count greater than 25% in the ascitic fluid after 24-48 hours of antimicrobial therapy is appropriate (Table 1).

Table 1.

Orderable Tests for Ascitic Fluid Evaluation
Routine Tests Optional Tests Unusual Tests
Cell count and differentialAlbumin concentrationTotal protein concentrationCulture in blood culture bottles Glucose concentrationLactate dehydrogenase concentrationAmylase concentrationGram stainCulture on solid media Bilirubin concentrationTuberculosis smear and cultureCytology

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

There are several factors that may affect laboratory results.

First, the volume of fluid submitted for culture has a significant impact on culture sensitivity. Studies have shown that 10-20 mL of fluid inoculated into blood culture bottles improves sensitivity of culture. If the blood culture bottle holds less than 10 mL of inoculum, the bottle should be inoculated with as much fluid as the vacuum will allow. Swabs of ascitic fluid submitted to the laboratory for culture are unacceptable.

Time to inoculation of blood culture bottles is also important. In one study, inoculation of ascitic fluid into blood culture bottles was performed immediately after paracentesis at the bedside, compared to a 4-hour delay in inoculation of the fluid in the laboratory by the technologists. Of the 29 blood culture sets that grew pathogens, 7 grew bacteria only from the bedside-inoculated specimen. However, in this study, a “set” was defined as 4 blood culture bottles, which is typically more than the usual 2-bottles set. Regardless, in many hospitals, the delay from bedside to laboratory may be even longer than 4 hours, favoring immediate inoculation of blood culture bottles by the clinicians or staff at the bedside.

Finally, previous administration of antibiotics may lead to negative cultures so antibiotics should ideally be withheld until paracentesis is complete.

What Lab Results Are Absolutely Confirmatory?

The diagnosis of SBP is established by a PMN count in the peritoneal fluid greater than 250 cells/mm3 even if culture of ascitic fluid is negative. A positive culture is likely confirmatory, but the Gram stain result must also be taken into account when interpreting the culture result. Likewise, a positive Gram stain may not be confirmatory because of potential contaminants. A Gram stain with several bacterial morphologies is indicative of a polymicrobic infection that is likely due to anaerobes or a perforated gut.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

Measurements of ascitic fluid pH and lactate were previously used to aid in the diagnosis of SBP. However, recent studies have failed to confirm the utility of such tests.

The bilirubin concentration may be measured if the ascitic fluid is dark orange or brown in color to assess for gallbladder perforation. In the absence of an elevated ascitic fluid amylase, an ascitic fluid bilirubin higher than the serum concentration and greater than 6 mg/dL suggests a perforated gallbladder.

A rapid test for leukocyte esterase in ascitic fluid using a reagent strip has been evaluated as a bedside test to screen for SBP. Leukocyte esterase is produced by white blood cells (WBCs). Previously, dipstick tests designed for testing urine were used for testing ascitic fluid, but the test performances were not generally good. Some studies have demonstrated a high negative predictive value of the strip but poor specificity. Ascitic fluid-specific dipsticks are currently in development.

Lactoferrin has been correlated with the presence of SBP, but further validation studies of its use as a marker of SBP must be performed. Lactoferrin is an iron-binding protein that protects against pathogens by depriving them of iron, an element often necessary for their growth.

Blood cultures may be drawn concurrently with ascitic fluid cultures, as bacteremia is present in 75% of patients with SBP that is due to aerobic bacterial organisms.

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

Inoculation of fluid into both aerobic and anaerobic blood culture bottles has been shown to increase the yield of the culture.

Cytocentrifugation of the ascitic fluid is of questionable significance, but many laboratories spin the fluid down before performing the Gram stain. It is believed that cytocentrifugation improves the visibility of the organism, but some laboratorians argue that potential contaminants may be introduced into the culture with manipulation of the fluid.

Many laboratories also culture a liquid broth medium in a tube in addition to the solid plated media. The liquid broth medium is used to recover anaerobes, as well as organisms that cannot grow well on solid media. Rarely, an organism only recovered from the broth and not from the solid media may be reported. This pattern of growth may be due to a contaminant or an anaerobe, an organism that may be present in such low numbers that it could be recovered only from the rich tube broth medium. Laboratories will often report such organisms followed by a qualifying phrase, such as “recovered from broth only.” It is the clinician’s responsibility to determine the significance of such organisms.

If tuberculous peritonitis or Coccidioides spp. peritonitis are suspected, the diagnosis is best accomplished by culture. However, the laboratory should be notified if Coccidioides spp. peritonitis is considered, as this organism is extremely infectious when grown as a mold in the microbiology laboratory.

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