Spontaneous bacterial peritonitis: Clinical sciences

Spontaneous bacterial peritonitis refers to a bacterial infection of ascitic fluid without an identifiable source. It typically affects individuals with liver cirrhosis, and it’s thought to be due to a bacterial migration from the gut. Based on ascitic fluid culture results, you can differentiate spontaneous bacterial peritonitis from secondary bacterial peritonitis, which is associated with an identifiable source of infection!

When a patient presents with a chief concern suggesting spontaneous bacterial peritonitis, first you should perform an ABCDE assessment to determine if the patient is unstable or stable. If unstable, stabilize the airway, breathing, and circulation, which means you may have to intubate the patient. Next, obtain IV access and put your patient on continuous vital sign monitoring, including heart rate, blood pressure, and pulse oximetry. Finally, if needed, don’t forget to provide supplemental oxygen!

Now, here’s a clinical pearl! Like any other infection, patients with spontaneous bacterial peritonitis can progress to septic shock. In that case, broad spectrum antibiotics with carbapenems are essential, and remember to also look for other potential sources of infection. In otherwise stable patients, a third generation cephalosporin is adequate.

Okay, now let’s go back to the ABCDE assessment and focus on stable patients. In this case, obtain a focused history and physical examination. Also, order labs including CBC, CMP, and serum albumin. Finally, don't forget to order an abdominal ultrasound. Your patient will likely report systemic symptoms, such as fever and chills, as well as abdominal pain and progressive abdominal distention, as well as rigidity.

There might also be a history of cirrhosis, with or without prior episodes of ascites. The physical examination will reveal mild abdominal tenderness and signs of ascites, like bulging flanks, a palpable fluid wave, and shifting dullness. In some cases, you might notice guarding as well. Finally, labs will usually show low serum albumin levels, and ultrasound will show free fluid in the peritoneal cavity. With these findings, you should suspect peritonitis!

Next, perform diagnostic paracentesis and send the ascitic fluid for analysis and cultures. Once the results of ascitic fluid analysis are available, calculate the Serum-Ascites Albumin Gradient, or SAAG for short. To do so, subtract the ascitic fluid albumin value from the serum albumin value. Now, if the SAAG is less than 1.1 grams per deciliter, and the ascitic fluid polymorphonuclear, or PMN count, is less than 250 cells per cubic millimeter, you should consider an alternative diagnosis.

Here’s a clinical pearl! When calculating PMNs, it should be corrected for any RBCs present in the ascitic fluid sample. In fact, 1 PMN is subtracted from the absolute PMN count for every 250 red cells/mm3.

Let’s go back to our SAAG! Now, a SAAG of 1.1 or greater is indicative of portal hypertension. So, if you see this finding, and the ascitic fluid polymorphonuclear count is greater than 250 cells per cubic millimeters, suspect bacterial peritonitis. Next, start empiric antibiotics covering gram-negative and anaerobic organisms. For example, you can start a third-generation cephalosporin, like cefotaxime or ceftriaxone.

Here’s a clinical pearl to keep in mind! While not necessary for the diagnosis of spontaneous bacterial peritonitis, checking LDH, glucose, and total protein levels can provide valuable insights while waiting for culture results.

For example, an LDH level above the upper limit of normal, a glucose level below 50 milligrams per deciliter, and a total protein above 1 gram per deciliter may suggest secondary bacterial peritonitis as the most likely diagnosis.