Spontaneous bacterial peritonitis (SBP) is diagnosed by testing a sample of abdominal fluid for infection. The defining threshold is a white blood cell count above 250 polymorphonuclear neutrophils (PMNs) per cubic millimeter in the fluid, regardless of whether bacterial cultures come back positive. This fluid is obtained through a procedure called paracentesis, where a needle is inserted into the abdomen to withdraw a small amount of the fluid that has accumulated there.
Who Needs Testing
SBP almost exclusively affects people with cirrhosis who have developed ascites, the buildup of fluid in the abdomen. Current guidelines from the American Association for the Study of Liver Diseases recommend that every patient with cirrhosis and ascites who is admitted to a hospital non-electively should have a diagnostic paracentesis performed. This applies even when infection isn’t the obvious reason for admission, because SBP can be the hidden driver behind other problems like confusion, worsening kidney function, or a sudden drop in blood pressure.
Certain factors put patients at especially high risk. An ascitic fluid total protein below 1.0 g/dL signals that the fluid has very little natural antibacterial defense. High bilirubin levels (above 2.5 mg/dL), a history of gastrointestinal bleeding, and any previous episode of SBP all raise the likelihood of developing a new infection. Patients with low-protein ascites combined with impaired kidney or liver function face the greatest risk.
Symptoms That Raise Suspicion
The classic presentation includes severe abdominal pain, fever, and chills, often alongside an elevated white blood cell count on routine bloodwork. But SBP is notorious for being subtle. Many patients present with nonspecific symptoms: new or worsening confusion, low blood pressure, or a decline in kidney function that looks like general decompensation of their liver disease. Because the symptoms can be vague, clinicians are trained to test the fluid rather than wait for a textbook presentation.
Why Timing Matters
Speed saves lives with SBP. Research has shown that performing paracentesis within 12 hours of hospital admission is associated with lower mortality, fewer days in intensive care, and better survival at three months. One large study found that each hour of delay in performing the procedure was associated with a 3.3% increase in in-hospital mortality, even after accounting for how sick the patient was. The message is straightforward: the earlier the tap, the better the outcome.
The PMN Count: The Core Diagnostic Marker
Once the fluid is collected, it goes to the lab for a cell count and differential. The single most important number is the absolute PMN count. A count above 250 cells per cubic millimeter confirms SBP, provided there is no obvious surgical cause of infection in the abdomen (like a ruptured appendix or perforated bowel). This threshold was chosen because it offers the best sensitivity, meaning it catches the most true cases. A higher cutoff of 500 PMNs per cubic millimeter is more specific, meaning fewer false positives, but it misses more real infections.
The diagnosis does not depend on culture results. Bacterial cultures of ascitic fluid come back negative in a large proportion of SBP cases. When the PMN count is elevated but cultures are negative, the condition is called culture-negative neutrocytic ascites. It is treated identically to culture-positive SBP with a full course of antibiotics, because outcomes are similar and delaying treatment while waiting for cultures is dangerous.
Getting Cultures Right
Although cultures aren’t required for diagnosis, identifying the specific bacteria helps guide treatment. The technique used to culture the fluid makes an enormous difference in whether anything grows. The standard recommendation is to inject the ascitic fluid directly into blood culture bottles at the bedside, rather than sending it to the lab in a sterile container for conventional plating.
Blood culture bottles contain nutrient broth optimized for growing bacteria from fluids with very low bacterial concentrations, which is exactly what ascitic fluid is. Studies have consistently shown that bedside inoculation into blood culture bottles detects bacteria far more often than conventional methods. In one key study, culture positivity jumped from 43% with conventional processing to 93% with direct bedside inoculation. The optimal volume is 8 to 10 mL of fluid per bottle, using one aerobic and one anaerobic bottle. Underfilling reduces the chance of growing an organism, while overfilling past 10 mL can also lower the yield.
Additional Fluid Tests
Beyond the cell count and culture, several other measurements from the fluid sample can provide useful information.
Ascitic fluid pH drops during infection. Normal ascitic fluid has a pH around 7.50, while in SBP the average drops to about 7.34. Lactate levels in the fluid also rise during infection, roughly tripling from normal values. However, specific cutoffs for pH (below 7.31) and lactate (above 33 mg/dL) were only met in a minority of confirmed SBP patients in early studies, which limits their usefulness as standalone diagnostic tools. They can support the diagnosis when the PMN count is borderline, but they don’t replace it.
Total protein and glucose levels in the fluid play a different role: they help distinguish SBP from secondary peritonitis, which is infection caused by a surgical problem like a bowel perforation. A set of criteria known as Runyon’s criteria suggests secondary peritonitis when two of three findings are present: fluid glucose below 50 mg/dL, fluid protein above 1 g/dL, or fluid LDH above the upper limit of normal for blood. This distinction matters because secondary peritonitis typically requires surgery, not just antibiotics. That said, these criteria have limited sensitivity (around 67%) and aren’t always reliable, so clinical judgment and imaging usually factor in as well.
Rapid Bedside Screening
In settings where lab turnaround is slow, urine dipstick reagent strips that detect leukocyte esterase have been studied as a rapid bedside screening tool. At a reading of 2+ on the strip, sensitivity for detecting SBP reaches 100% with a specificity of 94%, meaning a negative result at this threshold effectively rules out SBP. At a stricter reading of 3+, specificity hits 100%, meaning a positive result at that level confirms the diagnosis with high confidence, though sensitivity drops to about 76%.
These strips can give results in minutes and cost almost nothing, making them potentially valuable in emergency departments or resource-limited settings. They are not a replacement for formal cell counts and cultures, but they can help prioritize which patients need immediate treatment while lab results are pending.
Diagnostic Subtypes of Infected Ascites
The combination of PMN count and culture results creates three diagnostic categories that are worth understanding:
- Classic SBP: PMN count above 250/mm³ with a positive culture. The clearest diagnosis.
- Culture-negative neutrocytic ascites: PMN count above 250/mm³ with negative cultures. Treated the same as classic SBP.
- Bacterascites: Positive culture but PMN count below 250/mm³. This can represent early or resolving infection, or colonization without true peritonitis. Management depends on whether the patient has symptoms.
In practice, the PMN count drives the treatment decision. If it is above 250 and there is no surgical cause, antibiotics are started immediately without waiting for culture results. The culture, when it does grow an organism, helps narrow or adjust the antibiotic choice a day or two later.