Ascitic fluid neutrophils, not culture, diagnose SBP

Spontaneous bacterial peritonitis is diagnosed on an ascitic polymorphonuclear count of 250 cells/mm³ or more, and treatment starts on that number. Culture is negative in roughly 40% of true cases, so waiting for it delays antibiotics in the patients who most need them, and delayed antibiotics in SBP are associated with higher mortality.

The practical failures are all pre-analytical. Ascitic fluid must go into blood culture bottles at the bedside, inoculated with at least 10 mL per bottle, not sent in a plain container to be processed later — bedside inoculation roughly doubles the yield. A diagnostic tap should be done on every cirrhotic patient admitted with ascites, not only those with abdominal pain or fever, because the classic presentation is absent in a large minority and encephalopathy or renal impairment may be the only sign.

One correction worth making: a bloody tap does not invalidate the count. Subtract one polymorph for every 250 red cells and use the corrected figure.

• Diagnostic paracentesis on every cirrhotic admitted with ascites, regardless of symptoms.
• Inoculate blood culture bottles at the bedside with at least 10 mL each — plain containers halve the yield.
• Start antibiotics on a polymorph count of ≥250 cells/mm³ without waiting for culture.
• Correct the count for a traumatic tap: subtract 1 polymorph per 250 red cells.
• Give albumin with the antibiotic in patients with renal impairment or bilirubin above 68 µmol/L — it reduces hepatorenal syndrome and death.

Why it matters

The diagnosis is made on a cell count available in an hour, and the commonest reason it is missed is that the tap was not done.

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