As physicians, we all use diagnostic criteria when we're trying to determine a patient's condition, but it's important to make sure you know in which patients those diagnostic criteria have been validated. A great example of that is diagnosing spontaneous bacterial peritonitis.
There are many reasons that patients develop ascites; one of the most common reasons is liver disease, although other causes of hypervolemia, malignancy, intrabdominal infections, and other etiologies lead to ascites.
In patients with known liver disease with portal hypertension and ascites related to that, the diagnostic criterion for spontaneous bacterial peritonitis is 250 neutrophils per cubic millimeter in ascitic fluid. However, one of the pitfalls we can fall into is when we apply that diagnostic criterion to patients who don't have liver disease, like patients with tuberculous peritonitis. Certainly the criterion does fit in patients who might have portal hypertension for other reasons, but patients with malignancy very commonly will have many white blood cells in their ascitic fluid, and the risk of infection is lower than in patients with liver disease. Applying the diagnostic criteria in the right clinical context is one of the most important pearls we can have.
We can follow this pearl as primary care physicians but also as a consultant. We do this quite commonly with surgeons: They want to know if this is an infection. That's where we as internal medicine specialists use a diagnostic criterion, and one of the things we understand is how to construct that clinical context appropriately. When we walk into a room and think about this, we have to think about what signs, symptoms, or lab findings the patient has to go with liver disease. Or does the patient have normal platelets, normal clotting time, they don't have any spider angioma, they don't have splenomegaly? All these things arguing against liver disease can really help us think about how this might stem from another cause.
We often get called from the ER about patients with ascites, and the first step is to think about this patient's liver function before we think about whether there might be an infection and ascites. If the patient doesn't have evidence of cirrhosis or liver disease, then physicians really need to expand their differential and really not pay attention to 250 neutrophils as a main finding.