How should chronic diarrhea in a cat be worked up to distinguish small from large bowel disease?

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The first step in working up chronic diarrhea in a cat is localizing it to small bowel, large bowel, or a combination using clinical sign pattern recognition. Chronic disease is defined as signs present for more than 3 weeks or intermittently recurring for more than 3 weeks.1 Small bowel diarrhea is characterized by large volume, weight loss, and vomiting, while large bowel diarrhea is characterized by increased frequency, mucus, tenesmus, and dyschezia. Small bowel diarrhea carries a higher grade for weight loss and vomiting, whereas large bowel diarrhea carries a higher grade for mucus and tenesmus; general condition is more affected with small bowel disease.1 Diarrhea is a rare presenting sign in feline pancreatitis, so small bowel diarrhea with concurrent vomiting and weight loss should direct the workup toward primary intestinal disease rather than pancreatic disease.4 Once small bowel localization is established, the differential diagnosis is dominated by chronic enteritis (inflammatory bowel disease/chronic inflammatory enteropathy) and intestinal lymphoma. In cats with clinical signs of chronic small bowel disease and ultrasonographic small bowel thickening, chronic small bowel disease was diagnosed in 96% of cases, with chronic enteritis (n = 150) and intestinal lymphoma (n = 124) as the two most common diagnoses.8 These two conditions are clinically indistinguishable — both present with weight loss, decreased appetite, vomiting, and diarrhea.1112 The minimum database should include a complete blood count, serum biochemistry, urinalysis, total thyroxine (in cats over 7 years), fecal flotation, and abdominal ultrasonography. Fecal bacterial culture and PCR panels for enteropathogenic bacteria have low diagnostic value because putative enteropathogenic bacteria are detected at similar rates in cats with and without diarrhea, and diarrhea is not significantly associated with enteropathogenic infections in cats.9 Ultrasonographic small bowel thickening supports