When and What to Test for Diarrhea: Focus on Stool Testing

Aim: To review stool diagnostic tests in acute and chronic diarrhea. Method: Narrative review of published literature. Results: (A). In acute diarrhea, stool tests are indicated when there is strong pre-test probability of infectious etiology or C. difficile infection (CDI) suggested by > 3 unformed bowel movements per 24h, symptoms lasting >7 days, and circumstances that are suggestive of infection. Several commercially available rapid tests for bacterial, viral or protozoal infections may be offered in addition to traditional methods (e.g. culture, microscopy), and provide a result within 6 hours. For CDI infection, a highly sensitive test such as glutamate dehydrogenase test is required; however, this does not distinguish infection from carrier state. That differentiation requires specialized nucleic acid amplification test (for toxin B) or enzyme immunoassays for toxin A or B, which are unfortunately not generally offered by microbiology labs. (B). Chronic diarrhea may result from inflammatory, fatty, osmotic or secretory causes; the commonest causes are IBS-D/functional diarrhea. Current recommendations in societal guidelines or clinical practice updates regarding stool tests in IBS-D/functional diarrhea in the absence of alarm symptoms include testing for giardia, calprotectin, fecal immunochemical test (FIT), and bile acid diarrhea (BAD). Comprehensive stool biochemical analyses (osmolality, pH, electrolytes) differentiate osmotic from secretory diarrhea and identify laxative abuse. Specific stool diagnostic tests for BAD and exocrine pancreatic insufficiency can lead to specific diagnosis and treatments. Surrogate markers associated with high fecal output and rapid transit in chronic diarrhea are stool form and colonic transit. Conclusion: Fecal testing is still very relevant in the practice of gastroenterology and deserves introduction of advanced microbiological and biochemical tests.