Based on technical review and expert consensus, a guideline committee put together by the American Gastroenterological Association (AGA) released official recommendations on the laboratory evaluation of functional diarrhea and diarrhea-predominant irritable bowel syndrome (IBS-D) in immunocompetent adults. This report was published in Gastroenterology.

Members of the AGA guideline committee and authors of the technical review convened to formulate recommendations for diagnostic testing. Although quality of evidence was a crucial factor in determining the strength of each recommendation, the panel also considered the balance of risk-benefit, patient preferences, and resource utilization. Background information and subsequent guidelines were rated using Grading of Recommendations Assessment, Development, and Evaluation (GRADE) methodology. Final recommendations were formulated by consensus from AGA members, a GRADE methodologist, and a primary care physician.

Recommendation 1
The AGA conditionally recommends the use of fecal calprotectin or fecal lactoferrin testing to screen for markers of inflammatory bowel disease (IBD) in patients with chronic diarrhea. For optimal sensitivity, the threshold value suggested for calprotectin is 50 µg/g and for lactoferrin is between 4.0 and 7.25 µg/g. Conditional recommendation.


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Recommendation 2
Due to low diagnostic accuracy, the AGA recommends against the use of erythrocyte sedimentation rate and C-reactive protein testing to screen for IBD in patients with chronic diarrhea. It may be reasonable to consider C-reactive protein in a setting where fecal calprotectin or lactoferrin testing is unavailable or not covered by insurance. Conditional recommendation.

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Recommendation 3
As Giardia is a common culprit of watery diarrhea, the AGA strongly recommends testing for Giardia in patients with chronic diarrhea. Diagnostic tests that detect Giardia antigens or polymerase chain reaction of Giardia are suggested for their excellent sensitivity and specificity. Strong recommendation.

Recommendation 4
In the absence of recent travel or immigration from high-risk areas, the AGA recommends against testing patients’ stool for ova and parasites (other than Giardia) as these tests are unlikely to identify the source of watery diarrhea. Conditional recommendation.

Recommendation 5
The AGA strongly recommends testing patients with chronic diarrhea for celiac disease with IgA tissue transglutaminase using thresholds of 7 to 15 AU/mL. In the setting of IgA deficiency, secondary testing to detect celiac disease should include IgG tissue transglutaminase or test for IgG or IgA deaminated gliadin peptides. Celiac disease should be confirmed through a small bowel biopsy and serologic diagnosis. Strong recommendation.

Recommendation 6
The AGA conditionally recommends testing for bile acid diarrhea in patients with chronic diarrhea. In Europe, the homotaurocholic acid test may be used to identify bile acid malabsorption; in the United States, 48-hour stool collection or serum fibroblast growth factor 19 may be used to test for fecal bile acids or for defective feedback of bile acid synthesis, respectively. Because these tests are not widely available or approved by the Food and Drug Administration, the AGA considers an empiric trial of bile acid binders reasonable in patients for whom bile acid diarrhea is suspected. Conditional recommendation.

Summary
The AGA makes no recommendation for the use of serologic testing for the diagnosis of IBS-D in patients with chronic watery diarrhea. The only data currently available suggest that contemporary serologic tests lack the diagnostic accuracy required for routine use.

These practice guideline recommendations are intended to reduce practice variation and promote high-quality and high-value care for adults with functional diarrhea and IBS-D.

Reference
Smalley W, Falck-Ytter C, Carrasco-Labra A, Wani S, Lytvyn L, Falk-Ytter Y. American Gastroenterological Association guideline on mild-to-moderate ulcerative colitis on the laboratory evaluation of functional diarrhea and diarrhea-predominant irritable bowel syndrome in adults (IBS-D) [published online July 11, 2019). Gastroenterology. doi:10.1053/j.gastro.2019.07.004