A new guideline from the American Gastroenterological Association (AGA) aims to help physicians diagnose the cause of chronic watery diarrhea, particularly to exclude diagnoses other than functional diarrhea or diarrhea-predominant irritable bowel syndrome (IBS). The guideline, published in Gastroenterology, does not apply to patients with concerning presentations like weight loss/anemia, diarrhea with signs of fat malabsorption, bloody diarrhea, cases with a family history of inflammatory bowel disease (IBD), colon cancer, or celiac disease or to those who have traveled to diarrheal disease–related regions.
To rule out IBD, physicians can use either fecal calprotectin (threshold value of 50 microg/g, sensitivity, 0.81; specificity, 0.87) or fecal lactoferrin (threshold, 4.0-7.25 mcg/g; pooled sensitivity for IBD, 0.79; specificity, 0.93). Neither erythrocyte sedimentation rate (ESR) nor C-reactive protein (CRP) should be used to diagnose IBD because tests have shown low pooled sensitivity and specificity. CRP might be useful in settings in which fecal lactoferrin or calprotectin tests are not available or covered by insurance.
Patients should be tested for giardia infection, using the antigen test or PCR, because this pathogen is common in the United States and easily treated.
Patients who have not recently traveled to or from high-risk areas should not be tested for ova and parasites because this is unlikely to identify a culprit. There are other guidelines for treating patients who have traveled to high-risk countries.
Celiac disease should be tested for using immunoglobulin-A tissue transglutaminase (IgA tTG) and a second test (IgG tTG and IgG or IgA deamidated gliadin peptide, or DGP) in case the patient has IgA deficiency that could lead to a false negative on the primary test. Thresholds of 7-15 AU/mL in IgA tTG typically provide sensitivity and specificity greater than 90%. A quantitative IgA level found to be normal confirms the IgA tTG test. In abnormal findings, either IgG tTG or a test for IgG DGP can be used. If no information on IgA levels is available, IgG tTG or IgG DGP can be combined with IgA tTG. Positive celiac disease tests should be confirmed by duodenal biopsy.
Bile acid diarrhea should be tested for in the United States by measuring total bile acids in a 48-hour stool collection to document increased fecal bile acids, or serum fibroblast growth factor 19, to identify defective feedback of bile acid synthesis. The Selenium HomotauroCholic Acid Test (SeHCAT) has moderate diagnostic efficiency, but it is not available in North America. A measurement of serum levels of 7alpha-hydroxy-4-cholesten-3-one (C4), which measures bile acid synthesis, is not yet available.
No recommendation was made for using available serologic tests for the diagnosis of IBS because existing evidence suggests they lack the diagnostic accuracy needed for routine use.
The guideline development was funded by AGA and had no outside funding.
SOURCE: Smalley W et al. Gastroenterology. 2019 Jul 11. doi: 10.1053/j.gastro.2019.07.004.