Combined Fasting Serum C4 and Primary Bile Acids From a Single Stool Sample to Diagnose Bile Acid Diarrhea

Bile acid diarrhea (BAD) can affect up to 33% of patients with diarrhea-predominant irritable bowel syndrome (IBS-D).1 In the United States, the 48-hour fecal BA test is the gold standard diagnostic test for BAD. Fasting serum 7a-hydroxy-4-cholesten-3-one (C4) measures the product of the rate-limiting step of hepatic BA synthesis and increases in response to BA loss in the stool. Primary fecal BA (cholic acid [CA] and chenodeoxycholic acid [CDCA]) >10% is equivalent to increased total fecal BA (>2337 μmol/48 hours) in predicting stool weight >400 g/48 hours.