Primary Bile Acids

If you have been dealing with persistent loose stools and no one has been able to explain why, there is a specific cause your standard workup may have missed. Roughly half of people with chronic, unexplained, non-bloody diarrhea turn out to have too much bile acid reaching their colon.

Bile acid diarrhea is one of the most underdiagnosed causes of chronic loose stools, and one of the most treatable. Knowing your stool bile acid number tells you whether a well-established intervention is likely to fix the problem, or whether something else is driving your symptoms.

What This Test Actually Measures

Your liver makes primary bile acids, mainly CA (cholic acid) and CDCA (chenodeoxycholic acid), from cholesterol to help digest fats. After they do their job in your small intestine, about 95% are reabsorbed in the last segment of your small intestine (called the ileum) and recycled back to the liver. The remaining 5% reach your colon, where bacteria convert them into secondary bile acids before they leave your body in stool.

When this recycling system fails, more primary bile acids reach your colon than it can handle. They pull water into the bowel and irritate the lining, triggering urgent, loose, sometimes painful stools. A stool test for primary bile acids tells you whether that process is broken in you.

Bile Acid Diarrhea

This is the most common reason to order this test. In a study of 1,071 adults with chronic, unexplained, non-bloody diarrhea, 51% had increased bile acid excretion in stool. About 29% had specifically elevated primary bile acids (more than 25% of their total bile acids), and 22% had elevated total bile acids over a 48-hour stool collection (above 3,033 micromoles, a unit for very small amounts of these molecules).

What this means for you: if a roughly 1-in-3 chance of finding a treatable explanation for chronic loose stools sounds worth pursuing, this test is the most direct way to check. A high result points to a specific, well-established treatment pathway rather than another round of trial-and-error workups.

IBS-D and the Bile Acid Connection

Many people who have been labeled with IBS-D (the diarrhea-predominant form of irritable bowel syndrome) actually have undiagnosed bile acid diarrhea. In a study of 83 IBS-D patients, stool levels of cholic acid and chenodeoxycholic acid were markedly elevated compared to healthy controls. Higher fecal bile acid levels also correlated with more frequent bowel movements and greater visceral pain.

If you have been told your symptoms are simply IBS, measuring your stool primary bile acids can reveal whether a more specific, more treatable problem is hiding underneath that diagnosis.

Inflammatory Bowel Disease

In a study of 79 adults with inflammatory bowel disease, stool bile acid composition was disrupted in active ulcerative colitis, with shifts that distinguished it from Crohn's disease and that correlated with disease severity and diarrhea symptoms. Studies measuring serum bile acids (a related but different sample type) found that the ratio of primary to secondary bile acids stratified active disease from remission with an AUC of 0.84 (a measure of how well a test separates two groups, where 1.0 is perfect).

What this means for you: if you already have IBD (inflammatory bowel disease), elevated stool primary bile acids may reflect both inflammation and the disrupted microbial conversion that happens in a chronically inflamed gut. The number adds context to what your inflammation markers are showing.

Colorectal Cancer Signal

A 2025 systematic review and meta-analysis of fecal bile acid studies concluded that higher fecal bile acid concentrations are associated with a higher risk of colorectal cancer. The strongest signal comes from secondary bile acids, but persistently elevated primary bile acids reaching the colon are part of the same disrupted pattern. This is one reason to take a chronically high result seriously even if your diarrhea is mild.

Reference Ranges

There is no single universal cutoff for stool primary bile acids in healthy people. The most widely cited thresholds come from a study of 1,071 adults with chronic diarrhea using a 48-hour stool collection method, and they are designed to flag bile acid malabsorption rather than to define a healthy norm. They are useful orientation, not a definitive target. Your lab may report numbers in different units or use different cutpoints.

Source: Vijayvargiya et al., 2020. Compare your results within the same lab over time for the most meaningful trend, since assays and units vary.

Tracking Your Trend

A single stool bile acid measurement reflects what was happening in your gut during one collection window. Recent diet, gut infections, antibiotic use, and natural day-to-day variation can all shift the result. The most useful information comes from a trend, not a single number.

If you are testing for the first time, get a baseline. If you start a treatment or make a major dietary change, retest in 3 to 6 months to confirm the change is real and sustained. After that, annual testing is reasonable for anyone managing chronic gut symptoms or living with IBD, IBS-D, or a history of gallbladder removal.

When Results Can Be Misleading

• Incomplete stool collection: the standard threshold is based on a full 48-hour collection. Missing samples or a shorter window can underestimate your true bile acid output.
• Recent antibiotics: antibiotics disrupt the colonic bacteria that convert primary bile acids into secondary forms, which can artificially raise the primary fraction for weeks after a course.
• Acute gastroenteritis: a recent stomach bug can transiently change stool bile acid composition. Wait at least 2 to 4 weeks after a gut infection before testing.
• Bile acid sequestrants on board: if you are already taking cholestyramine, colesevelam, or colestipol, the drug binds bile acids in the gut and changes what the assay measures, making interpretation unreliable. Talk to your clinician before testing.

What to Do If Your Result Is High

An elevated result alone does not give you the full picture. The next step is to confirm whether the bile acid pattern explains your symptoms and to rule out other contributors:

• Pair with gut inflammation markers: calprotectin and pancreatic elastase help separate bile acid diarrhea from inflammatory bowel disease and pancreatic insufficiency, which can produce overlapping symptoms.
• Look at your secondary bile acids: if secondary bile acids are also low while primary is high, your colonic microbiome may not be converting bile acids properly, which has different implications than simple malabsorption.
• Review your history: prior gallbladder removal, ileal resection, or Crohn's disease affecting the last part of the small intestine are the classic causes. Pelvic radiotherapy is another.
• Bring it to a gastroenterologist: a clinician familiar with bile acid diarrhea can confirm the diagnosis with a structured trial of a bile acid sequestrant and decide whether further imaging or endoscopy is needed.