Test helps evaluate for under-recognized bile acid diarrhea
Bile acids are natural products of cholesterol metabolism that help solubilize and absorb dietary lipids. The primary bile acids, chenodeoxycholic acid (CDCA) and cholic acid (CA), are synthesized in the liver through the classic microsomal cholesterol 7α-hydroxylase (CYP7A1) pathway, conjugated to taurine or glycine, and stored in the gallbladder until released into the small intestine after eating.
About 95 percent of bile acids are eventually reabsorbed by the distal ileum and returned to the liver for recycling back to the small intestine. Only a small percentage of them reach the colon where they are deconjugated and dehydroxylated by bacteria to produce the secondary bile acids, deoxycholate and lithocholate. Most of these are then excreted in feces.
Interruption of the enterohepatic circulation, which allows excess bile acids to enter the colon, causes the classic symptoms of bile acid malabsorption (BAM), including watery stool, urgency and fecal incontinence. Although bile acid malabsorption has been associated with diarrhea for nearly 50 years, it remains an under-recognized and underdiagnosed cause of chronic diarrhea. Studies have demonstrated that BAM occurs in about one-third of patients diagnosed with irritable bowel syndrome with diarrhea (IBS-D), up to 50 percent of those with functional diarrhea and 35 percent of those with microscopic colitis.
Even so, BAM is seldom considered in most cases of chronic diarrhea encountered by gastroenterologists and primary care physicians. As a result, patients may be extensively investigated with colonoscopies, CT enterography and other stool studies, diagnosed with other causes of diarrhea, or considered to have irritable bowel syndrome or functional diarrhea by exclusion, thereby delaying specific treatment.
Testing for BAM in clinical practice
Michael Camilleri, M.D., a gastroenterologist at Mayo Clinic's campus in Minnesota, says the main problem in evaluating for BAM has been the limited availability of diagnostic tests.
"Bile acid diarrhea affects 4 or 5 percent of people in the community, but there have been few readily available methods that directly measure it. The 75selenium homotaurocholic acid retention test (SeHCAT), which is validated and used in most European countries, is not available in the United States. The alternative has been a therapeutic trial of a bile sequestrant. But some of these drugs are poorly tolerated, especially resin formulations such as cholestyramine, the response is variable and their use is difficult to justify without a definitive diagnosis," he explains.
Now, however, there are two tests for evaluating BAM in the U.S. One test, currently available only for research purposes, measures serum levels of the marker 7α-hydroxy-4-cholesten-3-one (C4), a downstream product of CYP7A1. Plasma C4 levels increase when bile acid synthesis increases, and C4 levels are substantially elevated in BAM patients with a sensitivity and specificity of 90 percent and 79 percent, respectively. C4 levels have also been shown to correlate well with SeHCAT retention. This makes fasting serum C4 attractive as a screening test for BAM, although it can produce false-positives and false-negatives in patients who have liver disease or are taking statins.
"Serum C4 was not developed at Mayo, but we have continued to validate and perfect the assays, proving that it's a useful test, and we plan to offer it as an orderable test soon," Dr. Camilleri says. An article describing Mayo's development of a serum C4 assay appeared in the March 2009 issue of Neurogastroenterology & Motility.
Fecal bile acid test
The second test, which can now be clinically ordered, is the fecal bile acid excretion test. It quantifies individual and total bile acids in a 48-hour stool collection. Leslie J. Donato, Ph.D., co-director for Cardiovascular Laboratory Medicine, Hospital Clinical Laboratory and Point of Care Testing at Mayo Clinic's campus in Minnesota, says increased total fecal bile acids are seen in patients with chronic functional diarrhea and higher levels of CA and CDCA are associated with IBS-D.
A clinical validation involving 94 healthy volunteers, 60 patients with IBS-D and 28 patients with IBS with constipation (IBS-C) found that the sum of CA and CDCA concentrations above 3.7 percent were indicative of IBS-D with 72 percent sensitivity and 90 percent specificity. In addition, the upper limit of normal total fecal bile acid excretion over the 48 hours has been defined by collaborative work in the labs headed by Drs. Camilleri and Donato.
"The chemical measurement of fecal bile acids involves several steps of extraction, separation and quantification," Dr. Donato explains. "Essentially, the stool specimen is homogenized with water and bile acids are extracted from the specimen to remove proteins, lipids, salts and solid materials. Quantification is accomplished using high-performance liquid chromatography and mass spectrometry, which is extremely accurate."
At Mayo Clinic, the same sample is used to measure bile acids and fecal fat — an advantage for the evaluation of diarrhea of unknown etiology Patients take the test at home, without the need for a visit to a tertiary care center. Test preparation includes a diet containing 100 grams of fat for two days prior to collection and during the 48-hour collection period. Samples must be kept frozen to preserve bile acid stability.
The fecal bile acid excretion test will likely have multiple uses, including evaluation of patients with Crohn's disease who have diarrhea after ileal resection or ileal inflammatory disease that impedes bile acid absorption in the distal ilium and those with post-cholecystectomy diarrhea.
The fecal bile acid test is available to physicians within and outside Mayo through Mayo Medical Laboratories (test ID: BA48F).
"At last we've conclusively demonstrated that a about a third of patients with chronic diarrhea thought to be due to IBS-D have BAM and need to be diagnosed properly and treated more specifically and effectively," Dr. Camilleri says. His comprehensive review of bile acid diarrhea appeared in the January 2014 issue of Expert Review of Gastroenterology & Hepatology.
For more information
Camilleri M, et al. Measurement of serum 7α-hydroxy-4-cholesten-3-one (or 7αC4), a surrogate test for bile acid malabsorption in health, ileal disease and irritable bowel syndrome using liquid chromatography-tandem mass spectrometry. Neurogastroenterology & Motility. 2009;21:734.
Camilleri M. Advances in understanding of bile acid diarrhea. Expert Review of Gastroenterology & Hepatology. 2014;8:49.