Bile Acid Malabsorption and Fecal Bile Acids Testing
Expires August 2021
Up to 30 percent of patients with diarrheal irritable bowel syndrome also suffer from bile acid malabsorption (BAM). Recently, a new test was introduced that can identify bile acid malabsorption from a stool sample. BAM alone can cause ileal dysfunction and impaired reabsorption, as well as complicate other gastrointestinal disorders. Treatment for BAM without identification of increased bile acids can result in adverse side effects. With the introduction of this new test, it is now possible to identify BAM in patients before initiating treatment.
Leslie Donato, Ph.D., Consultant in Cardiovascular Laboratory Medicine and Clinical Core Laboratory, and Instructor in Laboratory Medicine and Pathology at Mayo Clinic in Rochester, Minn.
Our speaker for this program is Dr. Leslie Donato, an Instructor in Laboratory Medicine and Pathology at Mayo Clinic, as well as a consultant in Cardiovascular Laboratory Medicine, and also the Clinical Core Laboratory. Dr. Donato provides an overview of the new fractionated fecal bile acids test.
Thank you for the introduction.
I have nothing to disclose
As you view this presentation, consider the following important points regarding Bile Acids fractionated, Fecal (BA48F).
- How is the testing going to be used in your practice?
- When should the tests be used?
- How will results impact patient management?
Irritable Bowel Syndrome
In this session, I will describe a new test for probing the etiology of a condition called irritable bowel syndrome or IBS. Irritable bowel syndrome is characterized by abdominal pain or discomfort with defecation and also changes in bowel habits or disordered defecation. It is quite common with 10% to 20% of the world-wide population displaying symptoms of IBS. It is estimated to be the cause of up to 50% of all GI referrals and is the second highest cause of work absenteeism, second only to the common cold. The diagnostic criteria used to identify IBS utilizes only patient symptoms and is defined by the Rome III criteria as outlined on the right-hand side of the screen.
There are 4 types of irritable bowel syndrome-IBS with predominantly diarrhea, IBS with predominantly constipation, mixed IBS with varying diarrhea and constipation, and unsubtyped IBS. We will focus today on IBS diarrhea or IBS-D.
IBS-D is not a straight-forward diagnosis. Stools may be solid but frequent. Straining to defecate can occur with soft or watery stools. Lastly, symptoms can overlap with celiac disease, food allergies, and even healthy individuals.
Patients with mild to intermittent symptoms may benefit from treatments including lifestyle and dietary modification (which would include avoiding foods that are gas producers, contain lactose or gluten, etc.)
Patients with moderate to severe symptoms often require pharmacological therapy such as the ones listed here for patients with IBS diarrhea.
Connection Between IBS-D and Bile Acids
Up to 30% of IBS-D patients have a condition called bile acid malabsorption. Let’s discuss the biological function of bile acids.
Bile acids aid in the emulsification and absorption of dietary fats. They are synthesized from cholesterol in the liver and stored in the gallbladder. Upon ingestion of a meal they are secreted into the intestine where they carry out their function to transport dietary fats through the GI system. A majority of bile acids, about 95%, are then reabsorbed in lower GI tract. Only the remaining 5% of bile acids can be found in stool.
Bile acids that are released from the gall bladder are called primary bile acids. As they make their way through the GI system, intestinal bacteria convert primary bile acids to secondary bile acids. The majority of bile acids found in stool are secondary bile acids, with a small percentage of primary bile acids remaining. The 5 most prevalent bile acids found in stool are shown in the table above.
Bile Acids Malabsorption (BAM)
Bile acid malabsorption is a condition in which insufficient bile acid reabsorption or overproduction of bile acids leads to increased bile acids in the stool. It can be separated into 3 types. Type 1 is caused by ileal dysfunction and impaired reabsorption (for example Crohn’s disease). Type 2 is primary or idiopathic caused by an increase of bile acid production. The last is type 3, which consists of other gastrointestinal disorders which affect absorption, such as small intestinal bacterial overgrowth, celiac disease, or chronic pancreatitis. So the question is, how can a physician differentiate cases of increased fecal bile acids from other causes of IBS diarrhea?
Increased Fecal Bile Acids in IBS-D
High fecal bile acids can be found in patients with IBS diarrhea from several causes including bile acid malabsorption, increased bile acid synthesis, and increased colonic transit. It is estimated that up to 30% of all patients with IBS diarrhea have increased bile acids in their stool.
The testing we offer to identify high fecal bile acids includes 2 results. The first is the total amount of fecal bile acids in which all 5 of the most abundant fecal bile acids are measured separately and then added together. The second reported result is the percentage of the primary bile acids, the cholic acid and chenodeoxycholic acid, as a percent of the total fecal bile acids. An elevated value of either total bile acids or primary bile acids is indicative of bile acid malabsorption. Total bile acids are increased because of the reasons stated on the top of this slide. The primary bile acids are increased most likely because of a decreased rate of conversion of primary to secondary bile acids as colonic transit rate increases.
The potential pharmaceutical treatments for patients with bile acid malabsorption include bile acid sequestrants, which complex to bile acids and prevent their reabsorption such that the body excretes more bile acids in the stool. Also, drugs called FXR agonists can be used, which function to decrease the synthesis of bile acids. Historically, if a patient was thought to have bile acid malabsorption a physician may choose to empirically treat with 1 of these treatments, most commonly a bile acid sequestrant, in the hopes to relieve the patient’s symptoms. However the empirical treatment of patients without knowing they have bile acid malabsorption can be problematic for a variety of reasons, including adverse side effects and poor patient tolerability. Therefore, identifying patients that could benefit from treatment would be ideal.
Colesevelam Treatment of Patients with IBS and High Fecal Bile Acids
In our own internal studies, we identified a cohort of patients diagnosed with IBS-D that also had elevated fecal bile acid values. They were enrolled in an open-labeled 10-day single dose trial of the bile acid sequestrant colesevalam. In this slide you’ll see that nearly all patients had an improvement of their stool form as measured by the Bristol Stool Form scale.
Bile Acids, Fractionated, Fecal (BA48F)
The test for fecal bile acids starts with a timed stool collection. The timed collection requires a 100-gram per day fat diet for 2 days prior to collection and also during the time of collection for a total of 4 days on the high-fat diet. The collection of all passed stools occurs on day 3 and 4 of the high-fat diet for a total of 48 hours. The combined sample is either brought to the laboratory if the patient is local or the sample is mailed to the lab. One important aspect of sample collection is that the collected stool must be kept cold, preferably frozen, until delivered to the lab. This means that mailed stool samples should be mailed frozen to the laboratory.
Once the 48-hour stool collection is received in the lab, the sample is homogenized, the bile acids are extracted from the stool, and are then measured using liquid chromatography separation with mass spectrometer detection. The assay measures the 5 most abundant fecal bile acids found in stool. As a reminder, the test results that the physician receives include 2 reported values: the first is the total amount of fecal bile acids contained in the entire 48-hour collection. And the second is the percent of the primary fecal bile acids. Bile acid malabsorption is suggested if either value is elevated.
To interpret the total bile acids in the sample, the upper limit of the central 95th percentile for total fecal bile acids of a healthy reference population is used. This upper limit is 2,619 micromoles per 48 hours. To interpret the percent of primary bile acids an internal study was performed that showed a value of 3.7% or more of primary bile acids in the feces is 90% specific and 72% sensitive to detect IBS-D over healthy controls.
Lastly, it is important to take note of a few important considerations when ordering the fecal bile acid test. First, patients currently taking or have recently been treated with systemic antibiotics will have reduced conversion of primary to secondary bile acids. This is because intestinal bacteria convert primary bile acids to secondary bile acids. Patients on antibiotics, which function to kill bacteria, will have extremely elevated primary bile acids in their stool that is not the result of bile acid malabsorption. It may take up to 3 months to restore the microbiome in these patients such that normal conversion of primary to secondary bile acids can occur.
Second, patients with severe liver disease or dysfunction will have low total bile acids. This is because bile acids are produced in the liver so patients with a malfunctioning liver will have low total bile acids in their stool.
Third, a random stool sample is not acceptable for testing. All laboratory and clinical validation studies were performed with 48-hour timed collections and an internal study proved that a random stool collection did not give the same results as the timed collection. Lastly, the fecal bile acids test can be ordered along with a 48-hour fecal fat test. This is because they require the same patient preparation and the same 48-hour stool collection. Physicians that want to order both a fecal fat test and a fecal bile acids test on the same patient to assess the cause of their chronic diarrhea can do so on the same order, if the fecal fat is a 48-hour duration.