Irritable bowel syndrome (IBS) is a nonspecific disorder of the large intestine characterized by bloating, cramping, diarrhea, and/or constipation. A common condition affecting 10% to 20% of the world’s population, IBS is the second highest cause of work absenteeism after the common cold, and contributes to 25% to 50% of all GI referrals. As of June 2016, IBS with diarrhea (IBS-D) is diagnosed using the Rome IV criteria,1 which consists exclusively of patient symptoms including weekly recurrent abdominal pain in the last three months accompanied by at least two of the following:
Given these non-specific factors, patient sequelae from several other non-IBS conditions tend to fit the Rome IV criteria (eg, viral or parasitic infections, intestinal inflammatory disease, celiac disease, steatorrhea, etc), and these must be ruled out before confirming IBS. For each of these non-IBS conditions, specific laboratory tests and treatments are available. However, there are few targeted laboratory tests to either diagnose IBS or subcategorize IBS patients into effective treatment groups.
An Underserved Population
Due in part to ambiguity over the causes of IBS, physicians often treat IBS-D symptoms without fully understanding the underlying disorder. Commonly, a “try it and see” approach is taken toward symptom reduction (eg, introduction of probiotics, opioid receptor antagonists, tri-cyclic antidepressants, antispasmodic drugs, etc) without any patient preselection. These approaches tend to treat the symptoms, but do not diagnose nor resolve the underlying disease state.
It is now appreciated that up to 30% of all patients with IBS-D have a condition called bile acid malabsorption (BAM) that results in excess bile acids (BAs) in the stool.2 Normally, BAs are synthesized in the liver from cholesterol and aid in the emulsification and absorption of dietary fats. Upon ingestion of a meal, BAs are released into the GI system where they bind with dietary fats and facilitate travel through the small intestine, down to the terminal ileum. At this point, nearly all (95%) of BAs are reabsorbed in the ileum, while only 5% travel through the colon (where intestinal bacteria convey primary BAs to secondary BAs) and are excreted in feces. The reabsorbed BAs negatively inhibit the synthesis of new BAs, as this synthesis and reabsorption pathway is highly regulated under normal circumstances.
Differentiate BAM from IBS
BAM occurs when the BA reabsorption process is deficient, resulting in excess BA in the large intestine that causes fluid secretion, increased motility, and watery stool (ie, diarrhea). The biological phenotype associated with this condition includes the following factors:
The gold standard for identifying patients with BAM is the 75SeHCAT retention assay, an assay that requires whole-body scanning to detect retention of an orally administered BA derivative not currently available in the US.3 To help address this gap, Mayo Clinic’s Department of Laboratory Medicine and Pathology (DLMP) in Rochester, Minnesota, has recently released two novel tests to aid in the evaluation of patients with possible BAM: The fecal BAs test and the serum 7aC4 test. Published literature has shown that measuring fecal BAs yields equivalent diagnostic information compared to the gold standard SeHCAT retention test where serum 7aC4 can be used as a surrogate marker for BAM to avoid a timed fecal collection.2,3 Both tests are performed using liquid chromatography mass spectrometry (LC-MS) and both can detect whether BA malabsorption is the cause of IBS-D.
These tests open up a new avenue for physicians to identify and treat patients who are diagnosed with BAM. If results are positive for BAM, the patient can be put on a specific and effective treatment plan; namely, a BA sequestrant, rather than treating the patient non specifically with a general anti-diarrheal. BA sequestrants bind to the excess BAs in the GI system and prevent them from producing a pro-diarrheal phenotype in the patient. In turn, patients who test negative can be put on a different treatment path.
The Fecal Bile Acids Test
While the fecal BAs test provides the most accurate diagnostic information, it is significantly more demanding of the patient than the serum 7aC4 test. The fecal test requires the patient to consume a 100 gram/day fat diet for four days. Self-collection of a stool sample must be taken on days 3 and 4 and stored frozen. On the fifth day, the samples are sent to the laboratory frozen (either dropped off to the testing laboratory if the patient is local or mailed on dry ice). Freezing is necessary because fecal BAs are not highly stable in stool samples, especially watery stools. Once the sample reaches the lab, the stool is homogenized, BAs are extracted, and the five most prevalent BAs are measured by LC-MS. Test results summate the following two reportables:
If either result is elevated, BAM should be considered. It is important to note that fecal BAs testing can be ordered along with fecal fat testing, since both tests require the same patient preparation and sample collection, so both BAM and steatorrhea can be assessed from the same fecal collection. When interpreting results, physicians and laboratorians should keep in mind that a patient who is currently taking antibiotics (or has done so recently) will have reduced conversion of primary to secondary BAs (it may take up to three months to restore the microbiome), and patients with severe liver disease/dysfunction will have low total BAs (impaired synthesis). A random stool sample is not acceptable for this test, according to an additional Mayo study.
The 7aC4 Test
Some patients are more than willing to follow the rather demanding steps required for a fecal BAs test, especially those plagued by chronic diarrhea. However, others are less willing when other options may be available. Accordingly, Mayo’s DLMP sought to develop a more practical and less demanding test, resulting in the serum 7aC4 test. Although this is only a screening test, it may be a more palatable choice for patients, as unlike the special fat diet and stool collection required of the fecal test, the 7aC4 test is performed on a single, fasting, morning serum sample.
As an intermediate in the BA synthesis pathway, 7aC4 is elevated in the serum of patients with BAM, owing to decreased reabsorption of colonic BAs and subsequent lack of negative feedback regulation of BA synthesis in the liver. Given the testing methodology, this test is highly specific for 7aC4 with no interferences from structurally related compounds. It is important to note that measurements should be performed on a morning fasting sample, when the most consistent concentration of 7aC4 can be found. Otherwise, diurnal variation could affect results.
For patients who have a result below the 17.6 ng/mL cutoff (which maximizes sensitivity), BAM is unlikely. Conversely, patients whose results are above the alternate 52.5 ng/mL cutoff (which maximizes specificity) most likely have BAM and can be prescribed BA sequestrants. If the result is between these two cutoffs, the health care provider can either order a confirmatory fecal BA test or consider a trial of BA sequestrants to determine whether they alleviate the symptoms.
The combination of the fecal BA test and the 7aC4 serum test is a lesson in utilization and matching a test to the appropriate patient population. Further, it is an exercise in how to improve the quality of overall patient care through laboratory test offerings, and most important, factoring into consideration the needs of the patient.
Leslie J. Donato, PhD, DABCC, is co-director of cardiovascular laboratory medicine and co-director of the Hospital Clinical Laboratory and Point of Care in the Department of Laboratory Medicine and Pathology at Mayo Clinic in Rochester, Minnesota.
Christoph Bahn is a freelance writer for Mayo Medical Laboratories, specializing in clinical research and discovery.