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Current Practice in the Diagnosis of Bile Acid Diarrhea

      Bile acids (BAs) are synthesized from the liver to aid in fat and fat-soluble vitamin solubilization and absorption. Approximately 95% of the unbound BAs are reabsorbed via the ileal BA transporter and transported through the portal circulation to the liver for recycling (Figure 1).
      • Hofmann A.F.
      • Small D.M.
      Detergent properties of bile salts: correlation with physiological function.
      • Camilleri M.
      Physiological underpinnings of irritable bowel syndrome: neurohormonal mechanisms.
      Figure thumbnail gr1
      Figure 1Primary BAs alone or in combination with total fecal BAs are equivalent to fecal BAs in the ability to detect elevated fecal weight, a validated correlate of BA diarrhea. Receiver operating characteristic (ROC) curves of total fecal BAs (central image), primary BAs (bottom left ROC), and primary BAs with total fecal BAs (lower right ROC) predict a fecal weight of >400 g. Adapted with permission from.
      • Vijayvargiya P.
      • Camilleri M.
      • Chedid V.
      • et al.
      Analysis of fecal primary bile acids detects increased stool weight and colonic transit in patients with chronic functional diarrhea.
      AUC, area under the curve; αC47, α-hydroxy-4-cholesten-3-one; BA, bile acid; C4, 7a-hydroxy-4-cholesten-3-one; CA, cholic acid; CDCA, chenodeoxycholic acid; DCA, deoxycholic acid; LCA, lithocholic acid.
      BA diarrhea (BAD) is a condition characterized by excess BAs within the colon, resulting in increased colonic motility and secretion. Patients typically present with chronic diarrhea, urgency, and abdominal cramping.
      • Camilleri M.
      Physiological underpinnings of irritable bowel syndrome: neurohormonal mechanisms.
      There have been a number of novel approaches to diagnosis of BAD over the past decade.
      There are 4 categories of BAD, based on the underlying etiology. Type I includes any ileal disease that would prevent the reabsorption of BAs from the terminal ileum, including Crohn’s disease, ileal resection, and radiation ileitis.
      • Fromm H.
      • Malavolti M.
      Bile acid-induced diarrhoea.
      Type II is considered idiopathic, with no clear underlying cause and presents typically as chronic diarrhea or diarrhea-predominant irritable bowel syndrome (IBS-D).
      • Fromm H.
      • Malavolti M.
      Bile acid-induced diarrhoea.
      Type III results from underlying diseases related to malabsorption or biliopancreatic diseases such as chronic pancreatitis, celiac disease, and cholecystectomy.
      • Fromm H.
      • Malavolti M.
      Bile acid-induced diarrhoea.
      Other diseases with normal terminal ileal BA absorption have been found to be associated with BAD. The best example is the spectrum of microscopic colitis; 60% of patients with lymphocytic colitis and 27% of those with microscopic colitis were diagnosed with BAD.
      • Fernandez-Banares F.
      • Esteve M.
      • Salas A.
      • et al.
      Bile acid malabsorption in microscopic colitis and in previously unexplained functional chronic diarrhea.
      The fourth category of BAD includes conditions that result in excessive BA synthesis without a clear source of impaired BA reabsorption within the gastrointestinal tract, as seen in patients with hypertriglyceridemia or those on metformin treatment.
      • Scarpello J.H.
      • Hodgson E.
      • Howlett H.C.
      Effect of metformin on bile salt circulation and intestinal motility in type 2 diabetes mellitus.

      Prevalence

      It is estimated that 5% of the population in developed countries has chronic diarrhea (defined as diarrhea for >4 weeks) at any point in time, with direct costs of $524 million per year and indirect costs of $136 million per year.
      • Fine K.D.
      • Schiller L.R.
      AGA technical review on the evaluation and management of chronic diarrhea.
      BAD has been reported in 25%–33% of patients who present with chronic diarrhea,
      • Wedlake L.
      • A'Hern R.
      • Russell D.
      • et al.
      Systematic review: the prevalence of idiopathic bile acid malabsorption as diagnosed by SeHCAT scanning in patients with diarrhoea-predominant irritable bowel syndrome.
      and the BAD prevalence estimate based on these calculations would be approximately 1% of the population.

      Diagnostic Tests to Identify BAD

      There are 3 major types of diagnostic tests for BAD: the 75selenium homotaurocholic acid test (75SeHCAT), serum biomarkers of hepatic BA synthesis, and total and individual fecal BAs.
      • Vijayvargiya P.
      • Camilleri M.
      • Shin A.
      • et al.
      Methods for diagnosis of bile acid malabsorption in clinical practice.
      A summary of all tests, required equipment, and potential future use is available in Table 1.
      Table 1Current and future BAD diagnostic tests
      Diagnostic Test75SeHCATFasting Serum C4Fasting Serum FGF19Total Fecal BAsPrimary BAs >4% + Total Fecal BAsFecal Primary BAs >10%Single or Combined Tests of Fecal Primary BAs + Fasting Serum C4
      What does it measure?Ileal capacity to reabsorb radiolabeled BA retention (%) on day 7Hepatic BA synthesisAmount of feedback inhibition to hepatic BA synthesisTotal fecal BA excreted from the colonAmount of BAs with secretory potential with total fecal BA excretionAmount of BAs that are directly synthesized from the liver with secretory potentialCombining serum and stool biomarkers to simplify diagnosis of BAD
      Diagnostic cutoffs<5% (severe)

      <10% (moderate)

      <15% (mild)
      >52.5 ng/mL<61.7 pg/mL>2337 μmol/48 hPrimary BA >4% + total fecal BA >1000 μmol/48 h>10% Primary BATo be determined
      Sensitivity relative to fecal wt >400 g/48 h15%28%59%46%49%
      Specificity relative to fecal wt >400 g/48 h86%75%92%97%91%
      Diet, radiation, and equipment required for measurementGamma camera with radiation exposure; 7-day testHPLC

      Measure before 9am
      ELISA

      Measure before 9am
      HPLC

      Requires 100-gram high fat diet x 4 days and 2-day stool collection
      HPLC + HPLC

      single, random stool sample + C4 measured before 9 AM
      Comment or pitfalls of testing? Best for type I BAD? diagnostic accuracy? diagnostic accuracyDirect way to analyze fecal BAs within colonIdentify additional patientsIdentify additional patientsIn development
      Note: Primary BAs are cholic acid and chenodeoxycholic acid.
      BA, bile acids; BAD, bile acid diarrhea; C4, 7a-hydroxy-4-cholesten-3-one; ELISA, enzyme-linked immunosorbent assay; HPLC, high-performance liquid chromatography.

       75SeHCAT

       What does it measure?

      75SeHCAT is an imaging test to evaluate terminal ileal reabsorption of BAs and retention over time. It is estimated that BAs undergo enterohepatic cycling about 5 times per day. Participants ingest a radiolabeled BA. With a gamma camera, an image of the abdomen is taken 3 hours after ingestion to establish a baseline. Seven days after ingestion, another image is obtained to determine the percentage of radiolabeled BA that is retained within the abdomen, including the gastrointestinal tract and biliary tree. Participants with decreased retention of the radiolabeled BA are considered to have an increased loss of BAs into the colon, indicating BAD.
      • Sciarretta G.
      • Fagioli G.
      • Furno A.
      • et al.
      75Se HCAT test in the detection of bile acid malabsorption in functional diarrhoea and its correlation with small bowel transit.

       Diagnostic Cutoffs

      75SeHCAT cutoffs and associated response to BA binder, cholestyramine, are as follows: <5% BA retention is severe BAD with a 96% response; <10% BA retention is moderate BAD with 80% response; and <15% BA retention is mild BAD with a 70% response to cholestyramine.
      • Wedlake L.
      • A'Hern R.
      • Russell D.
      • et al.
      Systematic review: the prevalence of idiopathic bile acid malabsorption as diagnosed by SeHCAT scanning in patients with diarrhoea-predominant irritable bowel syndrome.
      Currently, using the 75SeHCAT retention test, the most commonly used cutoff for the diagnosis of BAD is <10%, but one could also consider <15% retention depending on the patient’s response to the BA sequestrant.

       Serum Biomarkers of Hepatic BA Synthesis

      Fasting serum 7α-hydroxy-4-cholesten-3-one (C4) is a direct measure of hepatic BA synthesis and fibroblast growth factor 19 (FGF19), synthesized by ileal enterocytes when absorbed BAs stimulate the nuclear farnesoid X receptor, represents an indirect measure of ileal BA reabsorption and provides feedback inhibition on hepatic BA synthesis.
      • Shin A.
      • Camilleri M.
      • Vijayvargiya P.
      • et al.
      Bowel functions, fecal unconjugated primary and secondary bile acids, and colonic transit in patients with irritable bowel syndrome.
      • Wong B.S.
      • Camilleri M.
      • Carlson P.
      • et al.
      Increased bile acid biosynthesis is associated with irritable bowel syndrome with diarrhea.

       What does it measure?

      Upon enterocyte absorption in the terminal ileum, BAs activate the nuclear farsenoid X receptor and increase transcription of the enteroendocrine hormone, FGF19. FGF19 reaches the liver via the portal venous system to reach hepatocytes and bind hepatic receptors to decrease C4, an intermediate in the conversion of cholesterol to BA by the rate-limiting enzyme CYP7A1. Thus, when BAs are reabsorbed, more FGF19 is released from the enterocyte and serum C4 is decreased, reflecting a decrease in hepatic BA synthesis.
      • Camilleri M.
      Physiological underpinnings of irritable bowel syndrome: neurohormonal mechanisms.
      In patients with type I BAD, the decreased ileal BA absorption results in a decreased FGF19 and increased synthesis of hepatic BAs and serum C4 level.
      • Pattni S.
      • Walters J.R.F.
      Recent advances in the understanding of bile acid malabsorption.
      Less than 1% of patients with type II BAD have impairment of BA reabsorption due to a mutation in the gene for the ileal BA transporter,
      • Montagnani M.
      • Love M.W.
      • Rossel P.
      • et al.
      Absence of dysfunctional ileal sodium-bile acid cotransporter gene mutations in patients with adult-onset idiopathic bile acid malabsorption.
      have decreased synthesis of FGF19
      • Johnston I.M.
      • Nolan J.D.
      • Pattni S.S.
      • et al.
      Characterizing factors associated with differences in FGF19 blood levels and synthesis in patients with primary bile acid diarrhea.
      and feedback regulation by FGF19, or genetic variants in klothoβ or fibroblast growth factor receptor 4 (reviewed in
      • Camilleri M.
      Physiological underpinnings of irritable bowel syndrome: neurohormonal mechanisms.
      ), which impair the FGF19 inhibition of the CYP7A1 in the hepatocyte , resulting in increased hepatic BA synthesis.
      • Camilleri M.
      • Shin A.
      • Busciglio I.
      • et al.
      Genetic variation in GPBAR1 predisposes to quantitative changes in colonic transit and bile acid excretion.
      These mechanisms would result in increased serum C4. In addition, G protein-coupled BA receptor 1 (also known as GPR131, and referred to herein as TGR5) is a BA-responsive G-protein–coupled receptor, which is the target of BA effects in the colon alters functions that manifest as diarrhea or discomfort. Thus, TGR5 genetic variants are associated with altered colonic transit, symptom phenotype, and gas sensation ratings.
      • Camilleri M.
      • Shin A.
      • Busciglio I.
      • et al.
      Genetic variation in GPBAR1 predisposes to quantitative changes in colonic transit and bile acid excretion.
      • Camilleri M.
      • Vazquez-Roque M.I.
      • Carlson P.
      • et al.
      Association of bile acid receptor TGR5 variation and transit in health and lower functional gastrointestinal disorders.
      Thus, decreased FGF19 and increased C4 are diagnostic for BAD.
      • Camilleri M.
      Physiological underpinnings of irritable bowel syndrome: neurohormonal mechanisms.

       Caveats of testing

      C4 is measured by high-performance liquid chromatography (HPLC) with mass spectrometry,
      • Camilleri M.
      • Nadeau A.
      • Tremaine W.J.
      • et al.
      Measurement of serum 7alpha-hydroxy-4-cholesten-3-one (or 7alphaC4), a surrogate test for bile acid malabsorption in health, ileal disease and irritable bowel syndrome using liquid chromatography-tandem mass spectrometry.
      and FGF19 is evaluated with enzyme-linked immunosorbent assay.
      • Odunsi-Shiyanbade S.T.
      • Camilleri M.
      • McKinzie S.
      • et al.
      Effects of chenodeoxycholate and a bile acid sequestrant, colesevelam, on intestinal transit and bowel function.
      It is important to note that both C4 and FGF19 have diurnal variations, with a gradual increase after 9:00 AM.
      • Gälman C.
      • Angelin B.
      • Rudling M.
      Bile acid synthesis in humans has a rapid diurnal variation that is asynchronous with cholesterol synthesis.
      If samples are taken after 9:00 AM, the fasting serum biomarkers may result in higher values because of the diurnal variations, but not indicate true elevated hepatic BA synthesis.

       Diagnostic cutoffs and accuracy

      The following are accuracy assessments of serum C4 and FGF19, based on comparisons with either 75SeHCAT or total fecal BAs. Thus, relative to 75SeHCAT <10%, a fasting serum C4 of >48.4 ng/mL has a sensitivity of 90% and a specificity of 79%,
      • Sauter G.H.
      • Munzing W.
      • Ritter C.V.
      • et al.
      Bile acid malabsorption as a cause of chronic diarrhea diagnostic value of 7α-hydroxy-4-cholesten-3-one in serum.
      and fasting serum FGF19 of <145 pg/mL has a sensitivity of 58% and a specificity of 84%.
      • Pattni S.S.
      • Brydon W.G.
      • Dew T.
      • et al.
      Fibroblast growth factor 19 in patients with bile acid diarrhoea: a prospective comparison of FGF19 serum assay and SeHCAT retention.
      In addition, relative to total fecal BAs of >2337 μmol/48 hours, fasting C4 of >52.5 ng/mL (95th percentile of 184 healthy volunteers) has a sensitivity of 25% and a specificity of 90%,
      • Vijayvargiya P.
      • Camilleri M.
      • Carlson P.
      • et al.
      Performance characteristics of serum C4 and FGF19 measurements to exclude the diagnosis of bile acid diarrhoea in IBS-diarrhoea and functional diarrhoea.
      and fasting FGF19 of <61.7pg/mL (5th percentile in 50 healthy volunteers) has a sensitivity of 32% and a specificity of 78%.
      • Vijayvargiya P.
      • Camilleri M.
      • Carlson P.
      • et al.
      Performance characteristics of serum C4 and FGF19 measurements to exclude the diagnosis of bile acid diarrhoea in IBS-diarrhoea and functional diarrhoea.
      The difference in accuracy between 75SeHCAT and total fecal BAs is possibly due to differences in test sensitivity to identify the underlying cause of the BAD. Thus, patients with decreased retention on 75SeHCAT (which tests intestinal absorption) are likely to have abnormal FGF19 and C4 values. In contrast, the underlying mechanism of BAD in type II BAD may be more diverse (as explained elsewhere in this Commentary; e.g., genetic variations in TGR5) and may not necessarily directly involve abnormal C4 or FGF19.

       Total and Individual BAs in Serum or Stool

      There are 4 major individual fecal BAs found in human stool: chenodeoxycholic acid (CDCA), cholic acid (CA), lithocholic acid, and deoxycholic acid (DCA). These are classified as primary or secondary and secretory or nonsecretory. Primary BAs are directly synthesized by the liver and include CDCA and CA. Upon entering the colon, these primary BAs are deconjugated and then dehydroxylated by colonic bacteria, resulting in the formation of secondary fecal BAs, namely, DCA and lithocholic acid. Secretory fecal BAs stimulate both colonic secretion and motility and include CDCA and DCA.
      • Shin A.
      • Camilleri M.
      • Vijayvargiya P.
      • et al.
      Bowel functions, fecal unconjugated primary and secondary bile acids, and colonic transit in patients with irritable bowel syndrome.

       Serum total and individual BAs

      In healthy participants, serum BA measures the concentration of total and individual BAs, which represents recycled BAs that were reabsorbed from the terminal ileum. About 50% of newly synthesized BAs from the liver are stored in the gallbladder during fasting. The remaining BAs are secreted into the duodenum.
      • Mok H.Y.
      • Von Bergmann K.
      • Grundy S.M.
      Kinetics of the enterohepatic circulation during fasting: biliary lipid secretion and gallbladder storage.
      There is a selective hepatic uptake of individual BAs from the portal venous system, depending on the individual BA structure and the state of conjugation.
      • Dawson P.A.
      • Lan T.
      • Rao A.
      Bile acid transporters.
      • Setchell K.D.
      • Kritchevsky D.
      • Nair P.P.
      The bile acids: chemistry, physiology, and metabolism. Volume 4: methods and applications.
      The main purpose of measurement of serum BA is to identify cholestatic liver disease, biliary occlusion, and intrahepatic cholestasis during pregnancy. To date, there has been no association between 75SeHCAT retention and serum primary BAs.
      • Sciarretta G.
      • Fagioli G.
      • Furno A.
      • et al.
      75Se HCAT test in the detection of bile acid malabsorption in functional diarrhoea and its correlation with small bowel transit.

       Fecal total and individual BAs

      The measurement of fecal BAs is a direct measure of excess BAs exiting the colon. Within the colon, there is a proportion of fecal BAs passively absorbed and transformed into secondary fecal BAs via bacterial deconjugation and dehydroxylation. There was a significant correlation of total fecal BAs with 75SeHCAT, but no direct evaluation to determine sensitivity and specificity.
      • Sciarretta G.
      • Fagioli G.
      • Furno A.
      • et al.
      75Se HCAT test in the detection of bile acid malabsorption in functional diarrhoea and its correlation with small bowel transit.
      Currently, total fecal BAs of >2337 μmol/48 hours is the gold standard for diagnosis of BAD in countries where the 75SeHCAT retention test is not available.
      • Shin A.
      • Camilleri M.
      • Vijayvargiya P.
      • et al.
      Bowel functions, fecal unconjugated primary and secondary bile acids, and colonic transit in patients with irritable bowel syndrome.
      Primary fecal BAs (CDCA and CA) are significantly higher in patients with BAD and correlate with stool frequency and consistency.
      • Shin A.
      • Camilleri M.
      • Vijayvargiya P.
      • et al.
      Bowel functions, fecal unconjugated primary and secondary bile acids, and colonic transit in patients with irritable bowel syndrome.
      • Wong B.S.
      • Camilleri M.
      • Carlson P.
      • et al.
      Increased bile acid biosynthesis is associated with irritable bowel syndrome with diarrhea.
      Because 75SeHCAT is not available in the United States, we used the valid marker of diarrhea stool weight and showed that patients with BAD have higher stool weight compared with those with chronic diarrhea without BAD or healthy volunteers.
      • Vijayvargiya P.
      • Camilleri M.
      • Chedid V.
      • et al.
      Analysis of fecal primary bile acids detects increased stool weight and colonic transit in patients with chronic functional diarrhea.
      The sensitivity and specificity of serum and fecal biomarkers of BAD are listed in Table 1. Elevated primary BAs alone or in combination with total fecal BAs of >1000 μmol/48 hours have a similar diagnostic accuracy (P = .13) as total fecal BAs alone of >2337 μmol/48 hours in detecting elevated fecal weight in patients with BAD (Figure 1, right). After increasing the primary BA cutoff to 10% with total fecal BAs of >1000 μmol/48 hours, there was no significant gain in either sensitivity (41%) or specificity (97%) compared with the lower cutoff of primary BAs of >4% with total fecal BAs of >1000 μmol/48 hours.
      • Vijayvargiya P.
      • Camilleri M.
      • Chedid V.
      • et al.
      Analysis of fecal primary bile acids detects increased stool weight and colonic transit in patients with chronic functional diarrhea.
      Although one might expect a greater diagnostic usefulness of secretory fecal BAs, the sensitivity (9%) was significantly lower than with primary fecal BAs, possibly because DCA is a very significant component among the individual fecal BAs in stool. Thus, smaller changes in DCA are not sufficient to provide accuracy. In contrast, the typical proportion of primary fecal BAs in healthy volunteers is approximately 0.2%. Therefore, even at the lower cutoff values of >4%, primary fecal BAs are indicative of BAD, even though the total fecal BAs may be only 1000 μmol/48 hours.
      In an analysis of 986 patients who underwent 48-hour fecal BA measurement for chronic diarrhea, 26% of patients had elevated total fecal BAs, whereas 46% of patients had fecal primary BAs of >10%, indicating that measuring total fecal BAs alone will miss a subgroup of patients who have features of BAD.
      • Vijayvargiya P.
      • Camilleri M.
      • Chedid V.
      • et al.
      Analysis of fecal primary bile acids detects increased stool weight and colonic transit in patients with chronic functional diarrhea.

       Possible etiology for elevated fecal primary BAs

      Although further investigation is required, there are 3 possible etiologies for the increased primary fecal BAs in patients with BAD: increased hepatic BA synthesis resulting in increased delivery of primary BAs within the colon; an altered proportion of colonic bacteria with 7α hydroxylase activity resulting in differences in BA dehydroxylation or conversion of primary to secondary BAs; or acceleration of colonic transit. In a study of patients with IBS-D (75% of whom had elevated primary fecal BAs), there was a decrease in leptum microbial phylum with the capacity to dehydroxylate primary fecal BAs. However, there were still a number of microbial species in the stool with the capacity to dehydroxylate the primary fecal BAs that remained.
      • Duboc H.
      • Rainteau D.
      • Rajca S.
      • et al.
      Increase in fecal primary bile acids and dysbiosis in patients with diarrhea-predominant irritable bowel syndrome.
      Thus, future studies will need to determine what bacterial species are required to significantly impact 7 dehydroxylation. Last, it is possible that colonic transit may alter the ability to dehydroxylate primary fecal BAs; however, this factor seems unlikely, given that among patients with IBS-D, the mean ascending colon emptying T1/2 was 15 hours,
      • Camilleri M.
      • Busciglio I.
      • Acosta A.
      • et al.
      Effect of increased bile acid synthesis or fecal excretion in irritable bowel syndrome-diarrhea.
      suggesting that despite the observation of accelerated overall colonic transit at 24 or 48 hours in 48% of patients with IBS-D,
      • Camilleri M.
      • McKinzie S.
      • Busciglio I.
      • et al.
      Prospective study of motor, sensory, psychologic, and autonomic functions in patients with irritable bowel syndrome.
      there is still sufficient time for dehydroxylation of the primary BAs.

       Caveats of testing

      Total and individual fecal BAs are measured in a 48-hour stool sample taken during the last 2 days of a 4-day, 100-g fat diet. It is quantified via HPLC with mass spectrometry.
      • Shin A.
      • Camilleri M.
      • Vijayvargiya P.
      • et al.
      Bowel functions, fecal unconjugated primary and secondary bile acids, and colonic transit in patients with irritable bowel syndrome.
      Unlike with 75SeHCAT,
      • Wedlake L.
      • A'Hern R.
      • Russell D.
      • et al.
      Systematic review: the prevalence of idiopathic bile acid malabsorption as diagnosed by SeHCAT scanning in patients with diarrhoea-predominant irritable bowel syndrome.
      there are no randomized clinical trials that evaluate the response of BA binders to those with elevated total or primary fecal BAs.

      Which Test Should We Use to Diagnose BAD?

      75SeHCAT is considered the current gold standard to diagnose BAD and is an ideal test for those with BA reabsorption abnormalities to identify a proportion of patients with type II BAD. Unfortunately, 75SeHCAT is not available in the United States.
      The serum fasting biomarkers are convenient, but lack diagnostic accuracy on their own. Fasting serum C4 is appropriate as a screening tool and is available via commercial testing in the United States (Mayo Medical Laboratories and PROMETHEUS IBcause). A single, fasting FGF19 is not sensitive or specific enough to screen for BAD. Further research is required to assess CDCA stimulated FGF19 (acting as a stress test) in the diagnosis of BAD.
      • Borup C.
      • Syversen C.
      • Bouchelouche P.
      • et al.
      Diagnosis of bile acid diarrhoea by fasting and postprandial measurements of fibroblast growth factor 19.
      Measurements of total and individual fecal BAs provide the most direct method of analyzing colonic total fecal BAs and the changes of primary and secondary BAs within the colon. Measurements of total and individual fecal BAs are accurate and reproducible tests that provide a first-line option for the evaluation of chronic diarrhea.
      • Vijayvargiya P.
      • Camilleri M.
      • Chedid V.
      • et al.
      Analysis of fecal primary bile acids detects increased stool weight and colonic transit in patients with chronic functional diarrhea.

      Future Considerations

      Currently, measurements of total and individual fecal BAs require a 4-day, 100-g fat diet, with a 48-hour stool collection. Individual fecal BAs, particularly primary BAs, have a high concordance correlation coefficient (>0.8) between measurements of a 48-hour stool sample compared with individual BAs from a single, random stool sample collected simultaneously.
      • Camilleri M.
      • Acosta A.
      • Busciglio I.
      • et al.
      Effect of colesevelam on faecal bile acids and bowel functions in diarrhoea-predominant irritable bowel syndrome.
      Thus, there is the potential to replace the 48-hour stool collection, which requires the 4-day, 100-g fat diet, with a simple, random stool sample. Additionally, combining diagnostic tests such as fecal primary BA evaluation and fasting serum C4 may improve and simplify the diagnosis. In the current HPLC-MS method used to measure total and individual fecal BAs, we do not measure epimers of each individual fecal BA
      • Keely S.J.
      • Scharl M.M.
      • Bertelsen L.S.
      • et al.
      Bile acid-induced secretion in polarized monolayers of T84 colonic epithelial cells: structure-activity relationships.
      that may have different effects within the colon and may further help identify patients with BAD or individualize care.

      Conclusions

      At present in the United States, measurements of total and individual fecal BAs, collected over 48 hours, are the most direct way to identify patients with BAD among patients with chronic diarrhea or IBS-D. Primary fecal BA measurement has similar diagnostic usefulness as total fecal BA measurement and identifies a larger subset of patients with chronic diarrhea that may otherwise be missed by measuring total fecal BAs alone. We anticipate further simplification and usefulness of this diagnostic test.

      Acknowledgment

      The authors thank Mrs. Cindy Stanislav for excellent secretarial assistance.

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