Primary sclerosing cholangitis (PSC) is a chronic progressive inflammatory disease of the bile ducts that is surrounded by much uncertainty. First of all, it is a rare disease, with reported prevalence between 0 and 31.7 per 100,000 individuals,1Boonstra K. Beuers U. Ponsioen C.Y. Epidemiology of primary sclerosing cholangitis and primary biliary cirrhosis: a systematic review.J Hepatol. 2012; 56: 1181-1188Abstract Full Text Full Text PDF PubMed Scopus (374) Google Scholar,2Barner-Rasmussen N. Pukkala E. Jussila A. et al.Epidemiology, risk of malignancy and patient survival in primary sclerosing cholangitis: a population-based study in Finland.Scand J Gastroenterol. 2020; 55: 74-81Crossref PubMed Scopus (11) Google Scholar and establishing a diagnosis can be difficult. The primary pathology is hidden deep in the liver, and there is no specific and easy noninvasive diagnostic test for PSC. Histologic features of fibrosing cholangiopathy are present in <20% of liver biopsy specimens obtained from patients with PSC and can also be seen in cases of secondary sclerosing cholangitis. Current US and European guidelines do not recommend liver biopsy for making a diagnosis of PSC except for suspected cases of small duct PSC and pediatric cases.3Lindor K.D. Kowdley K.V. Harrison M.E. ACG clinical guideline: primary sclerosing cholangitis.Am J Gastroenterol. 2015; 110: 646-659Crossref PubMed Scopus (247) Google Scholar,4European Association for the Study of the LiverEASL clinical practice guidelines: management of cholestatic liver diseases.J Hepatol. 2009; 51: 237-267Abstract Full Text Full Text PDF PubMed Scopus (1230) Google Scholar In addition, biliary tract abnormalities can be found on magnetic resonance cholangiopancreatography (MRCP) or endoscopic retrograde cholangiography (ERC) in a wide array of conditions (see Supplementary Table 1). Furthermore, the course of PSC can be variable. Liver transplant–free survival ranges between 14.5 and 21.3 years in large cohorts.5Weismüller T.J. Trivedi P.J. Bergquist A. et al.Patient age, sex, and inflammatory bowel disease phenotype associate with course of primary sclerosing cholangitis.Gastroenterology. 2017; 152: 1975-1984Abstract Full Text Full Text PDF PubMed Scopus (197) Google Scholar,6Boonstra K. Weersma R.K. van Erpecum K.J. et al.Population-based epidemiology, malignancy risk, and outcome of primary sclerosing cholangitis.Hepatology. 2013; 58: 2045-2055Crossref PubMed Scopus (348) Google Scholar An important risk factor affecting survival is the development of cholangiocarcinoma (CCA). This has a cumulative incidence of approximately 20% at 20 years after diagnosis, but one third of CCA cases become manifest within 1 year after the diagnosis of PSC.5Weismüller T.J. Trivedi P.J. Bergquist A. et al.Patient age, sex, and inflammatory bowel disease phenotype associate with course of primary sclerosing cholangitis.Gastroenterology. 2017; 152: 1975-1984Abstract Full Text Full Text PDF PubMed Scopus (197) Google Scholar,6Boonstra K. Weersma R.K. van Erpecum K.J. et al.Population-based epidemiology, malignancy risk, and outcome of primary sclerosing cholangitis.Hepatology. 2013; 58: 2045-2055Crossref PubMed Scopus (348) Google Scholar Last but not least, the cause of PSC is still enigmatic, and consequently, there is no therapy that has been proven to modify disease progression. In recent years, there has been an upsurge of pilot studies and phase 2 clinical trials. Phase 2 and 3 trials are hampered by the lack of established surrogate endpoints for clinical outcome, the heterogeneity of the disease course, and the lack of clear definitions for many key clinical disease characteristics.7Ponsioen C.Y. Chapman R.W. Chazouillères O. et al.Surrogate endpoints for clinical trials in primary sclerosing cholangitis: review and results from an International PSC Study Group consensus process.Hepatology. 2016; 63: 1357-1367Crossref PubMed Scopus (95) Google Scholar While the available national and international clinical practice guidelines predominantly focus on clinical management, the International PSC Study Group (IPSCSG) has recognized that there is a vital need for clinical trials to use uniform definitions for the diagnosis of PSC and for assessing disease severity and outcomes.3Lindor K.D. Kowdley K.V. Harrison M.E. ACG clinical guideline: primary sclerosing cholangitis.Am J Gastroenterol. 2015; 110: 646-659Crossref PubMed Scopus (247) Google Scholar,4European Association for the Study of the LiverEASL clinical practice guidelines: management of cholestatic liver diseases.J Hepatol. 2009; 51: 237-267Abstract Full Text Full Text PDF PubMed Scopus (1230) Google Scholar,8Chapman M.H. Thorburn D. Hirschfield G.M. et al.British Society of Gastroenterology and UK-PSC guidelines for the diagnosis and management of primary sclerosing cholangitis.Gut. 2019; 68: 1356-1378Crossref PubMed Scopus (55) Google Scholar Therefore, the IPSCSG has commissioned a consensus process among its experts to issue a set of definitions to aid clinical trialists designing much needed clinical trials. To this end, an extensive consensus process was initiated, applying a hybrid between a Delphi process and a nominal group process, as was previously used by the IPSCSG and the European Crohn’s and Colitis Organization.7Ponsioen C.Y. Chapman R.W. Chazouillères O. et al.Surrogate endpoints for clinical trials in primary sclerosing cholangitis: review and results from an International PSC Study Group consensus process.Hepatology. 2016; 63: 1357-1367Crossref PubMed Scopus (95) Google Scholar,9Dignass A. Eliakim R. Magro F. et al.Second European evidence-based consensus on the diagnosis and management of ulcerative colitis part 1: definitions and diagnosis.J Crohns Colitis. 2012; 6: 965-990Abstract Full Text Full Text PDF PubMed Scopus (593) Google Scholar Where necessary, an additional survey was conducted among all IPSCSG members (see the Supplementary Methods for details). Results are presented as a comprehensive set of consensus statements (CSs) and diagnostic criteria for PSC. Although patients with PSC are increasingly diagnosed at earlier stages of disease based on asymptomatic serum liver enzyme abnormalities, at least 45% of patients PSC present with symptoms on either a constant or intermittent basis, which significantly reduces quality of life.10Broome U. Olsson R. Lööf L. et al.Natural history and prognostic factors in 305 Swedish patients with primary sclerosing cholangitis.Gut. 1996; 38: 610-615Crossref PubMed Google Scholar Furthermore, more than 22% of asymptomatic patients may develop symptoms within 5 years. The most common clinical symptoms associated with PSC are fatigue; pruritus; right upper quadrant (RUQ) abdominal pain; and emotional distress, which may include anxiety and depression symptoms. Pruritus and abdominal pain can fluctuate dramatically, and this may be suddenly worsened because of the development of a biliary obstruction or acute bacterial cholangitis. Therefore, clinical evaluation with laboratory testing and imaging may be necessary in the event of the sudden appearance or worsening of symptoms. Emotional distress for many patients is attributable to chronic anxiety about the uncertain etiology of the disease, the lack of effective therapy, and the risk of malignancy, among others.11Ranieri V. Kennedy E. Walmsley M. et al.The Primary Sclerosing Cholangitis (PSC) Wellbeing study: understanding psychological distress in those living with PSC and those who support them.PLoS One. 2020; 15e0234624Crossref PubMed Scopus (0) Google Scholar The incorporation of clearly defined patient-reported symptoms into the design of clinical trials is both desirable from a patient-centered perspective and highly encouraged by regulatory agencies12Ponsioen C.Y. Lindor K.D. Mehta R. et al.Design and endpoints for clinical trials in primary sclerosing cholangitis.Hepatology. 2018; 68: 1174-1188Crossref PubMed Scopus (24) Google Scholar (Box 1, CS 1.1). Recently, 2 such tools have emerged. One patient-reported outcome (PRO) dedicated to PSC showed excellent reliability and was able to discern patients with cirrhosis and those with a history of depression.13Younossi Z.M. Afendy A. Stepanova M. et al.Development and validation of a primary sclerosing cholangitis-specific patient-reported outcomes instrument: the PSC PRO.Hepatology. 2018; 68: 155-165Crossref PubMed Scopus (14) Google Scholar A more recently developed tool to assess patient-reported symptoms, the Simple Cholestatic Complaints Score, was developed through digital surveys of patients and showed good criterion and construct validity and a very high test-retest reproducibility.14Munster K.N. Dijkgraaf M.G.W. Gennep S. et al.The Simple Cholestatic Complaints Score is a valid and quick patient-reported outcome measure in primary sclerosing cholangitis.Liver Int. 2020; 40: 2758-2766Crossref PubMed Scopus (0) Google Scholar Further validation of these tools in larger and more diverse cohorts is needed.Box 1Consensus StatementsTabled 11.Clinical symptoms1.1.Assessment of symptoms in patients with PSC for clinical trial purposes should be performed through disease-specific and validated quantitative tools.1.2.Assessment of symptoms in patients with PSC should take into consideration and be distinguished from symptoms attributable to active inflammatory bowel disease and other coexistent conditions.2.Laboratory markers2.1.The presence of significant elevations of serum aminotransferase activity (>5 × ULN) and/or IgG (>2 × ULN) can be indicative of features of coexisting autoimmune hepatitis and should trigger consideration of liver biopsy for diagnostic and stratification purposes when considering enrolling a subject in a clinical trial.2.2.Serum IgG4 concentrations can be elevated in patients with PSC, in the absence of classical features of IgG4-related disease, but IgG4 elevations are not validated markers of disease diagnosis, severity, prognosis, or treatment.3.Imaging3.1.The pattern of PSC can be defined according to the appearances of cholangiography using high-quality magnetic resonance cholangiography (MRC) (at least 1.5 T):3.1.1.Small duct when a recent cholangiogram (not older than 1 year) is normal3.1.2.Intrahepatic when there are intrahepatic changes but the common ducts and first-order ducts are normal3.1.3.Extrahepatic disease when the common ducts and/or the first-order ducts are involved4.Histology4.1.Histologic findings in a liver biopsy sample from patient suspected to have PSC can be defined (classified) as follows:4.1.1.Typical of PSC: features of fibrosing cholangiopathy present (periductal fibrosis, fibro-obliterative duct lesions)4.1.2.Compatible with PSC: features of chronic biliary disease present (see text for further details) without typical bile duct lesions4.1.3.Atypical for PSC: features atypical for PSC present. Examples include•unusually prominent inflammatory activity suggesting the possibility of PSC-AIH overlap syndrome•features suggesting an alternative diagnosis, such as fatty liver disease, or IgG4-associated cholangitis.4.2.A liver biopsy specimen showing typical or compatible histologic features is required to establish a diagnosis of small duct PSC.4.3.In a person with PSC, a liver biopsy specimen showing at least moderate interface hepatitis in addition to features compatible with or diagnostic of PSC is required to establish a diagnosis of PSC with features of AIH or PSC-AIH overlap syndrome.4.4.The histologic features that indicate disease progression (staging) in PSC include fibrosis, bile duct loss, and evidence of chronic cholestasis (demonstrated by deposition of copper-associated protein in periportal hepatocytes).5.Concurrent IBD5.1.The definition and description of IBD in PSC should follow the European Crohn’s and Colitis Organization (ECCO) guidelines and Montreal classification. Any patient with established PSC should undergo a full ileocolonoscopy with histology before concurrent IBD may be excluded.5.2.When assessing the existence of backwash ileitis, it is recommended that the ECCO definition be followed. This defines backwash ileitis as continuous extension of macroscopic or histologic inflammation from the cecum into the most distal ileum.6.Staging6.1.Disease staging of PSC is defined as the histologic progression from initial disease onset to cirrhosis.6.2.A clinically significant change in disease stage is defined by a 1-point change in the Ludwig or Nakanuma system or a 2-point change in the Ishak system.6.3.Histologic cirrhosis is a clinically significant endpoint.6.4.Cirrhosis (stage 5–6 by Ishak) in PSC can be defined by an LS of >14.4 kPa measured by VCTE.6.5.Cirrhosis (stage 5–6 or greater by Ishak) in PSC can be defined by an ELF score of >9.8 or FibroTest of >7.3.7.Clinical endpoints7.1.Liver transplant:Liver transplant is an appropriate endpoint in PSC clinical trials. However, divergent indications for transplant listing (end-stage disease, dysplasia, CCA, symptoms, intractable cholangitis) and method of transplantation (deceased vs living donation) should not be grouped into a single endpoint because of the wide heterogeneity of clinical practice.7.2.Liver-related death:Liver-related death in the setting of PSC is defined as death from clinically significant portal hypertension, synthetic liver dysfunction, or specific complications from biliary disease such as hepatobiliary infections.8.Cancer development8.1.Cholangiocarcinoma:8.1.1.CCA in PSC requires pathologic confirmation and is classified according to CCA in general.8.1.2.Dysplastic changes considered precursors to CCA in PSC are defined by standard criteria.8.2.Colorectal carcinoma:8.2.1.CRC in the setting of PSC is defined according to standard criteria.9.Pediatric PSC9.1.Pediatric PSC is not a specific disease phenotype. PSC in children should be described in terms of small vs large duct involvement, whether features of overlap with AIH are present or absent, and whether IBD is present or absent, as in adult patients.9.2.Terms such as juvenile sclerosing cholangitis and autoimmune sclerosing cholangitis should be avoided.9.3.When gGT is substituted for ALP, the consensus definitions in this document are applicable to children.10.Recurrent PSC10.1.The diagnosis of rPSC is based on the presence of compatible radiologic changes (using high-quality MR) with/or without histologic findings, along with the exclusion of other causes of similar findings. Open table in a new tab Tabled 11.Clinical symptoms1.1.Assessment of symptoms in patients with PSC for clinical trial purposes should be performed through disease-specific and validated quantitative tools.1.2.Assessment of symptoms in patients with PSC should take into consideration and be distinguished from symptoms attributable to active inflammatory bowel disease and other coexistent conditions.2.Laboratory markers2.1.The presence of significant elevations of serum aminotransferase activity (>5 × ULN) and/or IgG (>2 × ULN) can be indicative of features of coexisting autoimmune hepatitis and should trigger consideration of liver biopsy for diagnostic and stratification purposes when considering enrolling a subject in a clinical trial.2.2.Serum IgG4 concentrations can be elevated in patients with PSC, in the absence of classical features of IgG4-related disease, but IgG4 elevations are not validated markers of disease diagnosis, severity, prognosis, or treatment.3.Imaging3.1.The pattern of PSC can be defined according to the appearances of cholangiography using high-quality magnetic resonance cholangiography (MRC) (at least 1.5 T):3.1.1.Small duct when a recent cholangiogram (not older than 1 year) is normal3.1.2.Intrahepatic when there are intrahepatic changes but the common ducts and first-order ducts are normal3.1.3.Extrahepatic disease when the common ducts and/or the first-order ducts are involved4.Histology4.1.Histologic findings in a liver biopsy sample from patient suspected to have PSC can be defined (classified) as follows:4.1.1.Typical of PSC: features of fibrosing cholangiopathy present (periductal fibrosis, fibro-obliterative duct lesions)4.1.2.Compatible with PSC: features of chronic biliary disease present (see text for further details) without typical bile duct lesions4.1.3.Atypical for PSC: features atypical for PSC present. Examples include•unusually prominent inflammatory activity suggesting the possibility of PSC-AIH overlap syndrome•features suggesting an alternative diagnosis, such as fatty liver disease, or IgG4-associated cholangitis.4.2.A liver biopsy specimen showing typical or compatible histologic features is required to establish a diagnosis of small duct PSC.4.3.In a person with PSC, a liver biopsy specimen showing at least moderate interface hepatitis in addition to features compatible with or diagnostic of PSC is required to establish a diagnosis of PSC with features of AIH or PSC-AIH overlap syndrome.4.4.The histologic features that indicate disease progression (staging) in PSC include fibrosis, bile duct loss, and evidence of chronic cholestasis (demonstrated by deposition of copper-associated protein in periportal hepatocytes).5.Concurrent IBD5.1.The definition and description of IBD in PSC should follow the European Crohn’s and Colitis Organization (ECCO) guidelines and Montreal classification. Any patient with established PSC should undergo a full ileocolonoscopy with histology before concurrent IBD may be excluded.5.2.When assessing the existence of backwash ileitis, it is recommended that the ECCO definition be followed. This defines backwash ileitis as continuous extension of macroscopic or histologic inflammation from the cecum into the most distal ileum.6.Staging6.1.Disease staging of PSC is defined as the histologic progression from initial disease onset to cirrhosis.6.2.A clinically significant change in disease stage is defined by a 1-point change in the Ludwig or Nakanuma system or a 2-point change in the Ishak system.6.3.Histologic cirrhosis is a clinically significant endpoint.6.4.Cirrhosis (stage 5–6 by Ishak) in PSC can be defined by an LS of >14.4 kPa measured by VCTE.6.5.Cirrhosis (stage 5–6 or greater by Ishak) in PSC can be defined by an ELF score of >9.8 or FibroTest of >7.3.7.Clinical endpoints7.1.Liver transplant:Liver transplant is an appropriate endpoint in PSC clinical trials. However, divergent indications for transplant listing (end-stage disease, dysplasia, CCA, symptoms, intractable cholangitis) and method of transplantation (deceased vs living donation) should not be grouped into a single endpoint because of the wide heterogeneity of clinical practice.7.2.Liver-related death:Liver-related death in the setting of PSC is defined as death from clinically significant portal hypertension, synthetic liver dysfunction, or specific complications from biliary disease such as hepatobiliary infections.8.Cancer development8.1.Cholangiocarcinoma:8.1.1.CCA in PSC requires pathologic confirmation and is classified according to CCA in general.8.1.2.Dysplastic changes considered precursors to CCA in PSC are defined by standard criteria.8.2.Colorectal carcinoma:8.2.1.CRC in the setting of PSC is defined according to standard criteria.9.Pediatric PSC9.1.Pediatric PSC is not a specific disease phenotype. PSC in children should be described in terms of small vs large duct involvement, whether features of overlap with AIH are present or absent, and whether IBD is present or absent, as in adult patients.9.2.Terms such as juvenile sclerosing cholangitis and autoimmune sclerosing cholangitis should be avoided.9.3.When gGT is substituted for ALP, the consensus definitions in this document are applicable to children.10.Recurrent PSC10.1.The diagnosis of rPSC is based on the presence of compatible radiologic changes (using high-quality MR) with/or without histologic findings, along with the exclusion of other causes of similar findings. Open table in a new tab A particular challenge in assessing clinical symptoms in patients with PSC is the frequent concurrence of inflammatory bowel disease (IBD), which itself can influence the clinical course of PSC and may also account for a significant proportion of symptoms, especially during periods of IBD exacerbations.15Cheung A.C. Patel H. Meza-Cardona J. et al.Factors that influence health-related quality of life in patients with primary sclerosing cholangitis.Dig Dis Sci. 2016; 61: 1692-1699Crossref PubMed Scopus (1) Google Scholar Existing tools, including the PSC PRO, do not specifically account for symptoms attributable to IBD. Although the Simple Cholestatic Complaints Score did not show a difference in PSC patient symptoms according to the presence or absence of IBD, a systematic evaluation of IBD symptoms in patients with PSC has not been performed, which can result in confounding findings. Therefore, assessment of PSC symptoms should be performed alongside specifically designed tools to detect symptoms from IBD in those with PSC-IBD or other relevant coexistent conditions (Box 1, CS 1.2) This is critically important for clinical trials that aim to use PRO measures as endpoints in PSC. Cholestatic serum liver tests are a characteristic, but nonspecific, feature of biliary disease irrespective of the nature of biliary involvement. Alkaline phosphatase (ALP) as the hallmark of cholestasis is elevated in the vast majority of patients with PSC.16Poupon R. Liver alkaline phosphatase: a missing link between choleresis and biliary inflammation.Hepatology. 2015; 61: 2080-2090Crossref PubMed Scopus (52) Google Scholar Previously, it was thought that to make a diagnosis of PSC, elevation of ALP, or gamma-glutamyl transpeptidase (gGT) in children (see the “Pediatric Primary Sclerosing Cholangitis” section) was a prerequisite. Recent studies have shown that elevated cholestatic serum parameters need not always be present. For more background, see the Supplementary Material. Aminotransferases are often moderately elevated, whereas at diagnosis, serum bilirubin and albumin levels are usually normal.3Lindor K.D. Kowdley K.V. Harrison M.E. ACG clinical guideline: primary sclerosing cholangitis.Am J Gastroenterol. 2015; 110: 646-659Crossref PubMed Scopus (247) Google Scholar Aminotransferase activity of more than 5 times the upper limit of normal (ULN) and a serum IgG of >2 × ULN should raise the suspicion of coexisting autoimmune hepatitis (AIH) and should trigger consideration of liver biopsy to identify histologic features that would support a diagnosis of AIH or PSC/AIH overlap, also called variant syndrome (Box 1, CS 2.1). This has management consequences, because patients with PSC/AIH commonly respond favorably to immunosuppressive medication such as corticoids and thiopurines, although not as well as patients with isolated AIH.17Boberg K.M. Chapman R.W. Hirschfield G.M. et al.Overlap syndromes: the International Autoimmune Hepatitis Group (IAIHG) position statement on a controversial issue.J Hepatology. 2011; 54: 374-385Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar In contrast to primary biliary cholangitis (PBC) and AIH, there is no pattern of autoantibody reactivity in patients with PSC of relevance to diagnosis or treatment. Positive titers of autoantibodies are common in PSC, including antinuclear antibody (8%–77%), smooth muscle antibody (0%–83%), and atypical perinuclear antibody (26%–94%).18Prideaux L. De Cruz P. Ng S.C. et al.Serological antibodies in inflammatory bowel disease: a systematic review.Inflamm Bowel Dis. 2012; 18: 1340-1355Crossref PubMed Scopus (121) Google Scholar,19Moiseev S. Cohen Tervaert J.W. Arimura Y. et al.2020 International consensus on ANCA testing beyond systemic vasculitis.Autoimmun Rev. 2020; 19: 102618Crossref PubMed Scopus (18) Google Scholar However, these are also frequently present in AIH, and perinuclear antineutrophil cytoplasmic antibodies can be found in ulcerative colitis (UC) in 41%–73% and in Crohn’s disease in 6%–38%.17Boberg K.M. Chapman R.W. Hirschfield G.M. et al.Overlap syndromes: the International Autoimmune Hepatitis Group (IAIHG) position statement on a controversial issue.J Hepatology. 2011; 54: 374-385Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar, 18Prideaux L. De Cruz P. Ng S.C. et al.Serological antibodies in inflammatory bowel disease: a systematic review.Inflamm Bowel Dis. 2012; 18: 1340-1355Crossref PubMed Scopus (121) Google Scholar, 19Moiseev S. Cohen Tervaert J.W. Arimura Y. et al.2020 International consensus on ANCA testing beyond systemic vasculitis.Autoimmun Rev. 2020; 19: 102618Crossref PubMed Scopus (18) Google Scholar Serum IgG4 levels can be elevated in patients with PSC, but this has no validated bearing on diagnosis or disease severity (Box 1, CS 2.2). For distinction from IgG4-sclerosing cholangitis, see the Supplementary Material. Biliary imaging is integral to the diagnosis and classification of PSC. MRCP has been established as the noninvasive imaging of choice.3Lindor K.D. Kowdley K.V. Harrison M.E. ACG clinical guideline: primary sclerosing cholangitis.Am J Gastroenterol. 2015; 110: 646-659Crossref PubMed Scopus (247) Google Scholar,4European Association for the Study of the LiverEASL clinical practice guidelines: management of cholestatic liver diseases.J Hepatol. 2009; 51: 237-267Abstract Full Text Full Text PDF PubMed Scopus (1230) Google Scholar Regarding the utility of magnetic resonance imaging (MRI) in PSC, the reader is referred to a recent publication from the IPSCSG Working Group on MRI in PSC.20Schramm C. Eaton J. Ringe K.I. et al.Recommendations on the use of magnetic resonance imaging in PSC-A position statement from the International PSC Study Group.Hepatology. 2017; 66: 1675-1688Crossref PubMed Scopus (58) Google Scholar A range of cholangiographic features are described in large duct PSC, including multifocal stricturing, focal dilatation or ectasia, ductal wall thickening within the intra- and/or extrahepatic biliary tree, obliteration of the peripheral intrahepatic ducts with pruning of the biliary tree, retraction of the ampulla, and periductal inflammation. These changes are, however, not pathognomonic of PSC, and differentiation from causes of secondary sclerosing cholangitis (especially in the absence of colitis) can be difficult. Saccular outpouchings in the extrahepatic bile ducts on cholangiography are thought to be highly specific for PSC. These diverticular outpouchings in the common hepatic and common bile duct, and—very rarely—in the cystic duct, are usually seen only in advanced disease and are reported in only 7.5% of cases in a large series of ERCs.21Ponsioen C.Y. Vrouenraets S.M.E. Prawirodirdjo W. et al.Natural history of primary sclerosing cholangitis and prognostic value of cholangiography in a Dutch population.Gut. 2002; 51: 562-566Crossref PubMed Scopus (0) Google Scholar True small duct PSC where cholangiography is normal is an infrequent finding in PSC. Two recent publications have reported on the natural history of small cohorts of small duct PSC patients followed prospectively. Progression to large duct PSC was described in 33% and 55%, respectively, after a mean of 11 years of follow-up in each study22Kozaka K. Sheedy S.P. Eaton J.E. et al.Magnetic resonance imaging features of small-duct primary sclerosing cholangitis.Abdom Radiol (NY). 2020; 45: 2388-2399Crossref PubMed Scopus (2) Google Scholar,23Ringe K.I. Bergquist A. Lenzen H. et al.Clinical features and MRI progression of small duct primary sclerosing cholangitis (PSC).Eur J Radiol. 2020; 129: 109101Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar (Box 1, CS 3.1.1). There was no correlation with any baseline MRI findings and progression to large duct PSC in either study. Strictures in the draining bile ducts are a common and clinically relevant sequela of the chronic inflammatory process that takes place in the biliary tree in PSC. From a consecutive series of 96 large duct PSC patients followed up with regular ERCs, the incidence of the development of so-called dominant strictures can be inferred to amount to approximately 10% in patients with large duct PSC.24Stiehl A. Rudolph G. Klöters-Plachky P. et al.Development of dominant bile duct stenoses in patients with primary sclerosing cholangitis treated with ursodeoxycholic acid: outcome after endoscopic treatment.J Hepatol. 2002; 36: 151-156Abstract Full Text Full Text PDF PubMed Scopus (229) Google Scholar After extensive careful deliberations (outlined in the Supplementary Material), the panel delineated a number of criteria that coalesced into the working definition of a dominant stricture, as outlined in Box 2.Box 2Working Definition of Dominant StrictureTabled 1A stricture is called a possible dominant stricture when it meets the following criteria before ERC:•Imaging–High-quality MRCP or ERC with narrowing of any length in extrahepatic or first-order intrahepatic ducts, often—but not necessarily—with upstream dilatationTOGETHER WITH•Symptoms and biochemistry–Significant worsening of cholestatic symptoms within last 2 months:jaundice , biliary infection, pruritus, or RUQ pain–and–Bilirubin and/or ALP: absolute level of >1.2 × ULN and recent increase, for example, >1.2 × baseline25Gotthardt D.N. Rudolph G. Klöters-Plachky P. et al.Endoscopic dilation of dominant stenoses in primary sclerosing cholangitis: outcome after long-term treatment.Gastrointest Endosc. 2010; 71: 527-534Abstract Full Text Full Text PDF PubMed Scopus (89) Google ScholarOR•Biochemistry alone–Bilirubin and/or ALP: increase within past 6 months, for example, >1.5 × baseline, or absolute level of >2 × ULN if no previous measurement within 6 months is available26