BACKGROUND AND AIM: A definitive diagnosis of Crohn's disease (CD) or ulcerative colitis (UC) is not always possible, and a proportion of patients will be diagnosed as inflammatory bowel disease unclassified (IBDU). The aim of the study was to investigate the prognosis of patients initially diagnosed with IBDU and the disease course during the following 5 years. METHODS: The Epi-IBD study is a prospective population-based cohort of 1289 IBD patients diagnosed in centers across Europe. Clinical data were captured prospectively throughout the follow-up period. RESULTS: Overall, 476 (37%) patients were initially diagnosed with CD, 701 (54%) with UC, and 112 (9%) with IBDU. During follow-up, 28 (25%) IBDU patients were changed diagnoses to either UC (n = 20, 71%) or CD (n = 8, 29%) after a median of 6 months (interquartile range: 4-12), while 84 (7% of the total cohort) remained IBDU. A total of 17 (15%) IBDU patients were hospitalized for their IBD during follow-up, while 8 (7%) patients underwent surgery. Most surgeries (n = 6, 75%) were performed on patients whose diagnosis was later changed to UC; three of these colectomies led to a definitive diagnosis of UC. Most patients (n = 107, 96%) received 5-aminosalicylic acid, while 11 (10%) patients received biologicals, of whom five remained classified as IBDU. CONCLUSIONS: In a population-based inception cohort, 7% of IBD patients were not given a definitive diagnosis of IBD after 5 years of follow-up. One in four patients with IBDU eventually was classified as CD or UC. Overall, the disease course and medication burden in IBDU patients were mild.

1st Department of Medicine Semmelweis University Budapest Hungary

Clinic of Gastroenterology University of Medicine 'Victor Babes' Timisoara Romania

Department of Biomedicine Institute of Pharmacology Faculty of Medicine of Porto University Porto Portugal

Department of Gastroenterology and Alimentary Tract Surgery Tampere University Hospital Tampere Finland

Department of Gastroenterology and Hepatology Soroka Medical Center and Ben Gurion University of the Negev Beer Sheva Israel

Department of Gastroenterology Centro Hospitalar de São João EPE Porto Portugal

Department of Gastroenterology Faculty of Medicine and Health Örebro University Örebro Sweden

Department of Gastroenterology Herlev and Gentofte Hospital University of Copenhagen Copenhagen Denmark

Department of Gastroenterology Hospital Alvaro Cunqueiro Instituto Investigación Sanitaria Galicia Sur EOXI de Vigo Vigo Spain

Department of Gastroenterology Hospital POVISA Vigo Spain

Department of Gastroenterology Medical Academy Lithuanian University of Health Sciences Kaunas Lithuania

Department of Gastroenterology Moscow Regional Research Clinical Institute Moscow Russian Federation

Department of Gastroenterology North Zealand Hospital University of Copenhagen Copenhagen Denmark

Department of Gastroenterology State University of Medicine and Pharmacy of the Republic of Moldova Chisinau Republic of Moldova

Department of Gastroenterology University Hospital of Ioannina Ioannina Greece

Department of Medicine Herning Central Hospital Herning Denmark

Department of Surgical Oncological and Gastroenterological Sciences Azienda University of Padua Padua Italy

Division of Gastroenterology and Hepatology University Hospital Center Zagreb Zagreb Croatia

Division of Gastroenterology Mater Dei Hospital Msida Malta

Division of Gastroenterology McGill University Health Center Montreal Canada

Division of Gastroenterology Tartu University Hospital University of Tarty Tartu Estonia

Focused Research Unit for Molecular Diagnostic and Clinical Research IRS Center Sonderjylland Hospital of Southern Jutland Aabenraa Denmark

Gastroenterology Department Odense University Hospital Odense Denmark

Gastroenterology Department Slagelse Hospital Slagelse Denmark

Gastroenterology Unit Epimad Registry CHU Amiens Sud Avenue Laennec Salouel Amiens University Hospital Amiens France

IBD Clinical and Research Centre ISCARE Prague Czech Republic

IBD Department St Marks Hospital Imperial College London London UK

IBD Unit Hull and East Yorkshire NHS Trust Hull UK

Institute for Digestive Research Medical Academy Lithuanian University of Health Sciences Kaunas Lithuania

Institute of Molecular Medicine University of Southern Denmark Odense Denmark

Institute of Pharmacology 1st Faculty of Medicine Charles University Prague Czech Republic

Lille Inflammation Research International Center LIRIC Lille University Lille France

Medical Department Regional Hospital of Viborg Viborg Denmark

Medical Department The National Hospital of the Faroe Islands Torshavn Faroe Islands

Nicosia Private Practice Nicosia Cyprus

Public Health Epidemiology and Economic Health Registre Epimad Lille University and Hospital Lille France

U O Gastroenterologia ed Endoscopia digestiva Hospital Morgagni Pierantoni Forlì Italy

University of Tampere Tampere Finland

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