Review ArticlesMeasuring Histologic Activity in Inflammatory Bowel Disease: Why and HowPai, Reetesh K. MD*; Lauwers, Gregory Y. MD†; Pai, Rish K. MD, PhD‡ Author Information *Department of Pathology, University of Pittsburgh Medical Center, Pittsburgh, PA †Department of Pathology, Moffitt Cancer Center, Tampa, FL ‡Department of Laboratory Medicine and Pathology, Mayo Clinic Arizona, Scottsdale, AZ The authors have no funding or conflicts of interest to disclose. All figures can be viewed online in color at www.anatomicpathology.com. Reprints: Rish K. Pai, MD, PhD, Department of Laboratory Medicine and Pathology, Mayo Clinic Arizona, 13400 East Shea Boulevard, Scottsdale, AZ 85253 (e-mail: [email protected]). Advances In Anatomic Pathology 29(1):p 37-47, January 2022. | DOI: 10.1097/PAP.0000000000000326 Buy Metrics Abstract Histology is used to confirm the diagnosis of inflammatory bowel disease, exclude superimposed infections, and to evaluate for dysplasia. Histology has rarely been used to measure disease activity and guide therapy despite evidence that histologic measurements have value in predicting important clinical outcomes. More recently, there have been numerous studies supporting a role for histologic disease activity measurements in predicting a variety of outcomes including relapse, hospitalizations, steroid use, and dysplasia. The histologic assessment was superior to endoscopic measurements in many of these studies. This review will summarize the recent literature regarding histologic disease activity measurements in ulcerative colitis and Crohn disease. A detailed description of histologic scoring systems will also be provided to provide pathologists with the necessary tools to accurately measure disease activity. Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.