further evaluation of the stomach and small bowel by upper endoscopy and cross-sectional imaging is not needed unless there are other symptoms or findings to suggest proximal GI involvement or a diagnosis of Crohn's disease (CD) rather than UC. Definitions of disease severity are needed to guide treatment decisions; definitions should be based on (i) patient-reported outcomes (PROs) (bleeding and normalization of bowel habits), (ii) inflammatory burden (endoscopic assessment including extent and severity and markers of inflammation), (iii) disease course (need for hospitalization, need for steroids, and failure to respond to medications), and (iv) disease impact (functionality and QoL). Fecal calprotectin (FC) can be used in patients with UC as a noninvasive marker of disease activity and to assess response to therapy and relapse. Recommendations We recommend stool testing to rule out Clostridioides difficile (C. diff) in patients suspected of having UC (strong recommendation, very low quality of evidence). We recommend against serologic antibody testing to establish or rule out a diagnosis of UC (strong recommendation, very low quality of evidence). We recommend against serologic antibody testing to determine the prognosis of UC (strong recommendation, very low quality of evidence). Summary of evidence Symptoms of bloody diarrhea, mucous, urgency, tenesmus, and abdominal cramping should trigger consideration of a UC diagnosis, particularly in the absence of an alternate cause. A full clinical history should include assessment of severity of disease, triggers precipitating onset, and potential alternate etiologies. Symptoms assessed should include frequency of bowel movements, including number of nocturnal bowel movements. Assessment of bleeding should include the proportion of bowel movements that are mixed with blood. Other important symptoms to assess include urgency, abdominal pain, cramping, and weight loss, a marker of severity of disease. In addition, a thorough history should assess the presence of extraintestinal manifestations, including joint, skin, ocular, and oral manifestations, and symptoms suggesting hepatobiliary involvement. Potential precipitants of UC may include recent smoking cessation (14), nonsteroidal anti-inflammatory drug (NSAID) use (15,16), and enteric infections (17). C. diff infection (CDI) is increasingly recognized as complicating a significant proportion of patients with UC and is associated with an increased risk of hospitalizations, surgery, and even mortality (18,19). The prevalence of CDI among patients with newly diagnosed or relapsing IBD ranges from 5% to 47%. Concomitant CDI with UC has worse outcomes including higher mortality (20,21). Testing for C. diff is typically performed by polymerase chain reaction (PCR) or enzyme-linked immunosorbent assay (ELISA) and has been reviewed in recent guidelines (22). Other enteric infections that could mimic UC include infection with Escherichia coli (E. coli O157:H7), Salmonella, Shigella, Yersinia, and Campylobacter and parasitic infections such as amebiasis in the right clinical setting. Therefore, an infectious etiology should always be suspected and excluded at the time of diagnosis in the right clinical setting. Several institutions use comprehensive intestinal pathogen testing through PCR-based assays that include many bacterial and viral pathogens. The prevalence and impact of non–C. diff intestinal pathogens