821 Background: CPI therapy has expanded rapidly in recent years and represents a major advancement in the treatment of many cancers, including hepatocellular carcinoma, gastric cancer, and colon cancer. However, these therapies are associated with significant toxicities. CPI colitis is one of the most common toxicities and can be fatal, especially when not diagnosed and treated promptly. The current gold standard for diagnosis is endoscopy with biopsy, an invasive procedure that is resource- and time-intensive. CT has emerged as a possible alternative. The primary objective of this study is to identify the diagnostic performance of CT in the evaluation of CPI colitis. Methods: With IRB approval, we conducted a retrospective cohort study of patients who received CPI therapy between 2009-2019 across a single healthcare system. Patients were included if they underwent both abdominal CT and upper/lower endoscopy with biopsy within 72 hours of each other. We reviewed the electronic medical record to identify possible cases of colitis based on either CT or pathology. All cases were labeled as either true positive or false positive based on pathology. We examined clinical characteristics, including CTCAE grade and treatment received. We calculated the sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of CT for diagnosing CPI colitis when compared to the gold standard of tissue diagnosis. Results: Of the 4,474 patients screened, 141 met inclusion criteria. Average age was 63 years (23 – 91); 43% were male. Most common tumor types were melanoma (36%) and NSCLC (20%). Seventy-four percent of patients were treated with anti-PD-1/PD-L1 monotherapy. Forty percent had signs of colitis on CT scan and 59% had biopsy-proven CPI colitis. Sensitivity and specificity of CT were 51% and 74%, respectively. PPV of CT was 74% and NPV was 51%. Of those with confirmed CPI colitis, 78% had symptoms that were classified as grade 3 or above. Seventy-three percent received IV steroids and 38% received infliximab. Conclusions: CT demonstrates moderate specificity and PPV and remains an important diagnostic test but does not replace endoscopy/biopsy in the evaluation of CPI colitis.