Abstract Background Antibiotic use is a risk factor for CDI. Few studies have correlated use of prior antibiotics with CDI severity in cancer patients. This study identified clinical and microbiology risk factors associated with severe CDI in patients with cancer. We hypothesized that previous antibiotic exposure and microbiome composition at time of CDI presentation, are risk factors for severe disease in cancer patients. Methods This non-interventional, prospective, single-center cohort study examined patients with cancer who had their first episode or first recurrence of CDI between Oct 27, 2016 and Jul 1, 2019. C. difficile was identified using nucleic acid amplification testing. Multivariate analysis was used to determine significant clinical risk factors for severe CDI as defined in the 2018 IDSA/SHEA guidelines. Alpha, and beta diversities were calculated to measure the average species diversity and the overall microbial composition. Differential abundance analysis and progressive permutation analysis were used to single out the significant microbial features that differed across CDI severity levels. Results Patient (n=200) demographics show mean age of 60 yrs., 53% female, majority White (76%) and non-Hispanic (85%). Prior 90 day metronidazole use (Odds Ratio OR 4.68 [1.47-14.91] p0.009) was a significant risk factor for severe CDI. Other factors included Horn’s Index > 2 (OR 7.75 [1.05-57.35] p0.045), Leukocytosis (OR 1.29 [1.16-1.43] p< 0.001), Neutropenia (OR 6.01 [1.34-26.89] p0.019) and Serum Creatinine >0.95 mg/dL (OR 25.30 [8.08-79.17] p< 0.001). Overall, there were no significant differences in alpha and beta diversity between severity levels. However, when identifying individual microbial features, the high presence of Bacteroides uniformis, Ruminococceae, Citrobacter koseri and Salmonella were associated with protection from severe CDI (p< 0.05). Table 1 - Results of multivariate logistic regression analysis of factors associated with severe CDI Figure 1. Microbiome features identified by progressive permutation analysis as seen in a volcano plot. Conclusion A number of risk factors for severe CDI were identified among this population, including prior 90 day metronidazole use. Also, increased relative abundance of Bacteroides uniformis, Ruminococceae, Citrobacter koseri and Salmonella were linked to protection from severe CDI. Reducing metronidazole use in patients with cancer may help prevent subsequent severe CDI. Disclosures Adilene Olvera, MPH MLS (ASCP), MERK (Grant/Research Support, Scientific Research Study Investigator) Kevin W. Garey, PharmD, MS, FASHP, Merck & Co. (Grant/Research Support, Scientific Research Study Investigator) Ryan J. Dillon, MSc, Merck & Co., Inc., (Employee) Engels N. Obi, PhD, Merck & Co. (Employee)