Background: Although uncomplicated acute diverticulitis has a benign disease course, some patients are at increased risk for complications. Identification of these patients may aid the selection of treatment strategies such as outpatient treatment. This study aimed to assess the rate and timing of a complicated disease course in initially computed tomography (CT)-proven uncomplicated diverticulitis, and to identify risk factors for the development of these diverticular complications. Patients and Methods: Computed tomography-proven, left-sided uncomplicated diverticulitis patients from two cohorts were included. Main outcome measure was complicated diverticulitis (perforation, abscess, obstruction, or fistula) within three months after presentation. Risk factors for diverticular complications were identified using multivariable logistic regression. Results: Of the 1,087 patients with initially CT-proven uncomplicated diverticulitis, 4.9% (53/1,087) developed complicated diverticulitis. Most perforations and abscesses (16/21) occurred during the first 10 days, whereas colonic obstruction and fistula occurred during three months of follow-up. Independent risk factors for the transition from uncomplicated to complicated diverticulitis were American Society of Anesthesiologists (ASA) classification 3/4 (odds ratio [OR] 4.43, 95% confidence interval [CI] 1.57-12.48), duration of symptoms before presentation longer than five days (OR 3.25, 95% CI 1.72-6.13), vomiting (OR 3.94, 95% CI 1.96-7.92), and C-reactive protein (CRP) above 140 mg/L (OR 2.86, 95% CI 1.51-5.43). Conclusion: Approximately one in 20 patients with CT-proven uncomplicated diverticulitis develops a complicated disease course within three months; perforation and abscess occur predominantly within 10 days after presentation. Patients with systemic comorbidity, symptoms for more than five days, those who vomit, or have high CRP levels at presentation are at risk for diverticular complications after an uncomplicated initial presentation and may warrant closer observation.

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