Here is the design of a clinical trial:

Official Title: INtravenous Contrast Computed Tomography Versus Native Computed Tomography in Patients With Acute Abdomen and Impaired Renal functiOn (INCARO) - a Multicentre, Open-label, Randomised Controlled Trial

Included conditions:
- Acute Abdomen
- Radiocontrast Nephropathy

Study Armgroups:
- {'label': 'Abdominal or body CT with intravenous contrast', 'type': 'EXPERIMENTAL', 'description': 'Abdominal or body CT with intravenous contrast', 'interventionNames': ['Diagnostic Test: Abdominal or body CT with intravenous contrast']}
- {'label': 'Abdominal or body CT without intravenous contrast (native CT)', 'type': 'ACTIVE_COMPARATOR', 'description': 'Abdominal or body CT without intravenous contrast (native CT)', 'interventionNames': ['Diagnostic Test: Abdominal or body CT without intravenous contrast (native CT)']}

Interventions:
- {'type': 'DIAGNOSTIC_TEST', 'name': 'Abdominal or body CT with intravenous contrast', 'description': 'Abdominal or body CT with intravenous contrast', 'armGroupLabels': ['Abdominal or body CT with intravenous contrast']}
- {'type': 'DIAGNOSTIC_TEST', 'name': 'Abdominal or body CT without intravenous contrast (native CT)', 'description': 'Abdominal or body CT without intravenous contrast (native CT)', 'armGroupLabels': ['Abdominal or body CT without intravenous contrast (native CT)']}

Primary Outcomes:
- {'measure': 'Mortality or renal replacement therapy', 'description': 'A composite outcome that combines all-cause mortality or renal replacement therapy (number of patients)', 'timeFrame': 'Within 90 days from CT'}

Please estimate the sample size based on the assumption: 
The study is designed with 90% power and a 5% significance level. Additionally, a 5% crossover rate from the native CT group to the intravenous contrast CT group is assumed.
Sample size calculation
        A previous study conducted at the Helsinki University Hospital on patients with diffuse peritonitis showed a 90-day overall mortality of 22%. In patients with eGFR less than 45âmL/min/1.73âm2, the mortality was 39%. Patients with any perforation of the gastrointestinal (GI) tract had an overall mortality of 13%, whereas patients with eGFR less than 45âmL/min/1.73âm2 had a mortality of 25%.16 The documented incidence of AKI in hospitalised patients is 1.9%, and in ICU patients, the incidence exceeds 40%.17 A recent meta-analysis of post-CT AKI showed an incidence of AKI of 7.17% in contrast CT group and 7.42% in native CT group. In contrast and non-contrast groups, RRT was necessary in 0.56% and 0.68% of patients, respectively.8 The incidence of AKI after major abdominal surgery is 13.4% and rates of postoperative RRT vary from 0% to 3% between studies.18 After general surgery procedure, the incidence of AKI is 1%, and the post-surgery RRT rate is 0.68%.19 Risk factors for postoperative AKI include emergency surgery, intraperitoneal surgery, and mild or moderate renal insufficiency,19 all leading to a higher than 1% rate of AKI and RRT.
        All patients included in this trial will not have a GI tract perforation. Some patients might not have abdominal pathology, although it was suspected when ordering the CT scan. Thus, the mortality rate in the trial cohort will likely be lower than 25%. On the other hand, a composite outcome comprising all-cause mortality, RRT and renal transplantation will likely increase the rate of the primary outcome. We estimate that the primary outcome rate is 27% in the native CT group and 9âpercentage points less (ie, 18%) in the intravenous contrast CT group. With 90% power and 5% alpha, the study requires 896 patients to show the difference. We estimate that 5% of patients randomised to native CT group will undergo intravenous contrast CT (cross-over from native CT to intravenous contrast CT group). Taking this into account, the final adjusted sample size will be 994.