BackgroundAcute interstitial nephritis (AIN) is an important and common cause of acute renal failure. There are no generally accepted guidelines for the treatment of AIN, due to the lack of prospective randomized trials. Since AIN is characterized by an enhanced immune response, immunosuppressive treatment could potentially improve prognosis by attenuating inflammation and subsequent fibrosis. Despite the limited evidence of effects of steroids and potential adverse effects, prednisolone is frequently used in the treatment of AIN and there is a strong need for clinical trials on the effects of immunosuppression, including steroids, in the treatment of AIN. We aimed to evaluate the effectiveness of prednisolone treatment in AIN, and hypothesized a positive effect of prednisolone treatment on renal function in AIN.MethodsThe study is a randomized, controlled, prospective, open label multicenter study, including incident adult patients with biopsy proven AIN. Patients will be randomized 1:1 to one of 2 treatment regimens: No prednisolone treatment (control group) andB) Oral prednisolone treatment staring with 60 mg daily tapered over 8 weeks.One hundred ten patients (55 in each group) are planned to be included and followed for 1 year. Primary outcome is renal function estimated by eGFR 3 months after inclusion. Secondary outcomes are renal function after 12 months and need for renal replacement therapy and quality of life after 3 and 12 months. In addition, with-in prednisolone group analysis are performed to estimate the importance of treatment delay. Exploratory analyses include analysis of biomarkers in urine and plasma and the evaluation of these biomarkers in relation to renal prognosis and re-evaluation of renal biopsies to identify possible renal prognostic factors.DiscussionStrengths and possible limitations in the design are evaluated. The study will provide important information on the effects of prednisolone treatment in AIN and as well as prognostic information relevant for future use of biomarkers and histology. Ultimately, this would lead to improved and evidence based clinical guidelines for the treatment of AIN.Trial registrationClinicalTrials.gov identifier NCT04376216 (Retrospectively registered on May 6, 2020).