5.1. Study Design and Participants

The present study was an ancillary analysis of the DRUGTOX study [31]. Briefly, the DRUGTOX study followed up 403 adult patients being monitored after kidney transplantation at Amiens University Medical Center (Amiens, France) and who underwent calcineurin inhibitor therapeutic drug monitoring between 4 August 2019, and 11 March 2020. The main inclusion criteria were age 18 or over, kidney transplantation, and available data on the patient’s prescription medications. Patients with acute graft rejection and/or who refused to participate were not included in the study.

For each patient, we recorded clinical data, laboratory data, the plasma concentrations of UTs (IxS, pCS, IAA, and TMAO), and prescriptions of drugs with OAT1/OAT3 inhibition activity at the time when the UT assay was performed.

The study was registered with the French National Data Commission (Commission Nationale de l’Informatique et des Libertés, Paris, France; registration number: PI2019_843_0060). In line with the French legislation on retrospective analyses of clinical practice, approval by an institutional review board was not required. However, patients were free to refuse to participate. The study was registered at ClinicalTrials.gov (15 July 2021) ({"type":"clinical-trial","attrs":{"text":"NCT04963673","term_id":"NCT04963673"}}NCT04963673).