Background: The transition from adolescence to young adult is a vulnerable period for the renal transplant patient. We aimed to identify the prevalence of non-adherence leading to graft loss among adolescents and young adults and analyze the influence of transfer from pediatric to adult health care on transplant outcomes. 
Methods: Retrospective cohort analysis of all renal transplantations in the period 2000-2020 in Norway. Inclusion criteria were standard immunological risk recipients (i.e., no donor specific antibodies (DSA)/panel reactive antibodies (PRA) <20%) engrafted <50 years of age with functioning graft > 6 months post-engraftment. The cohort was divided in two; recipients engrafted <26 years of age compared with recipients engrafted 26-50 years of age. Focus was set on graft loss during the transition phase, here defined as age 14.0 -25.9. The outcomes (graft loss/biopsy verified rejection/development of de novo DSA) in the group <26 years were registered if occurred during the transition phase. Data were retrieved from the Norwegian Renal Registry (NRR) and hospital charts.
Results: We identified 1832 kidney recipients engrafted <50 years; 373 (20%) were engrafted <26 years (64% male, 68% Tx with living donor, median age at engraftment 16.8 years) vs. 1459 engrafted >26 years (63% male, 43.5% with living donor, median age at engraftment 40.5 years). There were in total 300 (16%) graft-losses during the study period (80 Tx <26 years vs. 220 Tx >26 years). During the transition phase, 58% (21/36) grafts were lost due to verified medication non-adherence, vs. 12% (27/220) in the group engrafted >26 years of age (p<0.001). Of the 36 grafts lost in the transition phase, 29 (81%) were lost after transfer to the adult service, i.e., >18 years, with a mean age at graft loss of 21.3 ±3.1 years. Seventy-nine % of the rejections in the transition phase appeared after transfer to the adult service and de novo DSA corresponding results were 84%. In the group engrafted <26 years of age, the overall mean death censored graft survival time was 16.0 (95% CI 15.2-16.8) years vs. 17.2 (95% CI 16.8-17.6) years in the group engrafted >26 years of age (p=0.006).
Conclusions: Non-adherence was verified as main cause for kidney graft loss in the transition phase, responsible for 58% of the graft losses with 81% of the grafts lost after the transfer to adult service. Improved knowledge of non-adherence in the transition phase can contribute to reduce the risk of adverse health outcomes in this patient group.