Background: The rate of estimated glomerular filtration rate (eGFR) improvement in the early post renal transplant period and its effect on long-term graft function have not been well studied in pediatrics. This study aims to describe the rate of eGFR improvement and assess its impact on long-term outcomes.
Methods: A retrospective analysis of pediatric kidney transplantations in a tertiary children's hospital between 2005 and 2018 was performed. Rate of eGFR improvement was recorded and classified as immediate or slow graft function (SGF) based on several definitions explored. Delayed graft function (DGF) was defined as the need for dialysis in the first week post-transplant. The association between eGFR improvement, and graft loss at last follow-up (LFU) was examined via multivariate logistic regression and Cox regression analysis.
Results: A hundred and thirty-two pediatric kidney transplantations were enrolled in the study with a mean follow-up time of 8.48±3.45 years. Sixty-seven (50.7%) patients received kidneys from a living donor (LD). LD kidneys reached maximal eGFR by day 7 followed by an average decrease of 33.9 ml/min/1.73m² before stabilization of eGFR by 3 months post-transplant. Kidneys from deceased donors (DD) showed constant eGFR improvement until week 3 post-transplant with a subsequent stabilization. Mean eGFR at 3-months and 1-year post-transplant were similar between LD and DD. Ten patients (7.5%) experienced DGF, 9 of whom received kidneys from DD. At last follow-up (LFU) 25 (18.9%) patients experienced graft loss. DGF duration was longer in patients who lost their graft at LFU compared to patients who did not lose their graft (1.4±0.9 vs. 19±27 days p=0.027). Multivariate logistic regression analysis identified delayed graft function (OR=4.9, 95% CI 1.1-21.3, p=0.035), acute rejection (OR=4.7, 95% CI 1.8-12.5, p=0.002), and urologic pathology (OR=2.83, 95% CI 1-7.6, p=0.039) as significant predictors of graft loss on LFU. No definition of SGF was associated with reduced graft survival. On Cox survival analysis, only acute rejection remained a significant risk factor for graft loss (HR 2.98, 95% CI 1.3-7, p=0.012). When excluding patients with acute rejection, DGF (HR 7.6, 95% CI 1.4-42.6, p=0.021), urologic pathology (HR 4.6, 95% CI 1.3-16.9, P=0.021), and ethnicity (HR 3.8, 95% CI 1.2-12.5, p=0.027) were found to be risk factors for graft loss.  
Conclusion: Rejection is the main factor affecting long term graft survival in pediatric kidney transplantation. DGF also has an effect on long term graft survival. Rate of eGFR improvement at the early post-transplant period is significantly different between LD and DD, but does not appear to have an effect on graft survival. Future studies are required to determine the reasons for eGFR decline early post-transplant in pediatric LD recipients and whether preventing it can improve long term graft outcome.