Aims: Magnetic resonance imaging (MRI) using an arterial spin labelling technique uses protons from dissociated water molecules as an endogenous tracer.  Tracking the movement of protons through a series of images allows estimation of renal blood flow (RBF).  Measuring haematocrit and serum creatinine simultaneously, enables estimation of renal plasma flow (RPF), glomerular filtration rate (GFR) and filtration fraction.  The latter represents the proportion of plasma that is filtered at the glomerulus.  Haemodynamic changes and a lack of autoregulation may result in significant changes to the filtration fraction after kidney transplantation.
Methods: We aimed to measure filtration fraction in a prospective cohort of fourteen paediatric patients who received a kidney transplant for end-stage kidney disease.  The cohort had standard of care follow-up monitoring as well as an additional renal allograft MRI scan at 2 weeks and 2, 6, 9, 12, and 36 months after kidney transplantation.
Results: Strong negative correlation was observed between serum creatinine and filtration fraction (r = -0.81 (95% CI -0.88, -0.72) p < 0.0001).  No significant difference was detected in serum creatinine, RBF, RPF or filtration fraction between patients.  The median RBF and serum creatinine remained stable throughout the follow-up period.  The median filtration fraction was significantly higher in children post-transplantation compared to normal native kidneys.  A higher filtration fraction was associated with higher estimated glomerular filtration rate and improved renal allograft function.  No significant difference was detected in filtration fraction in patients who experienced rejection of their transplant during follow-up versus those who did not.
Conclusions: The estimated glomerular filtration rate was higher per ml/min of renal plasma flow in children with lower serum creatinine.  This may represent glomerular hyperfiltration due to altered haemodynamics.  It is unclear whether such glomerular hyperfiltration has adverse effects on long-term renal allograft outcomes.