Introduction
Kidney transplantation is the optimal treatment for most patients with chronic renal failure (1). However, the long term success of kidney transplantation is far from optimal (2). In 2017, 10-year all-cause graft failure was 49.7% for deceased donor kidney recipients and 34.1% for living donor kidney transplants (3). Immunological rejection, a major cause of graft failure, is driven by attack of the graft by T cells (T cell mediated rejection, or TCMR) or antibodies (antibody mediated rejection, or ABMR), or in some cases a combination of these two mechanisms (mixed rejection). A key early contributor to long-term graft loss is subclinical immune injury that leads to chronic damage of the renal allograft (4-8). Until recently there have been no commercially available fully validated non-invasive tests to monitor patients with stable renal function for silent rejection (9). As a result, a significant number of centers rely on surveillance (protocol) biopsies to detect early silent rejection, whereas other centers who choose not to perform these wait for clinical evidence of graft injury and damage (10, 11).