The time dependency of renal allograft histology

Much of the complexity of the histological appearance of kidney transplant biopsies depends on the time at which the biopsies are obtained. It is well established that many elementary histological lesions and diagnoses have a time-dependent occurrence. While some "active" inflammatory lesions are noted primarily early after transplantation, other lesions are "chronic" and accumulate over time post-transplant, sometimes closely related to the prior active inflammatory lesions. With time after transplantation, the complexity of histology increases, by the co-occurrence of chronic damage and specific diseases. This leads to difficulties in clinical interpretation of the histological picture. We discuss the time-dependent prevalence of active and chronic lesions in kidney allograft biopsies and their associations with outcome. We also elaborate on the importance of time post-transplant in the interpretation of complex histological lesions or mixed diagnoses and illustrate that further research is necessary to evaluate whether time post-transplant is important for prognostication of graft injury processes. Adding a multidimensional prognostic layer to the current diagnostic Banff classification, including graft functional characteristics and time after transplantation, could become an interesting aid in the interpretation of complex histological lesions and mixed diagnoses, and in therapeutic decision-making.