*Purpose: Simultaneous heart-kidney (SHK) transplantation has superior post-transplant survival compared to heart transplantation alone in end-stage heart failure patients who have dialysis dependent or non-dialysis dependent renal insufficiency. Renal allograft insufficiency following SHK is often presumed to be due to calcineurin inhibitor (CNI) toxicity but the use of CNI free regimen has not improved renal function. In this study, we reviewed the transplant kidney biopsies of SHK recipients with renal allograft insufficiency to understand the spectrum of kidney histopathology.

*Methods: Clinical transplant kidney biopsies of SHK patients at the University of Chicago Medical Center (2003-2018) were review by experienced renal pathologists.

*Results: Thirty-six SHKs were performed between 2003 and 2018. 15 of the 36 had kidney transplant biopsies for renal insufficiency. Median biopsy time from SHK was 4.5 months. Histopathology of the biopsies was noted as follows (Table 1): 5(33%) showed acute rejection, 3(20%) showed glomerular diseases (1 focal segmental glomerulosclerosis, 2 membranous nephropathy), 3(20%) showed polyomavirus nephropathy, 2(13%) showed thrombotic microangiopathy, 1(7%) showed isometric vacuolization of proximal tubules consistent with CNI toxicity and acute rejection, and 1(7%) showed medullary angiitis.

*Conclusions: Post SHK renal insufficiency has a varied histopathological spectrum. Renal biopsy should be considered in the event of renal insufficiency to guide further management. Our findings indicate that CNI toxicity should not be presumed.

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