Diagnose This! (April 17, 2017)

What is this stain and what does this finding mean in a transplant patient (Hint: It’s not a vascular marker).

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This is a C4d immunoperoxidase stain showing diffuse positivity within the peritubular capillaries. While this can be a non-specific finding in ABO incompatible transplants, in ABO compatible transplants (the vast majority of transplants in the US) it suggests antibody-mediated rejection, especially when found in concert with other antibody-mediated changes such as microcirculation inflammation (glomerulitis, peritubular capillaritis), transplant glomerulopathy, peritubular capillary multilayering, and transplant arteriopathy.