The H&E photomicrographs show acute tubular injury with eosinophilic ‘beaded’ casts (arrow). Immunohistochemical stains were strongly positive hemoglobin in the casts (pictured here) while myoglobin was completely negative. These findings are consistent with hemoglobin cast nephropathy and indicate the presence of intravascular hemolysis. The morphologic lesions present in hemoglobin cast nephropathy are indistinguishable from myoglobin cast nephropathy without immunohistochemical analysis. It is important when staining for suspected hemoglobin cast nephropathy to stain for both hemoglobin and myoglobin as positive hemoglobin staining in a myoglobin cast is commonly present and considered nonspecific. However, the converse is not true and hemoglobin casts will not stain positive for myoglobin. This patient ultimately proved to have Evans syndrome, an autoimmune disease with antibodies directed against RBCs and platelets.
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