Positive Congo red staining in tissue with apple-green birefringence under polarized light is diagnostic of the presence of amyloid fibrils. Image A shows a glomerulus with Congo red positive mesangial staining while image B shows a renal biopsy with diabetic nephropathy that is negative for Congo red. Accurate interpretation of this stain hinges upon proper optimization so that there is elimination of nonspecific background staining.
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