fibrin ring granuloma fibrin ring granuloma a fibrin ring granuloma, also known as doughnut granuloma, is a histopathological finding that is characteristic of q fever. on hematoxylin-eosin staining, the fibrin ring granuloma consists of a central lipid vacuole (usually washed-out during fixing and staining, leaving only an empty hole) surrounded by a dense red fibrin ring and epithelioid macrophages. fibrin ring granulomas may also be seen in hodgkin's disease and infectious mononucleosis.