Intravascular eosinophilic deposits-when common knowledge is insufficient to render a diagnosis.

In the course of daily sign-out, the diagnoses within a histopathologist's armamentarium are limited by the scope of the histopathologist's knowledge, that is, one cannot diagnose what one does not know. The subject of homogeneous intravascular eosinophilic deposits is used to illustrate this point. A histopathologist unaware that a tick bite reaction can induce intravascular eosinophilic deposits may misdiagnose the specimen as representing a manifestation of cryoglobulinemia. Furthermore, conventional teaching imparts that monoclonal cryoglobulinemia shows intravascular eosinophilic deposits (cryoprecipitates) histopathologically, whereas mixed cryoglobulinemia is histopathologically manifested as leukocytoclastic vasculitis. Although it is not well known, this is not always the case because mixed cryoglobulinemia may histopathologically present itself as intravascular eosinophilic deposits without leukocytoclastic vasculitis. In addition, it is not common knowledge that intravascular cryoprecipitates, when present, may be associated with an increased number of blood vessels. Examples of these phenomena are presented in conjunction with a discussion of relevant issues/lessons learned from such cases.