A diagnostic mystery solved by electron microscopy: a case of an "atypical" lymphoproliferative disorder

An elderly woman with a previous diagnosis of atypical chronic lymphocytic leukemia (CLL) was noted to have a strikingly abnormal blood film, with the lymphocytes displaying numerous large cytoplasmic granules. This appearance had not been described before in the literature to the best of the authors' knowledge. After a series of investigations, electron microscopy was eventually performed, which demonstrated that the abnormal granules were composed of immunoglobulin crystals. The immunofixation study confirmed that they were monoclonal IgM paraprotein. These results led to a change of diagnosis to lymphoplasmacytic lymphoma. This report illustrates how electron microscopy can be used as a valuable additional diagnostic tool in difficult cases.