Infectious mononucleosis. Diagnosis by in situ hybridization in two cases with atypical features.

Two cases of infectious mononucleosis are reported in which in situ hybridization studies were of use to distinguish this disorder from non-Hodgkin's lymphoma. One patient was an 80-year-old man with a tonsillar mass that histologically resembled non-Hodgkin's lymphoma and, on fixed tissue immunohistochemistry, appeared to contain a population of cells anomalously coexpressing the B-cell marker L26 and the T-cell marker Leu-22, suggesting diffuse large-cell non-Hodgkin's lymphoma. The second patient was a 43-year-old woman with inguinal lymphadenopathy that, on histologic examination, also mimicked diffuse large-cell lymphoma. In situ hybridization studies for Epstein-Barr virus revealed both cases to possess EBV DNA in a pattern characteristic of infectious mononucleosis. In addition, in situ hybridization studies for immunoglobulin light-chain mRNA demonstrated a polyclonal pattern of kappa and lambda mRNA expression. This report demonstrates the utility of the in situ hybridization technique as an adjunct to routine diagnosis.