Infectious mononucleosis (IM) may be suspected clinically but it is confirmed serologically. The heterophil antibody test, however, may be negative in up to 15% of adults and 35 to 40% of children. Because IM is caused by Epstein-Barr virus (EBV), a specific test for viral capsid antigen (VCA) of EBV is indicated when heterophil tests are negative. Acute IM is characterized by a VCA-IgM titer, and no VCA-IgG antibody or EBNA. Chronic EBV disease causes elevated VCA-IgG titers as well as high EA antibody titers.