The Epstein Barr Viral Capsid Antigen - IgG test is used for the diagnosis of Epstein Barr Viral (EBV), also known as human herpesvirus 4, infection by measuring the Epstein Barr Viral Capsid Antigen - IgG antibody levels in the blood. This test can help distinguish whether someone is susceptible to EBV infection or has a recent or past infection.