Elevated IgG antibody levels.
Elevated IgG antibody levels can suggest, but not prove active, chronic infection. In a 1996 study of HHV-6 in CFS patients, 89% of the patients with IgG titers of 1:320 and above were found to have
active infections by culture.
Stanford infectious disease specialist Jose Montoya believes that the best evidence of smoldering central nervous system (CNS) infection is the IgG antibody to the virus, and not the virus itself. In a pilot study, he found that when patients with high titers of HHV-6 IgG (1:320, 1:640 or higher) and EBV are treated with a potent antiviral; their titers fall substantially along with a significant improvement in symptoms.
Elevated IgG Antibodies to HHV-6 cannot tell you for certain that the infection is active, but high titers support a clinical diagnosis.
Similarly, elevated EBV VCA (late antibody) titers cannot indicate with certainly that an infection is active. However, EBV Early Antigen (EA) antibodies disappear rapidly after an infection is over, so elevated EBV EA antibodies do predict active infection.
Ref:
http://www.hhv-6foundation.org/research/hhv-6-testing