The presence of IgM antibody, usually along with IgG antibody, suggests current or recent infection. Equivocal index values indicate marginal antibody levels. A second sample should be submitted 2-3 weeks after the first. The most sensitive indicator of an acute infection is an increasing IgM or IgG index in two serum specimens bracketing the illness and tested at the same time. Appearance of an IgM antibody response normally occurs 7-14 days after the onset of disease.
Testing immediately post-exposure is of no value without a later convalescent specimen.
While the presence of IgM antibodies suggest current or recent infection, low levels of IgM antibodies may occasionally persist for more than 12 months post-infection or immunization. Such a residual IgM response may be distinguished from early IgM response to infection by testing sera from patients 3-4 weeks later for changing levels of specific IgM antibodies. In selected instances, due to suppressed or delayed immune response, serum antibody levels may not be a reliable means for establishing a firm diagnosis of human parvovirus infection. Therefore, antibody titers may not be the optimal way to diagnose PV infection in these patients. Since PV cannot be cultivated in routine clinical virology laboratory cell cultures and since antigen or DNA probe detection tests are not commercially available, the best method in these situations is detection of PV DNA in serum by PCR amplification and gel electrophoresis.