Varicella-zoster plaque assay and plaque reduction neutralization test by the immunoperoxidase technique

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Abstract
A new plaque assay for the quantitation of varicella-zoster virus and a plaque reduction neutralization test for the determination of neutralizing antibody titer were developed using the indirect immunoperoxidase technique. As compared with the classical plaque assay using a solid overlay, the test gives earlier results since plaque counting can be performed on day 3 after the inoculation of cell cultures. In 6 patients with zoster infection, neutralizing antibody titers ranged from 1:20 to 1:40 before the onset of infection and reached high levels (1:320-1:5120) during the convalescent phase of the disease. Complement fixing (CF) titers were all negative (< 1:8) in prezoster serum samples from the same patients and ranged from 1:128 to 1:2048 in the convalescent-phase sera. In the 2 cases in which late serum samples were available, neutralizing antibody titers matched the pre-illness levels; CF titers dropped to undetectable levels. Neither neutralizing nor CF antibody was detected in 2 sera from individuals with no history of varicella-zoster infection. No differences in virus titers or neutralizing antibody titers were observed between the immunoperoxidase and the classical plaque assays. The appropriate characterization of reagent specificity is required before routine application of the test.