Comparison of the neutralizing-antibody titers obtained by cPNT and pPNT in different age groups. The distributions of neutralizing-antibody titers obtained by cPNT and pPNT in the indicated age groups are shown. The numbers of serum samples examined for each age group are shown above the graphs in parentheses. The threshold values for pPNT were 20%, 2.5%, and 10% for type 1, 2, and 3 PV, respectively.