Comment "Plaque formation may not take place or cease at an early stage when the average number of IU which determines the spread of the infection is small. The concentration of serum to reduce the number of plaques by 50% compared to the serum free virus gives the measure of how much antibody is present or how effective it is. This measurement is denoted as the PRNT50 value.See also:- titre [ IATE:859230 ]"