Final answer:
In a virus neutralization test, the titer is the highest serum dilution at which there is no viral activity due to the neutralizing effect of antibodies, measured by a 50% reduction in plaque formation or blocking of hemagglutination.
Step-by-step explanation:
The titer of a virus neutralization test is the highest dilution of patient serum resulting in no viral activity. This means that it is the dilution at which virus-specific antibodies present in the serum can neutralize the virus and prevent it from infecting host cells, which is observed as a 50% reduction in the formation of plaques (areas of infected cells) in a plaque reduction assay. In a hemagglutination inhibition assay (HIA), the titer is the highest dilution that blocks the agglutination of red blood cells by the virus, indicating the presence of neutralizing antibodies.