Final answer:
A serum titer involves performing a serial dilution of a serum sample and determining the highest serum dilution that still provides a measurable effect, such as the prevention of viral replication or agglutination, to quantify the level of antibodies or virus.
Step-by-step explanation:
A serum titer specifically involves Serial Dilution of Serum Sample (option a) and determining the Highest Dilution of Serum that still gives a measurable response, such as the inhibition of viral replication or agglutination (option b). In the context of the information provided, the highest dilution of patient serum at which there is no detectable viral DNA, or where agglutination is blocked, is the titer. For instance, if a titer measures the level of antibodies present in the serum, it would be the highest serum dilution that blocks agglutination (as mentioned in Figure 20.20). The titer is expressed as a whole number and represents the reciprocal of the highest dilution that accomplishes this. The method applied is often used to estimate the quantity of a virus in cell culture or to track the immune response to an infection over time.