Final answer:
The approximate viral concentration of a sample can be determined by the titer, which is a measure obtained through titration methods such as neutralization assays and hemagglutination assays to quantify viruses or antibodies in a sample.
Step-by-step explanation:
The approximate viral concentration of a sample may be determined by C) the titer. A titer is the concentration obtained by titration and is a measure of the amount of virus or the level of antibodies against the virus present in a sample. The neutralization assay is one such method where serial dilutions of patient serum are mixed with a known amount of virus. The titer in this case is defined as the reciprocal of the highest dilution showing a 50% reduction in plaques. For instance, if a 1/64 dilution gives a 50% reduction in plaques, the titer would be 64. In a hemagglutination assay (HA), a sample is serially diluted and mixed with red blood cells; the highest dilution that still causes agglutination represents the virus titer. Neutralization assays are sensitive and can quantify the level of virus-specific antibody by the decrease in hemagglutination after patient serum is mixed with the virus.