Question

What is the end point of a titer, and how is it reported? Think about the tubes involved.

A) The end point is the highest dilution showing a reaction, and it is reported as a reciprocal value.
B) The end point is the lowest dilution showing a reaction, and it is reported as a direct value.
C) The end point is the midpoint of the dilution series, and it is reported as an average.
D) The end point is the dilution where the reaction is most intense, and it is reported as an absolute value.

Answer 1

The correct option is A. The end point of a titration is the point at which the indicator changes color, marking the completion of the reaction. In serological tests, it is the highest dilution showing a reaction and is reported as the reciprocal of the dilution factor as a whole number.

Step-by-step explanation:

The endpoint of a titration is identified as the point when the indicator changes color, signaling completion of the reaction between the titrant and the analyte. This is observed, for example, when phenolphthalein changes to a faint pink color. However, the slightly different equivalence point is the theoretical point when equivalent quantities of reactants have reacted. It is important that the endpoint closely matches the equivalence point for accurate titration results.

In serological tests such as a plaque reduction assay, the titer is defined by the highest dilution of serum that shows a reaction, such as a 50% reduction in plaques when titrating viral antibodies. It is reported as the reciprocal of the dilution factor, as a whole number. To illustrate, if a 1/64 dilution is the highest that shows the required level of reaction, the reported titer would be 64.