In assay terminology, what describes the Endpoint measurement?

• A. Reaction is stopped and measured
• B. Continuous measurement as a function of time
• C. Measures at several intervals
• D. Initial rate

Answer: (A)

Endpoint measurement means you stop the reaction at a defined time and read the final signal. This approach yields a single data point that reflects how much product has formed by that moment, which is then related to the analyte concentration. It differs from continuous or real-time monitoring, where the signal is tracked as it changes over time to produce a full curve; it also differs from taking readings at several intervals or calculating the initial rate, which relies on information from early time points rather than a single end-point reading. In many colorimetric assays, for example, you halt the reaction and read the absorbance to obtain a stable end-point signal that correlates with the amount of product produced.