What type of assay is used for the quantification of proteins in a sample?

The enzyme-linked immunosorbent assay, commonly known as ELISA, is designed specifically for the quantification of proteins in a sample. This assay utilizes antibodies that are specific to the proteins of interest, allowing for the measurement of their concentration within a mixture. The principle behind ELISA involves the binding of the target protein to a solid surface followed by the application of an enzyme-linked antibody. Upon the addition of a substrate for the enzyme, a detectable signal is produced, typically resulting in a color change that correlates with the amount of protein present.

In contrast, flow cytometry is primarily used for cell counting and analysis, utilizing fluorescence to measure and analyze physical and chemical characteristics of cells or particles as they flow in a fluid stream. Polymerase chain reaction (PCR) is a technique used to amplify DNA sequences and is not focused on protein quantification. Gel electrophoresis is used for separating macromolecules like DNA, RNA, or proteins based on their size and charge, but it does not provide quantitative data directly; additional steps must be taken to quantify proteins after separation. Thus, ELISA stands out as the most appropriate method for protein quantification in this context.