What is the primary method used in Enzyme-Linked Immunosorbent Assay (ELISA) to detect the presence of an analyte?

The primary method used in the Enzyme-Linked Immunosorbent Assay (ELISA) to detect the presence of an analyte is a color change upon substrate reaction. In this technique, an enzyme linked to an antibody catalyzes a reaction with a substrate to produce a colored product. The intensity of the color produced is directly proportional to the amount of analyte present in the sample, allowing for quantitative measurement.

This colorimetric detection is highly sensitive and provides a clear visual indication of the presence or concentration of the target molecule. Because the color change can be easily measured using a spectrophotometer, it enables precise quantification, making ELISA a widely used method in immunology, laboratory diagnostics, and various research applications.

Other methods listed, such as fluorescence measurement, involve different principles and are not the primary detection strategy employed in traditional ELISA formats. Similarly, pH level detection and magnetic particle interaction are not relevant to the fundamental mechanism of ELISA, which revolves around the enzyme-substrate reaction facilitating color development.