In a homogeneous enzyme immunoassay, which statement is true?

• A. Enzyme activity is altered when binding to antibody occurs.
• B. Enzyme label is on the antibody.
• C. Enzyme activity is directly proportional to the amount of patient antigen present.
• D. Most enzymes can be used in this type of assay.

Answer: (C)

In a homogeneous enzyme immunoassay, the readout comes from enzyme activity produced in solution without any separation step. The test is designed so that the amount of patient antigen drives the formation of antigen–antibody complexes that carry the enzyme, so more antigen means more enzyme-bearing complexes and a stronger enzymatic signal. That direct relationship between antigen amount and enzyme activity is what makes the readout directly proportional to the amount of patient antigen present. While enzyme activity can sometimes be influenced by binding interactions in other formats, this configuration yields a straightforward, proportional readout to antigen concentration.