Which immunoassay type is homogeneous and relies on enzyme activity proportional to the analyte concentration?

• A. Enzyme Multiplied Immunoassay Technique
• B. Microparticle Enzyme Immunoassay
• C. Fluorescence Polarization Immunoassay
• D. Enzyme-Linked Immunosorbent Assay

Answer: (A)

In a homogeneous immunoassay, all reactions occur in solution without a washing or separation step, so the signal comes from what happens in the liquid phase rather than from a bound–unbound fraction on a solid surface. The enzyme-based homogeneous approach uses the enzyme’s activity as the readout, and that activity is modulated by antibody binding in the presence of the analyte.

Enzyme Multiplied Immunoassay Technique fits this exactly. It uses an enzyme-labeled version of the analyte and an antibody. The analyte in the sample competes with the enzyme-labeled analyte for antibody binding. When more analyte is present, more antibody binds the free analyte and less binds the enzyme-labeled analog, leaving more active enzyme in solution. This results in greater enzyme-catalyzed product formation, so the measured signal increases proportionally with the analyte concentration. No separation step is needed, so it’s a homogeneous assay.

Fluorescence polarization immunoassay relies on changes in fluorescence polarization, not enzyme activity. ELISA requires a solid phase and washing steps (heterogeneous). Microparticle enzyme immunoassay involves particles and typically a separation step as well.