Which immunoassay type is heterogeneous and uses microparticles in a solid phase with enzyme-labeled antibodies?

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

Answer: (A)

The concept being tested is how a heterogeneous immunoassay uses a solid support to separate bound from free reactants, specifically when microparticles provide that solid phase for enzyme-based detection. In Microparticle Enzyme Immunoassay, tiny beads carry the capture component on their surface; after binding the target, an enzyme-labeled antibody binds, and after washing, a substrate produces a measurable signal. The solid-phase on the microparticles requires washing to remove unbound material, defining it as heterogeneous. This distinguishes it from homogeneous formats like EMIT, where no separation step is needed, and from fluorescence polarization methods that rely on optical signals rather than enzyme labels. ELISA is also a solid-phase immunoassay, but it uses a microplate well rather than microparticles, so the explicit microparticle solid phase is the key identifying feature here.