Which immunoassay for thyroxine testing corresponds to a microparticle EIA with an ELISA sandwich format?

• A. Fluorescence polarization immunoassay
• B. Fluorescent substrate-labeled inhibition immunoassay
• C. Chemiluminescent microparticle immunoassay
• D. Microparticle EIA (ELISA sandwich)

Answer: (D)

A microparticle EIA using an ELISA sandwich format captures the thyroxine (T4) molecule between two antibodies on a solid phase: one antibody is bound to a microparticle, and a second enzyme-labeled antibody binds to another site on the same molecule. When substrate is added, the enzyme produces a color change that is measured spectrophotometrically. This setup—microparticle solid phase with a two-antibody (sandwich) arrangement and colorimetric readout—matches the described format precisely.

Other approaches differ in format or detection: fluorescence polarization immunoassay uses a fluorescent signal and a competitive mechanism, not a sandwich ELISA; fluorescent substrate-labeled inhibition immunoassay is a competitive inhibition approach; chemiluminescent microparticle immunoassay uses microparticles but detects via chemiluminescence rather than a colorimetric ELISA readout.