Which immunoassay for thyroxine testing is indirectly proportional?

• A. Fluorescence polarization immunoassay
• B. Fluorescent substrate-labeled inhibition immunoassay
• C. Chemiluminescent microparticle immunoassay
• D. Enzyme immunoassay

Answer: (A)

An indirect (inverse) relationship between the signal and the analyte concentration is typical for competitive immunoassays used with small molecules like thyroxine. In fluorescence polarization immunoassay, a fluorescently labeled thyroxine tracer binds to an antibody, and when bound, the tracer rotates slowly, producing a high polarization signal. As thyroxine from the sample increases, it competes with the tracer for antibody binding, displacing more tracer. The displaced tracer is free to rotate rapidly, lowering the polarization signal. So, higher thyroxine concentration yields a lower measured signal, demonstrating an inverse relationship. This readout characteristic is the most direct way to see the indirect proportionality among the options.