In fluorescence polarization immunoassay, polarization is inversely related to the analyte concentration.

• A. Directly proportional to concentration
• B. Inverse relationship to concentration
• C. No relation to concentration
• D. Exponential relationship to concentration

Answer: (B)

In fluorescence polarization immunoassay, the measured polarization depends on how fast the fluorescent tracer tumbles in solution. A tracer bound to a large antibody rotates slowly, giving a high polarization signal. When analyte concentration increases in a competitive format, it competes with the tracer for the antibody, so more tracer remains unbound and free in solution. Free tracer tumbles quickly, producing a lower polarization signal. Therefore, as the analyte concentration goes up, the polarization goes down—inverse relationship. The other patterns (direct, none, or exponential) don’t describe this competitive FP format for small molecules.