What is the principle of a competitive immunoassay used in initial drug screening?

What is the principle of a competitive immunoassay used in initial drug screening?

• A. Analyte in the sample competes with a labeled analog for antibody binding; higher analyte concentration reduces signal; results compared to cutoff.
• B. Analyte binds to antibodies only if present in the sample; higher concentration increases signal.
• C. Antibody captures the antigen robustly regardless of concentration; the signal is constant.
• D. The assay uses a sandwich format with two antibodies binding the analyte.

Answer: (A)

In a competitive immunoassay for drug screening, the analyte in the sample competes with a labeled version of the same drug for a limited antibody binding site. If the sample has little or no drug, the labeled analog binds most of the antibodies, producing a strong signal. If the sample contains more drug, it occupies more antibody sites, reducing the binding of the labeled analog and thus lowering the signal. The measured signal is then compared to a predefined cutoff to decide positive or negative for the drug. This inverse relationship—more drug leads to less signal—is the hallmark of competitive formats and suits small-molecule drug detection where a sandwich format isn’t feasible.

In a competitive immunoassay for drug screening, the analyte in the sample competes with a labeled version of the same drug for a limited antibody binding site. If the sample has little or no drug, the labeled analog binds most of the antibodies, producing a strong signal. If the sample contains more drug, it occupies more antibody sites, reducing the binding of the labeled analog and thus lowering the signal. The measured signal is then compared to a predefined cutoff to decide positive or negative for the drug. This inverse relationship—more drug leads to less signal—is the hallmark of competitive formats and suits small-molecule drug detection where a sandwich format isn’t feasible.