Which type of assay uses a solid phase to capture an antigen with a subsequent labeled detector antibody?

• A. Competitive binding assay
• B. Sandwich (capture) enzyme assay
• C. Direct binding assay
• D. Chemiluminescent assay

Answer: (B)

A sandwich assay uses a solid-phase antibody to capture the antigen, then a second, labeled detector antibody binds to a different epitope on that same antigen, forming a “sandwich.” The detector antibody carries a label (often an enzyme) that produces a measurable signal, allowing quantification of the antigen. This format requires two antibodies and high specificity, which is why it’s ideal for accurately detecting and measuring antigens in samples—the antigen is captured on the solid surface and then revealed by the labeled detector antibody.

The other approaches don’t fit this setup. Competitive binding relies on antigen competing with a labeled component for a limited antibody, giving a signal inversely related to antigen amount and not a sandwich of two antibodies. Direct binding uses a single antibody to detect the antigen, with no second antibody forming a sandwich. Chemiluminescent describes the detection method (light produced), not the specific sandwich capture format.