Which method characterizes third-generation TSH assays?

• A. Radioimmunoassay
• B. Immunoassay
• C. Chemiluminescent microparticle immunoassay
• D. Enzyme-linked immunosorbent assay

Answer: (C)

Third-generation TSH assays are defined by using a chemiluminescent detection method on microparticle immunoassay technology. In this approach, capture antibodies are attached to magnetic or other microparticles and bind TSH from the patient’s serum. A second antibody labeled with a chemiluminescent tag then binds the TSH, forming a sandwich. After washing, triggering the chemiluminescent reaction produces light, and the instrument measures the light output, which is proportional to the TSH concentration over a wide range. This setup offers very high sensitivity and specificity, aided by monoclonal antibodies and automated processing, allowing detection of very low TSH levels.

Radioimmunoassay relies on radioactive labels and older technology, which is now largely obsolete for routine TSH testing. ELISA describes a broad colorimetric or sometimes chemiluminescent readout format and isn’t the defining characteristic of the modern third-generation platform. Immunoassay is a general term; what distinguishes third-generation TSH assays is the chemiluminescent microparticle immunoassay approach with high sensitivity and automation.