Which of the following is a homogeneous immunoassay where separation of the bound from the free labeled species is not required?

• A. Radioimmunoassay
• B. Enzyme-linked immunosorbent assay
• C. Immunoradiometric assay
• D. Enzyme-multiplied immunoassay technique

Answer: (D)

Homogeneous immunoassays do not require a separation step to distinguish bound from free labeled components; the signal is produced directly in solution as the binding event alters the label’s activity. Enzyme-multiplied immunoassay technique fits this approach because it uses an enzyme-labeled molecule whose activity is modulated by binding. When the analyte is present, it competes for the antibody or changes the enzyme’s ability to act on its substrate, so the measured signal changes without needing to separate bound from free species. In contrast, the other methods rely on separating bound from free components—such as washing away unbound label or precipitating complexes—so they are heterogeneous assays. Therefore, this option exemplifies a homogeneous immunoassay where separation is not required.