What causes a 'hook effect' in immunoassays and how can it be mitigated?

• A. Low antigen levels; concentrate sample.
• B. Extremely high antigen levels saturate antibodies, yielding falsely low results; mitigated by sample dilution and alternative assay formats.
• C. Interference from heterophile antibodies; use blocking reagents.
• D. Cross-reactivity with similar antigens; use a more specific antibody.

Answer: (B)

In immunoassays, the hook (or prozone) effect occurs when the antigen level is extremely high. That abundance saturates both the capture and detection antibodies, preventing the proper antibody–antigen–antibody sandwich from forming. Because the sandwich isn’t assembled correctly, the signal drops and the result appears falsely low despite a high real antigen concentration.

To mitigate this, dilute the sample to bring the antigen concentration into the assay’s linear range and re-measure; performing serial dilutions can reveal the true level if the signal increases with dilution. Another approach is to use a different assay format or antibodies with different binding characteristics that are less prone to this effect, helping maintain accurate readings at high antigen concentrations.