In the ELISA test, the principle involves antigen-antibody interaction detected through an enzyme-linked secondary antibody. In a competitive ELISA, if the patient sample contains a HIGH concentration of the target antigen, the absorbance reading will be:

• A High — more antigen means more antibody-antigen complex and stronger enzyme signal
• B Low — in competitive ELISA, patient antigen competes with labelled antigen for limited antibody binding sites; more patient antigen = less labelled antigen bound = lower signal ✓
• C Unchanged — competitive ELISA signal is independent of analyte concentration
• D Variable — competitive ELISA is only valid for antibody detection, not antigen quantification

Explanation

Competitive (inhibition) ELISA works by competition: a fixed amount of enzyme-labelled antigen and the patient's unlabelled antigen compete for a limited number of capture antibody sites. When patient antigen concentration is HIGH, it occupies more binding sites, displacing the labelled antigen — resulting in LESS labelled antigen bound to the antibody-coated plate — and therefore LOWER absorbance (low signal = high concentration). This is the inverse relationship characteristic of competitive ELISA, contrasting with the direct (sandwich) ELISA where higher antigen → higher signal.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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