Competitive ELISA
Definition
An ELISA format in which sample antigen competes with a fixed quantity of labelled antigen for a limited number of antibody binding sites. High sample antigen concentration reduces signal; the relationship between concentration and signal is inverse. Used for small molecules (haptens) such as drugs that lack two distinct epitopes.
- ELISA format
- Competitive
- Signal relationship
- Inversely proportional to sample antigen
- Best suited for
- Small molecules (haptens) such as drugs
- Reason
- Haptens lack two distinct epitopes for a sandwich format
Common questions
Why can't a sandwich ELISA be used for small drug molecules instead?+
A sandwich format needs two antibodies binding two separate, non-overlapping epitopes on the same molecule at once, which small haptens are generally too small to present, so a competitive format, needing only one binding site, is used instead for drugs and other low-molecular-weight analytes.
How is a competitive ELISA result distinguished from a false negative?+
A run-specific standard curve and negative and positive controls are included so a low signal, indicating high analyte, can be distinguished from a genuinely negative sample or an assay failure such as reagent degradation, since both would otherwise look similarly high in signal.
Related terms
- Cross-Reactivity
- The capacity of an antibody raised against one analyte to bind structurally related compounds. In RIA, cross-reactivity is the main driver of...
- Enzyme Conjugate
- An antibody (or antigen) chemically linked to a reporter enzyme, most commonly horseradish peroxidase (HRP) or alkaline phosphatase (AP). The enzyme catalyses...
- Sandwich ELISA
- An ELISA format in which a capture antibody immobilised on the plate binds one epitope of the target antigen, and a separate...
- Solid Phase
- The inert surface to which an antigen or antibody is adsorbed at the start of an ELISA. Usually the well walls of...
- Substrate
- The compound converted by the reporter enzyme into a coloured product. For HRP, 3,3',5,5'-tetramethylbenzidine (TMB) is the standard substrate, producing a blue...