Competitive vs Non-Competitive Format
Definition
Competitive: labelled antigen and sample antigen compete for a limited number of antibody sites; signal is inversely proportional to analyte. Non-competitive (sandwich): excess antibody captures all analyte and a second labelled antibody reports; signal is directly proportional.
- Competitive signal
- Inversely proportional to analyte
- Non-competitive signal
- Directly proportional to analyte (sandwich format)
- Competitive use case
- Small molecules, single epitope analytes
- Non-competitive use case
- Larger analytes with multiple epitopes
Common questions
Why does analyte size determine which format is chosen?+
A sandwich, non-competitive format needs the analyte to present two distinct epitopes so a capture antibody and a separate detection antibody can bind simultaneously, which large proteins can do but small molecules typically cannot, forcing the choice of a competitive format for those small analytes.
Which format generally gives better sensitivity at low analyte concentrations?+
Non-competitive sandwich formats generally give better sensitivity and a wider dynamic range at low concentrations, because the signal is generated by an added labelled antibody proportional to captured analyte, whereas competitive formats work by signal loss, which is harder to resolve precisely near zero.
Related terms
- Antibody
- An immunoglobulin (typically IgG) raised to bind a specific antigen. Source decides specificity: polyclonal antibodies recognise many epitopes, monoclonal antibodies recognise one.
- Antigen
- A molecule recognised by the adaptive immune system through specific binding to an antibody or T-cell receptor. Defined by immunogenicity (capacity to...
- ELISA
- Enzyme-Linked Immunosorbent Assay. Heterogeneous, plate-based, enzyme label (HRP, alkaline phosphatase), colour or chemiluminescent readout. Four standard formats: direct, indirect, sandwich, competitive.
- Epitope and Hapten
- Epitope is the small surface patch on an antigen the antibody actually binds. A hapten is a small molecule (most drugs of...
- Homogeneous vs Heterogeneous
- Homogeneous assays (EMIT, FPIA) need no wash step, so they run fast on clinical analysers. Heterogeneous assays (ELISA, RIA) require a wash...
- Label
- The reporter coupled to either the antigen or the antibody so the binding event becomes measurable. Three families dominate: radioisotopes (RIA), enzymes...
- Lateral-Flow Immunoassay
- A point-of-care competitive immunochromatographic format on a nitrocellulose strip. In the absence of drug, colloidal gold-labelled antibody is captured at a test...
- RIA
- Radioimmunoassay. Berson and Yalow, 1959. Heterogeneous, competitive, radioisotope label (most often 125-I). Highest sensitivity historically, now displaced in most Indian labs by...