Competitive ELISA is useful when the analyte is small, has limited antibody-accessible epitopes, or when only one suitable antibody is available. Unlike direct, indirect and sandwich formats, the signal is typically inversely related to analyte concentration: more target in the sample produces less assay signal.
Competitive ELISA can be configured in several orientations. The workflow below describes an antigen-down competitive ELISA, where plate-bound antigen competes with sample antigen for a limited amount of antibody. Antibody-down assays are also valid and may be preferable depending on which reagents can be conjugated reliably.
Protocol at a glance
| Best suited to | Small analytes, single-epitope targets, antibody or antigen competition studies and assays where a sandwich pair is unavailable. |
|---|---|
| Signal relationship | Usually inverse: more sample analyte produces less signal. |
| Core optimisation | Competitor concentration, antibody concentration, matrix matching and standard-curve range. |
| Analysis | Use an appropriate non-linear inhibition curve; do not interpret inverse OD linearly across the full range. |
| Critical warning | Different competitive ELISA orientations are not interchangeable; define exactly what is immobilised and what competes. |
Equipment and reagents
- High-binding ELISA plate
- Fixed/competitor antigen suitable for plate coating
- Target-specific antibody
- Enzyme-conjugated primary antibody or enzyme-linked secondary detection system
- Standards prepared from the target analyte
- Sample matrix-matched diluent where possible
- Blocking and wash buffers
- Substrate and stop solution
- Plate reader
Assay design
- Use a standard curve that spans the expected sample range.
- Keep the antibody concentration constant across standards and unknowns.
- Where small molecules do not adsorb efficiently to the plate, a carrier conjugate may be required for the plate-bound competitor.
- Optimise the amount of plate-bound competitor and antibody together; the most sensitive region is where competition produces a measurable change without saturating the assay.
Antigen-down competitive ELISA procedure
- Coat the plate with fixed antigen. Dilute the coating antigen or carrier conjugate in coating buffer and add approximately 100 µL per well. Incubate overnight at 4°C or use another validated coating condition.
- Wash three times.
- Block the plate. Add blocking buffer and incubate for approximately 1–2 hours at room temperature.
- Prepare standards and unknowns. Use a dilution buffer that minimises matrix differences between standards and samples. Include a zero standard containing no competing analyte.
- Prepare the antibody mixture. Add the same concentration of target-specific antibody to every standard and sample. Depending on the assay design, pre-incubate sample/standard with antibody before transferring to the plate, or add them together to the coated wells.
- Allow competition. Incubate under the validated assay conditions so sample antigen and plate-bound antigen compete for the limited antibody.
- Wash thoroughly. Remove unbound sample components and antibody.
- Add detection reagent if required. If the competing antibody is not directly enzyme-conjugated, add an appropriate enzyme-linked secondary antibody and incubate under validated conditions, then wash again.
- Add substrate. Develop the plate consistently across all standards and samples.
- Stop and read. For HRP/TMB, stopped absorbance is commonly measured at 450 nm.
Expected relationship
The zero standard should give the highest specific signal because the antibody is free to bind the plate-associated competitor. As analyte concentration increases, less antibody remains available for the plate and the signal falls. Fit the standard curve using an appropriate non-linear model such as a four- or five-parameter logistic model when validated for the assay.
Controls
- Zero standard: no competing analyte; expected to give high signal.
- High standard: high target concentration; expected to give low signal.
- Matrix blank: evaluates non-specific matrix signal.
- No-antibody or no-detection control: evaluates substrate and plate background.
- Known positive sample: monitors run-to-run assay performance.
Recommendations and warnings
- Define the competition architecture before optimisation. Antigen-down, antibody-down, direct competitive and indirect competitive formats can generate different background and sensitivity profiles.
- Matrix-match standards and samples where possible. Serum, plasma, culture medium and extraction buffers can alter apparent competition.
- Do not assume the zero standard is a blank. In many competitive ELISAs the zero-analyte well gives the maximum signal.
- Use enough standards around the IC50 region. The central, steep portion of the curve carries most of the quantitative information.
- Assess cross-reactivity deliberately for small molecules. Structurally related compounds can compete even when conventional specificity controls look clean.
Curve fitting and reporting
Competitive assays are generally best handled with a four- or five-parameter logistic model after confirming monotonic inhibition. Report the validated working range rather than the entire fitted curve. Samples above or below the working range should be repeated at an appropriate dilution or concentration rather than extrapolated.
Limitations
- The inverse signal relationship makes optimisation and interpretation less intuitive than a sandwich ELISA.
- Small differences in OD at the low-concentration end can produce relatively large concentration differences.
- Cross-reactivity with structurally related analytes can reduce specificity.
- Matrix effects can change apparent competition unless standards and samples are well matched.
Troubleshooting
| Problem | Possible cause | What to try |
|---|---|---|
| Little separation between standards | Antibody or fixed-antigen concentration not in a sensitive range | Titrate both reagents and adjust competition/incubation conditions. |
| High background | Poor blocking or detection reagent too concentrated | Optimise blocker, washes and detection-reagent concentration. |
| Non-monotonic curve | Pipetting error, inconsistent matrix or saturation | Prepare fresh serial standards, match matrix and reduce reagent concentrations. |
| Low zero-standard signal | Insufficient plate coating or inactive antibody/detection system | Verify coating, antibody activity and detection chemistry. |
| Poor recovery in real samples | Matrix interference | Test sample dilution, spike recovery and matrix-matched standards. |
Frequently asked questions
Why does signal decrease as analyte concentration increases?
Sample analyte competes with the immobilised or labelled assay component for a limited amount of antibody. More sample analyte therefore leaves less detector bound in the signal-generating complex.
When is competitive ELISA preferable to sandwich ELISA?
It is especially useful for small molecules or targets with only one accessible epitope, and where a matched capture/detection antibody pair is unavailable.
What should the zero standard look like?
In a conventional inhibition assay, the zero-analyte standard commonly gives the highest specific signal because there is no soluble analyte competing with the plate-associated assay component.
Why is spike recovery important?
It reveals matrix effects that can alter apparent competition and therefore bias the calculated sample concentration.
Related ABMIUM protocols
References and primary methods
- Engvall E, Jonsson K, Perlmann P. Enzyme-linked immunosorbent assay II: quantitative assay of protein antigen using enzyme-labelled antigen and antibody-coated tubes. Biochim Biophys Acta. 1971.
- Kohl TO, Ascoli CA. Direct Competitive Enzyme-Linked Immunosorbent Assay (ELISA). Cold Spring Harb Protoc. 2017.
- Kohl TO, Ascoli CA. Indirect Competitive Enzyme-Linked Immunosorbent Assay (ELISA). Cold Spring Harb Protoc. 2017.
- Hock BD et al. Development of an ELISA-based competitive binding assay for analysis of drug concentration and antidrug antibody levels. Ther Drug Monit. 2016.
Relevant ABMIUM products
Competitive ELISA reagent needs depend on the assay orientation. In an indirect competitive design using mouse or rabbit primary antibody, ABMIUM carries HRP secondary options such as ABM2976 Goat anti-Mouse IgG-HRP and ABM2983 Goat anti-Rabbit IgG-HRP. These are not automatically suitable for every competition format; confirm the assay architecture first. For target-specific antigen or antibody sourcing, use Find your MATCH.
Explore more with ABMIUM
Competitive formats are highly assay-specific. Use Find your MATCH if you need help sourcing the antigen, antibody or detection pair, or compare the Direct, Indirect and Sandwich ELISA protocols before choosing the format.