Overview
The Enzyme Linked Immunosorbent Assay (ELISA), also called Enzyme Immuno Assay (EIA), is a rapid, widely used test for detecting and quantifying antibodies or antigens against viruses, bacteria, and other materials. It is central to the diagnosis of infectious, autoimmune, and other diseases, and is performed on a solid matrix (typically 96-well polystyrene microtitre plates).
Before ELISA, Radioimmunoassay (RIA), developed by Rosalyn Yalow and Solomon Berson in 1960, used radioactively labelled antigens/antibodies for detection. Because radioactivity posed health risks, enzyme-based reporters (like peroxidase) were substituted, leading to the development of ELISA/EIA by Perlmann, Engvall, Schuurs, and van Weemen, published independently in 1971.
Learning Objectives
After this lesson you will be able to…- Define ELISA/EIA and describe its history
- Discuss the general process and principle of ELISA
- Describe the various types of ELISA (indirect, sandwich, competitive, antigen/antibody capture)
- Describe the method of each ELISA type using HIV testing as an example
- Explain internal quality control practices for ELISA
- List the applications of ELISA in medicine, food industry, and toxicology
Clinical Story
Why This MattersA pregnant woman comes for routine antenatal screening. Her blood sample is tested for HIV antibodies using an indirect ELISA — a rapid, sensitive method that will determine whether she needs immediate counselling and antiretroviral prophylaxis to prevent mother-to-child transmission. The result must be accurate, reproducible, and delivered quickly.
Core Concepts
Antigen is fixed to the solid phase, allowing specific antibodies in the patient's serum to bind. These bound antibodies are then detected by an enzyme-labelled anti-human immunoglobulin and a substrate system, producing a colour change. Used mainly to detect antibody in a sample (e.g. HIV antibody testing).
A "capture" antibody is bound to the solid phase. Antigen in the sample binds this antibody; a labelled "detection" antibody then binds the antigen, completing the sandwich. Used to determine antigen concentration in unknown samples with high accuracy.
Antibodies in the test specimen compete with enzyme-conjugated antibodies in the reagent for binding to the antigen on the solid phase. If the specimen contains the specific antibody, less labelled antibody binds — producing faint or no colour on substrate addition.
A monoclonal antibody bound to the solid phase captures the standard antigen supplied in the kit. The diluted test specimen is then added — antibodies present bind to the captured antigen. This method is more specific than the standard indirect assay.
Laboratory Principle
ELISA combines the specificity of an antigen-antibody reaction with the sensitivity of enzyme assays. An antigen (or antibody) is adsorbed onto a solid matrix (microtitre plate well). A specific antibody, linked to an enzyme with high turnover number, binds the target. A substrate specific to the enzyme is then added, and the enzyme converts it into a detectable colour signal proportional to the amount of antigen/antibody present. Every step is followed by washing to remove unbound reactants, so only specific high-affinity binding contributes to the final signal.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Coated antigen/antibody plate | Kit-specific | Solid phase capture | 2–8°C, per manufacturer |
| Enzyme conjugate | Diluted per kit instructions | Detection reagent (linked to antibody/antigen) | 2–8°C, protect from light |
| Substrate solution | e.g. TMB or ABTS | Converted by enzyme to produce colour | 2–8°C, light-sensitive |
| Stop solution | e.g. Sodium hypochlorite / dilute acid | Halts enzyme reaction for accurate reading | Room temperature |
| Wash buffer | Mild detergent solution | Removes unbound reactants between steps | Room temperature |
Step-by-Step Procedure
Add an appropriate amount of diluted patient sample to the antigen-coated wells and incubate for the required time at room temperature (indirect ELISA example).
Empty the plate, tap out residual liquid, and wash 3–5 times to remove unbound material.
Add appropriately diluted enzyme-labelled anti-human immunoglobulin conjugate and incubate as specified.
Wash the plate 3–5 times, then add the substrate solution to each well and incubate as specified to allow colour development.
Add stop solution, then read the plate on the ELISA reader (or visually) as OD (optical density) value; interpret against cut-off values.
Flow Diagram
Quality Control
Use only licensed, quality-checked ELISA kits. Perform the test strictly as per manufacturer's guidelines with internal controls checked in every run. Include an external control in the daily run and calculate its E-ratio (OD of external control ÷ kit cut-off value) — it must fall within the specified acceptable range for the run to be valid.
Participate in External Quality Assessment Schemes (EQAS) to monitor inter-laboratory performance and identify systematic errors over time.
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| OD above cut-off (indirect ELISA) | Specific antibody detected — reactive/positive | Confirm with a second/confirmatory test as per protocol |
| OD below cut-off | No specific antibody detected — non-reactive/negative | Report negative; consider window period if risk is high |
| Faint/no colour (competitive ELISA) | Antibody present in specimen, competing with labelled antibody | Interpret per kit-specific competitive assay criteria |
Common Errors & How to Avoid Them
Cause: Residual unbound conjugate/antibody left in wells causes false high OD values.
Prevention: Follow the specified number of wash cycles (3–5) strictly and ensure complete removal of wash buffer before the next reagent.
Cause: Uneven evaporation or temperature across the plate, especially at edge wells, alters reaction kinetics.
Prevention: Use a humidified incubation chamber and avoid leaving plates uncovered for long periods.
Cause: Enzyme conjugates and substrates degrade with time/temperature abuse, giving weak or false results.
Prevention: Check kit expiry dates and storage conditions before use; monitor controls each run.
Laboratory Tips from the Bench
Always add substrate and stop solution in the exact same time sequence across all wells (use a timer and consistent pipetting pattern) to avoid introducing timing-related OD variability.
Read the plate promptly after adding stop solution — colour can continue to change slightly over time even after the enzyme reaction is halted.
Remember ELISA's four basic steps with "C-P-P-S": Coat, Plate-block, Probe, Signal — the universal ELISA workflow regardless of type.
Important Notes
Because a monoclonal antibody first captures the antigen before the test specimen is added, antigen capture ELISA offers greater specificity than a standard indirect assay.
Fluorogenic, electrochemiluminescent, and real-time PCR-based reporters have been developed as ELISA-like techniques with greater sensitivity, though enzyme-based ELISA remains the classical and widely used approach.
Interactive Quiz
Test Your KnowledgeFlashcards
Tap to flipClick or tap any card to reveal the answer. Use arrow keys to navigate in single-card mode.
Clinical Case Study
Apply Your KnowledgePriya is 20 weeks pregnant and attends her routine antenatal screening. As part of standard prenatal panel testing, an indirect ELISA for HIV antibodies is performed on her serum sample.
The patient's OD is well above the cut-off, and controls performed as expected, confirming assay validity. Repeat testing on a fresh sample remains reactive, indicating a true screening-positive result that requires confirmatory testing before a final diagnosis is made.
- →A single reactive ELISA is a screening result, not a final diagnosis.
- →Controls validate the entire ELISA run and must always be checked before interpreting patient results.
- →Confirmatory testing protects the patient from receiving an incorrect diagnosis based on screening alone.
Frequently Asked Questions
They are essentially the same technique — ELISA (Enzyme Linked Immunosorbent Assay) is often used interchangeably with EIA (Enzyme Immuno Assay). Both use an enzyme-labelled antibody/antigen and a substrate to produce a detectable colour signal.
RIA uses radioactive labels, which pose health hazards, require special disposal, and need licensed handling facilities. ELISA uses enzyme labels that are safer, cheaper, and easier to work with while offering comparable sensitivity.
Yes — ELISA is also used in home pregnancy tests, food industry allergen detection (e.g. milk, peanuts, eggs), and toxicology as a rapid presumptive drug screen.
Quick Revision
10-Minute ReviewKey Takeaways
- ELISA/EIA is an antigen-antibody reaction using enzyme labels and specific substrates to produce a colour-based detectable signal.
- RIA, its radioactive predecessor, was replaced due to safety concerns.
- Four main ELISA types exist: indirect, sandwich, competitive, and antigen/antibody capture.
- Coating, blocking, probing, and signal measurement are the universal ELISA process steps.
- Results are reported as OD value, correlated against a kit-specific cut-off.
- ELISA has broad applications beyond infectious disease — including pregnancy testing, food allergen screening, and toxicology.
Competency Checklist
Track Your MasteryReferences
- NIOS Microbiology Module — Lesson 63: EIA and RIA.
- Yalow RS, Berson SA. Radioimmunoassay of Plasma Insulin. 1960 (historical reference).
- Standard textbooks of Medical Microbiology and Immunology (Ananthanarayan & Paniker; Baveja).