ELISA Workflow: From Sample to Data

Protocol boundary

Use this as general guidance

Cell-line-specific instructions, assay kit documentation, institutional biosafety requirements, and your lab's validated SOP should take priority when they differ from this general workflow.

Materials

Materials and reagents

Protocol

Step 1: Coat Plate with Capture Antibody

Dilute the capture antibody in an appropriate coating buffer. A practical starting concentration for many sandwich ELISAs is approximately 1–10 µg/mL.

Add the coating solution to each well and incubate according to the validated workflow. Overnight coating at approximately 4°C is common, although shorter room-temperature coating procedures can also be used.

After coating, aspirate the antibody solution and wash the plate. Consistent coating across wells is important because variation in immobilized capture antibody can directly affect signal.

Avoid allowing the plate to dry during subsequent steps.

Protocol

Step 2: Block and Add Samples

Blocking covers unoccupied binding sites on the plate and reduces nonspecific adsorption.

Common blocking solutions include approximately:

• 1% BSA
• 5% nonfat milk, when compatible with the assay

Incubate the blocking solution for approximately 1 hour at room temperature, then wash the plate.

Prepare a dilution series of the purified target analyte to generate a standard curve. Include a blank and appropriate negative controls.

Add standards and experimental samples to the coated wells. Sample incubation commonly lasts 1–2 hours at room temperature or overnight at 4°C, depending on assay sensitivity requirements.

Following incubation, wash approximately 3–5 times with PBST, typically PBS containing a low concentration of nonionic detergent.

Thorough washing is one of the most important factors for reducing ELISA background. Sandwich ELISA workflows commonly use multiple wash cycles between each binding stage. (Bio-Rad)

Protocol

Step 3: Add Detection Antibody and Enzyme Conjugate

Add the detection antibody at a validated concentration and incubate, commonly for approximately 1 hour at room temperature.

The detection antibody should recognize an epitope distinct from the capture antibody's binding site.

Wash the plate 3–5 times before adding the enzyme-linked detection component.

Depending on assay design, the enzyme may be directly conjugated to the detection antibody or introduced through a secondary binding system. Horseradish peroxidase is commonly used because it generates a sensitive colorimetric response with suitable substrates.

After enzyme-conjugate incubation, wash thoroughly again. Residual unbound enzyme is a major source of high background.

Protocol

Step 4: Add Substrate and Measure Absorbance

Add the appropriate chromogenic substrate and allow color to develop.

A typical incubation period is approximately 15–30 minutes at room temperature, often protected from strong light.

Monitor color development rather than relying solely on a fixed time. Excessive development can saturate high-concentration wells and reduce quantitative accuracy.

Add stop solution to terminate the enzymatic reaction. For commonly used peroxidase-based colorimetric assays, measure absorbance at approximately 450 nm shortly after stopping the reaction.

Protocol

Standard Curve and Data Analysis

Plot the absorbance values of known standards against analyte concentration.

A four-parameter logistic curve is commonly appropriate for ELISA because the full dose-response relationship is often sigmoidal rather than linear.

Determine unknown sample concentrations by interpolation from the standard curve and multiply by any sample dilution factor.

Do not report concentrations for samples outside the validated standard range. Instead, dilute high samples and repeat them when necessary.

Technical duplicates or triplicates can help identify pipetting or well-to-well variability.

Troubleshooting

Common problems to check

High Background

Possible causes include inadequate washing, excessive detection antibody, insufficient blocking, or excessive substrate development.

Weak Signal

Check capture and detection antibody concentrations, sample integrity, incubation time, and enzyme activity.

Poor Standard Curve

Confirm serial dilutions, mixing, pipetting accuracy, and standard stability.

Large Differences Between Replicates

Uneven plate washing, bubbles, inconsistent pipetting, or edge effects may contribute.

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