ELISA Protocol

ELISA (Enzyme-Linked Immunosorbent Assay) is a sensitive biochemical technique for detecting and quantifying proteins, antibodies, hormones, and other biomolecules in biological samples.

Various types of ELISA exist, including direct, indirect, sandwich, and competitive ELISA procedures. Sandwich ELISA is particularly beneficial for detecting low-abundance proteins in complex samples, as it employs two antibodies for improved specificity and sensitivity.

Reliable results depend heavily on protocol adherence, quality reagents, and optimized conditions. Boston BioProducts offers high-quality reagents specifically designed for consistent, accurate ELISA outcomes.

Here's everything you need to carry out our ELISA protocol. In addition to Boston BioProducts' high-quality buffers and reagents, you'll need the following materials and equipment:

• 96-well microplate
• Capture and detection antibodies
• Plate reader (spectrophotometer)
• Multichannel pipettes and tips
• Plate sealer films

This ELISA protocol outlines key steps, from plate coating to final signal detection, for a reproducible and optimized ELISA workflow. While this protocol primarily details the Sandwich ELISA workflow, it also provides insights into the other ELISA types to help you select the best assay for your research needs.

Sample Preparation

Your sample preparation may vary, but optimizing this step is a critical part of the ELISA protocol. It ensures antigen integrity and reduces background interference, leading to better results.

Our general advice by sample type:

• Serum or plasma: Centrifuge to remove debris and dilute in blocking buffer or sample diluent to fall within the assay’s dynamic range.
• Cell culture supernatant: Clarify by centrifugation and store on ice or at -20°C until use.
• Tissue or cell lysates: Use a lysis buffer compatible with ELISA (avoid detergents or agents that interfere with antibody binding). Centrifuge lysates and use the supernatant.

Coating

Coating the plate with the capture antibody enables specific immobilization of the target antigen, forming the foundation for sensitive and selective detection in Sandwich ELISA. If you're using a kit with pre-coated plates, you can proceed directly to the blocking step.

Blocking

Blocking prevents non-specific binding by saturating uncoated areas of the well, helping to reduce background noise and improve assay sensitivity.

Sample Addition

During this step, the antigen from your sample binds specifically to the immobilized capture antibody. Accurate sample dilution and incubation are essential to ensure optimal binding and reliable quantification within the assay's dynamic range.

Not doing a sandwich ELISA?

Here's how direct, indirect, and competitive ELISA procedures differ.

Mini direct ELISA protocol

Mini indirect ELISA protocol

Mini competitive ELISA protocol

Detection Antibody Incubation

The detection antibody binds to a second epitope on the captured antigen, forming the "sandwich" complex. When enzyme-conjugated, this antibody enables signal generation upon substrate addition, allowing quantification of your target analyte.

Substrate Incubation

The substrate reacts with the enzyme linked to the detection antibody to produce a measurable signal-typically a color change. Therefore, the enzyme used dictates which kind of substrate to choose.

Stop Reaction

Stopping the enzymatic reaction preserves the developed signal and prevents further color change. The choice of stop solution depends on the enzyme used and the substrate system, with acidic or denaturing agents commonly employed to halt activity.

Data Analysis

Once the plate has been read, ELISA results are analyzed by comparing sample absorbance values to a standard curve. Accurate curve fitting and replicate consistency are essential for quantifying analyte concentrations with confidence.

My background signal is too high

The problem:

High background can result from non-specific binding, insufficient washing, or suboptimal blocking.

What to do:

• Increase wash stringency (more washes, higher % Tween-20))
• Try a different blocking agent (e.g., BSA vs. casein)
• Ensure incubation conditions aren’t too long or warm

My signals are weak or inconsistent

The problem:

Inadequate antigen capture or low antibody affinity may reduce signal intensity.

What to do:

• Confirm antibody concentrations and storage conditions
• Increase sample incubation time
• Ensure the antigen or sample wasn’t degraded during prep

My replicates are highly variable

The problem:

Pipetting error, evaporation, or inconsistent plate handling can affect reproducibility.

What to do:

• Use a multichannel pipette
• Pre-wet tips
• Seal plates during incubation
• Avoid edge effects by not analyzing outer wells unless blocked and filled properly