ELISA Protocol: Sandwich Method Step by Step

LabProtocol.co Team·2026-03-22·7 min read
ELISAimmunoassayprotocolssandwich-ELISA

ELISA Protocol: Sandwich Method Step by Step

The enzyme-linked immunosorbent assay (ELISA) is the gold standard for quantifying proteins, cytokines, hormones, and antibodies in biological samples. Among the various ELISA formats, the sandwich ELISA offers the best combination of specificity and sensitivity — making it the go-to choice for most quantitative applications.

This guide covers the complete sandwich ELISA protocol from plate coating through data analysis, with real concentrations, incubation times, and solutions to common problems.

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How Sandwich ELISA Works

In a sandwich ELISA, the target analyte is sandwiched between two antibodies:

  1. Capture antibody — coated onto the plate, binds the analyte from solution.
  2. Detection antibody — binds a different epitope on the captured analyte, conjugated to biotin or an enzyme (HRP/AP).
  3. Enzyme-substrate reaction — produces a colorimetric, fluorescent, or chemiluminescent signal proportional to analyte concentration.

The key advantage: two antibodies must recognize the same protein, dramatically reducing background and increasing specificity.

Materials and Reagents

Antibodies and Standards

  • Capture antibody (purified, unconjugated; typically 1-10 ug/mL for coating)
  • Detection antibody (biotinylated or HRP-conjugated; typically 0.5-2 ug/mL)
  • Streptavidin-HRP conjugate (if using biotinylated detection antibody; 1:200 to 1:5000 dilution)
  • Recombinant protein standard (for generating a standard curve)

Buffers and Solutions

  • Coating buffer: 0.1 M sodium carbonate/bicarbonate buffer, pH 9.6 (or PBS pH 7.4 for some antibodies)
  • Wash buffer: PBS + 0.05% Tween-20 (PBS-T)
  • Blocking buffer: 1% BSA in PBS, or 5% non-fat dry milk in PBS (avoid milk for biotin-streptavidin systems)
  • Sample/Reagent diluent: 1% BSA in PBS-T
  • TMB substrate solution (3,3'5,5'-tetramethylbenzidine; e.g., R&D Systems DY999)
  • Stop solution: 2 N sulfuric acid (H2SO4)

Equipment

  • 96-well high-binding ELISA plate (e.g., Corning 9018 or Nunc MaxiSorp)
  • Multichannel pipette (20-200 uL)
  • Plate washer (or wash bottle for manual washing)
  • Microplate reader (450 nm, with 540 or 570 nm correction)
  • Plate sealers or adhesive film
  • Reagent reservoirs

Step-by-Step Sandwich ELISA Protocol

Step 1: Coat the Plate with Capture Antibody

  1. Dilute the capture antibody to the recommended concentration (typically 1-4 ug/mL) in coating buffer.
  2. Add 100 uL per well to a 96-well high-binding plate.
  3. Seal the plate and incubate overnight at 4 degrees C. Alternatively, incubate at 37 degrees C for 2 hours (overnight at 4 degrees C gives more consistent coating).

Step 2: Block Non-Specific Binding

  1. Aspirate the capture antibody solution. Do not wash at this stage.
  2. Add 200-300 uL blocking buffer per well.
  3. Incubate at room temperature for 1-2 hours.
  4. Wash 3 times with 300 uL PBS-T per well. Ensure complete aspiration between washes.

Step 3: Prepare Standards and Samples

  1. Reconstitute the recombinant protein standard per the manufacturer instructions.
  2. Prepare a 7-point standard curve with 2-fold serial dilutions in reagent diluent. Include a blank (diluent only).
    • Example range for a cytokine ELISA: 2000 pg/mL down to 31.25 pg/mL
  3. Dilute samples in reagent diluent. Typical dilutions:
    • Cell culture supernatant: neat to 1:10
    • Serum/plasma: 1:2 to 1:100 (depends on expected analyte concentration)

Step 4: Add Standards and Samples

  1. Add 100 uL of each standard and sample per well in duplicate.
  2. Seal the plate and incubate at room temperature for 2 hours on a rocker, or overnight at 4 degrees C.
  3. Wash 4 times with PBS-T.

Step 5: Add Detection Antibody

  1. Dilute the biotinylated detection antibody to the recommended concentration in reagent diluent.
  2. Add 100 uL per well.
  3. Incubate at room temperature for 1-2 hours.
  4. Wash 4 times with PBS-T.

Step 6: Add Streptavidin-HRP

(Skip this step if your detection antibody is directly HRP-conjugated.)

  1. Dilute streptavidin-HRP in reagent diluent (typically 1:200 to 1:1000).
  2. Add 100 uL per well.
  3. Incubate at room temperature for 20-30 minutes. Avoid direct light exposure — cover the plate.
  4. Wash 4 times with PBS-T. This wash is critical — residual streptavidin-HRP causes high background.

Step 7: Substrate Development

  1. Add 100 uL TMB substrate per well.
  2. Incubate at room temperature in the dark for 10-30 minutes. Monitor color development — the wells with highest standard should turn deep blue.
  3. Add 50 uL stop solution (2 N H2SO4) per well. The color changes from blue to yellow.

Step 8: Read and Analyze

  1. Read absorbance at 450 nm within 30 minutes of adding stop solution.
  2. Apply wavelength correction at 540 nm or 570 nm (subtracts background from plate imperfections).
  3. Subtract the blank OD from all readings.
  4. Plot the standard curve (log concentration vs. OD) and fit with a 4-parameter logistic (4PL) regression.
  5. Interpolate sample concentrations from the curve. Multiply by any dilution factor.

Troubleshooting Sandwich ELISA Problems

High Background

  • Insufficient washing: Increase washes to 5x, ensure complete aspiration, and add a 30-second soak between washes.
  • Blocking buffer inadequate: Try a different blocker (switch from milk to BSA or vice versa). Add 0.05% Tween-20 to the blocking buffer.
  • Streptavidin-HRP too concentrated: Titrate down to find the optimal dilution.

Low Signal

  • Capture antibody inactive: Verify antibody activity. Check that the coating buffer pH is correct (some antibodies coat poorly at pH 9.6 — try PBS pH 7.4).
  • Detection antibody too dilute: Titrate the detection antibody to find the optimal concentration.
  • TMB substrate expired or contaminated: TMB should be colorless. If it has a blue tint, discard it.
  • Samples denatured: Avoid repeated freeze-thaw. Aliquot samples upon collection.

Poor Standard Curve

  • Standard degraded: Reconstitute fresh standard. Aliquot and store at -80 degrees C.
  • Pipetting errors: Use calibrated multichannel pipettes. Mix serial dilutions thoroughly (8-10 times).
  • Insufficient incubation: Ensure plates are fully sealed and incubated at the correct temperature.

High Variability Between Replicates

  • Inconsistent washing: Use a plate washer for uniform results.
  • Edge effects: Temperature gradients cause edge wells to behave differently. Avoid using the outermost wells, or pre-incubate plates at room temperature before adding reagents.
  • Bubbles in wells: Centrifuge the plate briefly (200 x g, 1 min) after adding reagents, or pop bubbles with a clean needle.

Pro Tips for ELISA Success

  • Matched antibody pairs are essential — capture and detection antibodies must recognize different, non-overlapping epitopes. Use validated pairs from the same vendor when possible.
  • Run samples in duplicate — if CV is greater than 15% between duplicates, re-run.
  • Spike-and-recovery — add a known amount of standard to your sample matrix to check for matrix interference. Recovery should be 80-120%.
  • Dilution linearity — test 3-4 serial dilutions of your sample. If the calculated concentration is not consistent across dilutions, you have matrix effects.
  • Store coated plates — coated and blocked plates can be dried and stored at 4 degrees C with desiccant for weeks (seal tightly). This saves time for repeat experiments.

Common Mistakes to Avoid

  1. Using the wrong plate type — high-binding plates (MaxiSorp, polystyrene) are required. Regular tissue culture plates will not work.
  2. Skipping the wavelength correction — always subtract 540/570 nm to correct for optical imperfections.
  3. Using milk with biotin-streptavidin systems — milk contains endogenous biotin that competes with your detection antibody. Use BSA instead.
  4. Extrapolating beyond the standard curve — samples above the highest standard or below the lowest must be re-run at a different dilution.
  5. Inconsistent incubation temperatures — a plate near a window vs. the center of the bench can give very different results. Use an incubator or a consistent location.

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