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Sandwich ELISA: Step-by-Step Protocol & Troubleshooting

Written by Seán Mac Fhearraigh, PhD  •  Updated:  •  ~10 min read

Quick Answer

A sandwich ELISA uses two antibodies — a capture antibody immobilised on a 96-well plate and an enzyme-linked detection antibody — to bind two different epitopes on the same target protein. Signal is directly proportional to analyte concentration. A typical sandwich ELISA takes 3–4 hours and detects analyte at pg/mL to ng/mL levels.

What is a sandwich ELISA?

A sandwich ELISA is the most common immunoassay format used in research, drug development, biomarker validation and clinical diagnostics. It detects and quantifies a specific protein, cytokine or other large analyte in samples like serum, plasma, cell culture supernatant, or tissue lysate.

The name comes from the geometry: the target analyte is "sandwiched" between two antibodies — a capture antibody immobilised on a 96-well polystyrene plate, and a detection antibody added in solution that binds a second, non-overlapping epitope. The detection antibody carries (or recruits via biotin–streptavidin) an enzyme — most often horseradish peroxidase (HRP) — that converts a substrate into a coloured product. The amount of colour is directly proportional to how much analyte was in the sample.

Schematic of a sandwich ELISA showing capture antibody bound to a 96-well plate, target antigen captured, biotinylated detection antibody bound, and streptavidin-HRP producing a colorimetric signal
Figure 1 — The sandwich ELISA principle: capture antibody, analyte, biotinylated detection antibody, streptavidin-HRP

When to use a sandwich ELISA

The sandwich format is the right choice when your analyte has at least two non-overlapping antibody-binding epitopes. That covers nearly all proteins, cytokines, growth factors and antibodies. Use sandwich ELISA when you need:

  • High sensitivity — typically low pg/mL to ng/mL lower limit of detection.
  • High specificity — using two antibodies in series filters out non-specific binding.
  • Quantitative results — direct proportionality between signal and concentration, with a 2–3 log dynamic range.
  • Tolerance for complex samples — works well in serum, plasma, urine, CSF and crude lysates without sample purification.

For small molecules under ~5 kDa (hormones, steroids, drugs) that cannot host two antibodies, switch to a competitive ELISA instead. For antibody screening from serum, use an indirect ELISA. For comparison against Western blot, see ELISA vs Western Blot.

The principle in detail

Every sandwich ELISA follows the same five-step rhythm:

  1. Coat. The plate is coated with the capture antibody in a high-pH carbonate buffer that promotes adsorption onto the polystyrene surface. (In a pre-coated kit, this step has already been done for you.)
  2. Block. Remaining unbound plate surface is saturated with a blocker (typically 1% BSA or 5% non-fat dry milk) to prevent non-specific binding of subsequent reagents.
  3. Bind the analyte. Sample is added and the capture antibody pulls the target out of solution onto the plate.
  4. Bind the detection antibody. A biotinylated or directly enzyme-conjugated detection antibody is added and binds a second epitope on the analyte. (In biotin–streptavidin formats, a streptavidin-HRP conjugate is then added.)
  5. Develop and read. TMB substrate is added; HRP catalyses its conversion to a blue product. Stop solution turns the reaction yellow. Absorbance at 450 nm is read on a microplate reader.

Between steps 1 and 5, the plate is washed multiple times to remove unbound material. Wash quality is the single biggest determinant of assay quality.

Pre-coated sandwich ELISA protocol

Use this protocol with any pre-coated sandwich ELISA kit (the capture antibody is already immobilised on the plate). Total time: ~3 hours. Always defer to the specific kit datasheet for kit-specific volumes and incubation times.

Download protocol PDF
Prepare the plate layout 5 min Set standard, blank (zero) and sample wells on the pre-coated plate. Reserve columns 1–2 for the 7-point standard curve and blank wells. Lay out samples in duplicate from column 3 onwards.
Add standards and samples 10 min Pipette 100 μL of standard solutions into the standard wells, 100 μL of sample/standard dilution buffer into the blank (zero) well, and 100 μL of properly diluted sample (serum, plasma, lysate, etc.) into the sample wells. Run all in duplicate.
First incubation 90 min at 37 °C Seal the plate with a plate cover and incubate at 37 °C for 90 minutes. Avoid drafts — the plate temperature must be uniform.
First wash 5 min Discard plate contents, blot on absorbent paper to dry, then wash 3× with 300 μL wash buffer per well, soaking 1 minute per wash. Blot between cycles.
Add biotinylated detection antibody 5 min + 60 min at 37 °C Add 100 μL of biotin-detection antibody working solution to every well. Seal and incubate at 37 °C for 60 minutes.
Second wash 5 min Discard plate contents and wash 3× with 300 μL wash buffer, soaking 1 minute per wash.
Add SABC (streptavidin-HRP) 5 min + 30 min at 37 °C Add 100 μL of SABC working solution to each well. Seal and incubate at 37 °C for 30 minutes.
Final wash 7 min Discard contents and wash 5× with 300 μL wash buffer, soaking 1 minute per wash. This final wash is critical for low background.
Develop with TMB substrate 10–20 min at 37 °C in the dark Add 90 μL of TMB substrate to each well. Seal and incubate at 37 °C in the dark. Watch the top of the standard curve develop a clear blue colour — stop the reaction when the top standard reaches an OD of ~1.5–2.0 (before saturation).
Stop the reaction 2 min Add 50 μL of stop solution to each well in the same order you added TMB. The colour will turn from blue to yellow immediately.
Read absorbance 2 min Read OD at 450 nm on a microplate reader immediately after stopping. Optionally take a reference reading at 540 nm or 570 nm and subtract from the 450 nm value to correct for plate imperfections.
Step-by-step schematic of a sandwich ELISA protocol on a pre-coated plate: add standards, incubate, wash, add biotin detection antibody, add SABC, wash, develop with TMB substrate, stop reaction, read absorbance at 450 nm
Figure 2 — Sandwich ELISA protocol on a pre-coated plate (step-by-step)

DIY sandwich ELISA protocol

Use this protocol with an antibody-pair development kit (e.g. Assay Genie SuperSet) where you coat your own plate. Total time: ~1 working day plus an overnight coating step.

Day 1 — Coat & block

Coat the plate with capture antibody 15 min + overnight at 4 °C Dilute the capture antibody in coating buffer (Na₂CO₃ / NaHCO₃, pH 9.6) to the manufacturer's recommended concentration (typically 1–4 μg/mL). Add 100 μL per well, cover, and incubate overnight at 4 °C.
Wash 5 min Aspirate the coating solution and wash 3× with 300 μL wash buffer (PBS + 0.05% Tween-20) per well.
Block 5 min + 2 hours at RT Add 100 μL of blocking buffer (1% BSA in PBS, or commercial blocker) per well. Cover and incubate at room temperature (18–25 °C) for 2 hours.
Wash 5 min Wash 3× with 300 μL wash buffer per well.
Store or proceed — For immediate use, continue to Day 2 below. To store coated and blocked plates, bench-dry at room temperature, seal in a foil pouch with desiccant, and store at 4 °C for up to 4 weeks.

Day 2 — Run the assay

Prepare standard curve 15 min Reconstitute the standard in dilution buffer and prepare a 7-point, 2-fold serial dilution in microcentrifuge tubes (300 μL transfers).
Add standards and samples 10 min Add 100 μL of each standard, sample and zero (standard dilution buffer) to the appropriate wells in duplicate.
Add detection antibody 5 min + 60 min at RT Add 50 μL of diluted biotinylated detection antibody to all wells. Cover and incubate at room temperature for 1 hour.
Wash 5 min Wash 3× with 300 μL wash buffer per well.
Add streptavidin-HRP 5 min + 30 min at RT Add 100 μL of streptavidin-HRP solution to all wells. Cover and incubate at room temperature for 30 minutes.
Wash 7 min Wash 5× with 300 μL wash buffer per well. Soak 1 minute per cycle.
Develop, stop, read 15 min Add 100 μL ready-to-use TMB substrate per well. Incubate 5–15 minutes in the dark at room temperature. Add 100 μL stop solution. Read OD at 450 nm immediately.
Step-by-step schematic of a DIY development sandwich ELISA: coat plate with capture antibody, block, add sample plus detection antibody, add streptavidin-HRP, develop with TMB, stop, read absorbance
Figure 3 — DIY sandwich ELISA protocol using a matched antibody pair

Choosing your capture & detection antibody pair

The single biggest determinant of assay performance is the antibody pair. The two antibodies must:

  • Bind non-overlapping epitopes on the analyte (otherwise they can't both engage at once).
  • Be specific to the target with minimal cross-reactivity against closely related family members.
  • Tolerate the sample matrix — verify performance in serum or plasma if that's your real sample type.

Monoclonal vs polyclonal

Antibody type Strengths Trade-offs Best as
Monoclonal Single epitope, lot-to-lot reproducibility, lowest cross-reactivity Lower analyte capture per molecule, more sensitive to epitope masking Capture antibody (especially for regulated assays)
Polyclonal Multiple epitopes → higher analyte capture, robust to denaturation Lot-to-lot variation, higher background risk in complex samples Detection antibody (where signal amplification matters)
Recombinant Defined sequence, infinite reproducibility, no animal sourcing Higher per-unit cost; fewer targets available Either — increasingly the format used by pharma R&D

A common, reliable starting combination is a monoclonal capture + polyclonal biotinylated detection pair. Assay Genie's SuperSet ELISA development kits ship optimised pairs already validated to work together.

Detection chemistry: HRP-TMB vs biotin-streptavidin

Two detection chemistries dominate modern sandwich ELISAs:

  • Direct HRP-conjugated detection antibody. Faster (one fewer step) and lower background, but less sensitive. Suits high-abundance targets like albumin and IgG.
  • Biotinylated detection antibody + streptavidin-HRP (SABC). Signal amplification from multiple biotin–streptavidin interactions makes this format 5–20× more sensitive. The dominant format for cytokine ELISAs and other low-abundance analytes.

For limits of detection in the fg/mL range, switch from HRP-TMB to a chemiluminescent (CLIA) substrate.

Standard curve & data analysis

  1. Subtract the blank. Relative OD450 = (OD of well) − (OD of zero/blank well).
  2. Plot the standard curve. Relative OD on the y-axis, standard concentration on the x-axis. Use a log scale on the x-axis.
  3. Fit a four-parameter logistic (4PL) curve, not a linear or log-linear fit. GraphPad Prism, MyAssays, MARS (BMG Labtech), and most plate-reader software fit 4PL natively.
  4. Interpolate sample concentrations from the 4PL fit. Discard any sample that falls below the lowest standard or above the highest — those points are outside the assay's validated range.
  5. Multiply by the dilution factor if you diluted samples before the plate.
  6. Calculate CV. Aim for intra-assay CV < 10% and inter-assay CV < 15%.

Troubleshooting matrix

The five issues you'll see 90% of the time, and how to fix each:

Symptom Likely cause Fix
High background across the plate Insufficient washing; under-blocking; detection antibody too concentrated Increase wash volume to 300 μL and add 1-min soak; switch blocker; titrate detection antibody down 2-fold
No signal even at top standard Capture antibody not coated; detection antibody not added; substrate expired Confirm coating with an antibody-pair colour test; check pipetting; test substrate against a fresh aliquot
Hook effect (high samples read low) Analyte exceeds the top of the curve, saturating both antibodies separately Run samples at two dilutions (e.g. 1:2 and 1:20) whenever very high concentrations are possible
Poor standard curve fit (low r²) Pipetting error in serial dilution; standard not fully reconstituted; bubbles Re-prepare standards in a fresh tube; vortex stock standard 30 s; tap plate to remove bubbles before reading
High CV between replicates Pipetting variability; uneven plate temperature; edge effects Recalibrate pipette; pre-warm plate; use only inner 60 wells for unknowns; or run samples in triplicate

For a deeper troubleshooting walkthrough including hemolysis effects and matrix interference, see the ELISA troubleshooting guide.

Sandwich ELISA kits & reagents

Assay Genie supplies pre-coated sandwich ELISA kits, antibody-pair development kits, and the reagents you need to build your own assays:

Pre-coated

PharmaGenie ELISA Kits

High-sensitivity pre-coated sandwich kits validated to ISO 9001:2015 with NIBSC standard calibration. The default choice for routine quantification.

Explore PharmaGenie →
DIY / Development

SuperSet DIY ELISA Kits

Matched antibody pair + recombinant standard. Coat your own plates, optimise the dilution series, and scale up flexibly.

Explore SuperSet →
Multiplex

Multiplex ELISA Kits

Measure 4–100+ analytes per well — ideal for cytokine panels and biomarker discovery on limited samples.

Browse Multiplex →
Catalogue

All Sandwich ELISA Kits

5,000+ targets across human, mouse, rat and more. Search by target, species or analyte family.

Browse ELISA Kits →

Frequently asked questions

What is a sandwich ELISA?
A sandwich ELISA is an immunoassay format in which two antibodies — a capture antibody immobilised on a microplate and a detection antibody added later — bind two different epitopes on the same target analyte, 'sandwiching' it between them. The detection antibody is linked to an enzyme (usually HRP) that produces a colorimetric signal proportional to the amount of analyte present.
How long does a sandwich ELISA take?
A typical pre-coated sandwich ELISA takes 3–4 hours including incubations and washes. A DIY sandwich ELISA where you coat your own plate adds 1–2 hours of coating (often overnight at 4 °C) plus 2 hours of blocking, bringing the total to about 1 working day or an overnight + half day.
What is the difference between sandwich ELISA and competitive ELISA?
Sandwich ELISA uses two antibodies binding two different epitopes on the analyte — signal is directly proportional to analyte concentration. Competitive ELISA uses one antibody and a labelled competitor analyte that competes with the sample — signal is inversely proportional. Sandwich is more sensitive and preferred for proteins; competitive is used for small molecules with only one antibody-binding site.
Should I use monoclonal or polyclonal antibodies?
Monoclonal capture antibodies give the highest specificity and lot-to-lot reproducibility. Polyclonal antibodies bind multiple epitopes and pull down more analyte, giving higher sensitivity at the cost of some specificity. A common compromise is a monoclonal capture paired with a polyclonal detection antibody.
What is the hook effect?
The hook effect (or high-dose hook effect) is a paradoxical drop in signal at very high analyte concentrations, caused when excess analyte saturates both the capture and detection antibodies separately before they can form the sandwich. It produces false-low results in highly concentrated samples. The fix is to run samples at two dilutions whenever very high concentrations are possible.
Why is my sandwich ELISA background so high?
High background usually means insufficient washing, under-blocking, detection antibody cross-reactivity, or substrate over-development. Increase wash volumes to 300 μL with a 1-minute soak, extend blocking, titrate the detection antibody down 2-fold, and time the substrate step against the standard curve.
What is the sensitivity of a sandwich ELISA?
A well-optimised sandwich ELISA typically achieves a lower limit of detection in the low pg/mL range and a working dynamic range of 2–3 logs. Sensitivity depends on antibody affinity, detection chemistry (biotin-streptavidin-HRP is more sensitive than direct HRP conjugates), and the substrate (chemiluminescent CLIA substrates can push sensitivity into fg/mL).
How do I calculate sandwich ELISA results?
Calculate relative OD₄₅₀ for each well as (OD of well) − (OD of zero well). Plot the standard curve and fit a four-parameter logistic (4PL) regression. Interpolate sample concentrations from the 4PL fit, then multiply by any dilution factor. Aim for intra-assay CV below 10%.
Can sandwich ELISA be used for tissue lysates?
Yes. Lyse tissue in a non-denaturing buffer with protease inhibitors, normalise to total protein concentration using a Bradford or BCA assay, then dilute to fall within the linear range of the standard curve. Most lysis buffer detergents (1% NP-40, 1% Triton X-100) are tolerated at the final assay dilution — always check the kit datasheet.
What is a biotin-streptavidin sandwich ELISA?
A biotin-streptavidin sandwich ELISA uses a biotinylated detection antibody followed by streptavidin conjugated to HRP. The signal amplification from multiple biotin–streptavidin interactions makes this format 5–20× more sensitive than direct HRP-conjugated detection antibodies.
20th May 2026 Seán Mac Fhearraigh, PhD

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