ELISA: Principles and Workflow

The Enzyme-Linked Immunosorbent Assay, or ELISA, is one of the most widely used analytical techniques in biomedical research, clinical diagnostics, and pharmaceutical quality control. By coupling the exquisite specificity of antibodies with the signal amplification power of enzyme catalysis, ELISA can detect proteins, hormones, antibodies, and small molecules at picogram-per-milliliter concentrations in complex biological matrices such as serum, urine, or cell culture supernatant.

Core Principle: Antibody Specificity and Enzyme Amplification

An antibody binds its target antigen with high affinity and selectivity. On its own, this binding event is invisible. ELISA makes it visible by attaching an enzyme — most commonly horseradish peroxidase (HRP) or alkaline phosphatase (AP) — to a detection antibody. When a substrate is added, the enzyme converts it into a colored product at a rate proportional to the amount of enzyme present, and therefore proportional to the amount of target antigen captured. The color intensity is measured as absorbance, typically at 450 nm for HRP/TMB substrate systems.

The Sandwich ELISA Workflow

The sandwich ELISA is the most common format for protein quantification. It uses two antibodies directed at different epitopes on the same antigen, allowing high sensitivity and specificity even in crude samples.

Microplate well surface HRP HRP Capture Ab Antigen Detection Ab + Enzyme Color Sandwich ELISA: Capture – Bind – Detect – Read
Sandwich ELISA: capture antibodies immobilized on the plate bind antigen from the sample; enzyme-labeled detection antibodies bind the captured antigen; substrate addition yields a color proportional to antigen concentration.
  • Coat: A capture antibody specific to the target is adsorbed to the wells of a polystyrene microplate overnight at 4 degrees C.
  • Block: Remaining binding sites are saturated with BSA or casein to prevent non-specific binding of sample proteins.
  • Bind: The sample (serum, cell lysate, etc.) is added; the target antigen binds to the capture antibody. Unbound sample is washed away.
  • Detect: An enzyme-conjugated detection antibody is added, binding to a second epitope on the captured antigen. Another wash removes unbound antibody.
  • Develop: Substrate solution (e.g. TMB for HRP) is added; the enzyme catalyzes a color change. A stop solution (typically sulfuric acid) halts the reaction at a defined endpoint.
  • Read: Absorbance at 450 nm (with a 570 nm reference subtraction) is measured on a microplate reader. A standard curve of known concentrations converts OD values into analyte concentrations.

Other ELISA Formats

Beyond the sandwich format, direct ELISAs coat the antigen itself onto the plate and detect it with a single enzyme-labeled antibody — faster but less selective. Competitive ELISAs are used for small molecules (haptens) where two-antibody sandwiching is not feasible; signal decreases as analyte concentration increases, the reverse of sandwich behavior. Indirect ELISAs use a primary antibody followed by a secondary enzyme-labeled anti-species antibody, amplifying signal and reducing reagent cost when many different primary antibodies must be screened.

Reading ELISA Plates

Accurate ELISA results depend on fast, uniform plate reading. The K LAB MRX A2000 microplate reader measures a full 96-well plate in approximately 8 seconds using a xenon flash lamp, eliminating the drift that occurs when slow readers allow the stop reaction to continue producing color between the first and last wells. The MRX supports 6- to 384-well formats, endpoint and kinetic absorbance modes, and wavelength selection across the full UV-Vis range — covering not only the 450 nm TMB endpoint but also 405 nm pNPP (alkaline phosphatase) and 490 nm OPD substrates. For researchers running Bradford or BCA protein assays alongside ELISA on the same plate layout, the MRX reads all assays in a single run without reconfiguration.

Key Sources of ELISA Variability

  • Incomplete washing is the single largest contributor to high background signal
  • Non-uniform plate temperature during incubation creates edge effects — use a plate sealer and temperature-controlled incubator
  • Pipetting consistency across replicate wells determines the coefficient of variation (CV); automated dispensers reduce CV below 5%
  • Hook effect: extremely high antigen concentrations saturate both antibodies, paradoxically reducing signal — dilute unexpected high-end samples and re-run

ELISA remains the method of choice for protein biomarker quantification because it combines femtomolar-range sensitivity with practical throughput in a standard microplate format — and because the colorimetric readout requires nothing more than a reliable absorbance reader.

Related Articles

← Back to Technology