enzyme immunoassay

enzyme immunoassay
Name	Enzyme Immunoassay
Acronym	EIA
Classification	Immunoassay, Bioanalytical method
Analytes	Antigens, antibodies, Hormones, Proteins
Manufacturers	Abbott Laboratories, Roche Diagnostics, Siemens Healthineers
Related	Enzyme-linked immunosorbent assay, Radioimmunoassay, Chemiluminescence immunoassay

enzyme immunoassay. An enzyme immunoassay is a biochemical analytical method that utilizes the specific binding of an antibody to its target antigen, with detection achieved through an enzyme-labeled reagent that produces a measurable signal. This technique is a cornerstone of modern clinical diagnostics, food safety testing, and biomedical research, allowing for the sensitive and quantitative detection of a vast array of molecular targets. Its development represented a significant advancement over earlier methods like radioimmunoassay, eliminating the need for radioactive materials while maintaining high specificity and sensitivity.

Principle and mechanism

The fundamental principle relies on the formation of a stable immunocomplex between an antibody and its specific antigen. A critical component is the conjugation of an enzyme, such as horseradish peroxidase or alkaline phosphatase, to either an antibody or antigen. Following the binding event, a colorless substrate specific to the conjugated enzyme is added. The enzyme catalyzes a reaction that converts this substrate into a colored, fluorescent, or chemiluminescent product. The intensity of this signal, measured by a spectrophotometer or luminometer, is directly proportional to the concentration of the target analyte present in the original sample. This core mechanism allows for the quantification of substances even at very low concentrations, such as troponin in suspected myocardial infarction or human chorionic gonadotropin in pregnancy tests.

Types and formats

Several distinct formats have been developed to suit different analytical needs. The most common is the enzyme-linked immunosorbent assay, which is typically performed on a solid phase like a polystyrene microtiter plate. Within ELISA, common formats include the direct, indirect, sandwich, and competitive assays. Other important types include the microparticle enzyme immunoassay, used in automated analyzers from companies like Abbott Laboratories, and the western blot, which combines gel electrophoresis with EIA for protein detection. Homogeneous formats, such as the enzyme-multiplied immunoassay technique, do not require a separation step and are used in systems like those from Roche Diagnostics for therapeutic drug monitoring.

Applications and uses

Enzyme immunoassays have pervasive applications across multiple fields. In clinical pathology, they are indispensable for diagnosing infectious diseases like HIV, hepatitis B, and SARS-CoV-2, monitoring autoantibodies in conditions like systemic lupus erythematosus, and measuring hormone levels such as thyroid-stimulating hormone. They are critical in food industry laboratories for detecting allergens like peanut proteins or pathogens such as Salmonella. Furthermore, environmental monitoring agencies employ EIAs to screen for contaminants like pesticide residues, and they are a staple tool in pharmaceutical research for drug development and biomarker discovery.

Procedure and steps

A standard procedure begins with coating a solid surface with a capture antibody or antigen. After a blocking step to prevent non-specific binding, the patient sample or standard is added and allowed to incubate. The plate is then washed, a process crucial for removing unbound material. Next, an enzyme-conjugated detection antibody is added, forming the complete immunocomplex. Following another wash, the enzyme substrate is introduced. The reaction is stopped after a defined period, often with an acid like sulfuric acid, and the resulting signal is read by an instrument such as a plate reader. The concentration of the unknown sample is determined by comparison to a standard curve generated from samples of known concentration.

Advantages and limitations

Key advantages include high specificity due to antibody-antigen interactions, excellent sensitivity capable of detecting picogram quantities, and the ability to process many samples rapidly in a high-throughput screening format. They are generally cost-effective and avoid the hazards associated with radioisotopes. However, limitations can include the potential for cross-reactivity leading to false positives, the occurrence of hook effect at very high analyte concentrations, and the requirement for specialized reagents that may have limited shelf lives. Performance can also be affected by matrix effects from complex sample types like serum or soil extracts.

History and development

The technique was pioneered in the 1960s as a non-radioactive alternative to the radioimmunoassay invented by Rosalyn Yalow and Solomon Berson. Early work by researchers like Anton H. Coons on labeled antibodies laid important groundwork. The first practical EIA formats were described independently by Eva Engvall and Peter Perlmann in Sweden and by Anton Schuurs and Beatrijs van Weemen in the Netherlands in 1971. The subsequent development of the monoclonal antibody technique by César Milstein and Georges Köhler greatly enhanced the specificity and reproducibility of available reagents, propelling the widespread adoption of EIA in laboratories worldwide.

Category:Immunologic tests Category:Laboratory techniques Category:Enzymes