ELISA – Enzyme-Linked Immunosorbent Assay
ELISA (Enzyme-Linked Immunosorbent Assay) is a laboratory diagnostic method used to detect and quantify antibodies, antigens, hormones, or other proteins in blood or other body fluids.
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ELISA (Enzyme-Linked Immunosorbent Assay) is a laboratory diagnostic method used to detect and quantify antibodies, antigens, hormones, or other proteins in blood or other body fluids.
What is ELISA?
The ELISA (Enzyme-Linked Immunosorbent Assay) is one of the most widely used immunological testing techniques in medical laboratory diagnostics. It is designed to detect and measure specific proteins, antibodies, antigens, hormones, or other biomolecules in biological samples such as blood serum, plasma, urine, or other body fluids. The test combines the high specificity of antibody-antigen interactions with the sensitivity of enzymatic detection reactions.
Mechanism of Action
The core principle of ELISA is based on the specific binding between an antigen and an antibody. One component is labeled with an enzyme that, upon addition of a suitable substrate, produces a measurable color change. The intensity of the color is directly proportional to the concentration of the target molecule in the sample.
Common ELISA Formats
- Direct ELISA: The antigen is immobilized on a microplate and detected directly by an enzyme-linked antibody.
- Indirect ELISA: A primary antibody binds the antigen, and a secondary enzyme-linked antibody binds the primary antibody. This format offers increased sensitivity.
- Sandwich ELISA: The target antigen is captured between two antibodies. This format is especially suited for detecting proteins and hormones.
- Competitive ELISA: The sample antigen and a labeled antigen compete for binding to a fixed antibody. Higher antigen concentration in the sample results in a lower signal.
Clinical Applications
ELISA is used across a broad range of medical and scientific fields:
- Infectious diseases: Detection of antibodies against HIV, Hepatitis B and C, SARS-CoV-2, Borrelia burgdorferi (Lyme disease), Toxoplasma gondii, and many others.
- Autoimmune diagnostics: Measurement of autoantibodies, such as anti-transglutaminase antibodies in celiac disease or antibodies in systemic lupus erythematosus.
- Hormone testing: Quantification of hormones including TSH, LH, FSH, cortisol, and insulin.
- Allergy diagnostics: Detection of allergen-specific IgE antibodies.
- Tumor markers: Measurement of cancer markers such as PSA (prostate-specific antigen) or CA-125.
- Food and environmental testing: Detection of contaminants, allergens, or pollutants.
How the Test is Performed
A biological sample (e.g., blood serum) is added to the wells of a microtiter plate pre-coated with capture antibodies or antigens. After a series of incubation and washing steps, an enzyme-linked detection antibody is applied. A substrate is then added, triggering a color reaction that is measured by a photometer (plate reader). Results are expressed as optical density values and compared against reference standards.
Clinical Interpretation
A positive ELISA result indicates the presence of the target substance in the sample. Because ELISA is highly sensitive, false-positive results can occasionally occur due to cross-reactivity with structurally similar molecules. For this reason, positive ELISA results for certain conditions (e.g., HIV infection) are typically confirmed with a secondary test such as the Western blot.
A negative result generally rules out infection or disease, provided the test has sufficient diagnostic sensitivity and the diagnostic window period -- the interval between infection and detectable antibody production -- has been taken into account.
Advantages and Limitations
Advantages
- High sensitivity and specificity
- Cost-effective and scalable for routine laboratory use
- Allows simultaneous testing of many samples
- Capable of both qualitative and quantitative analysis
Limitations
- Risk of false-positive or false-negative results
- Diagnostic window period must be considered in infectious disease testing
- Requires specialized laboratory equipment and trained personnel
- Cross-reactivity may affect specificity in certain assays
References
- Lequin, R. M. (2005). Enzyme Immunoassay (EIA)/Enzyme-Linked Immunosorbent Assay (ELISA). Clinical Chemistry, 51(12), 2415-2418. PubMed PMID: 16179424.
- World Health Organization (WHO). HIV Diagnostics and Laboratory Technology. Available at: https://www.who.int/teams/global-hiv-hepatitis-and-stis-programmes/hiv/treatment/diagnostics
- Crowther, J. R. (2001). The ELISA Guidebook. Methods in Molecular Biology, Vol. 149. Humana Press.
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Related search terms: ELISA + Enzyme-linked Immunosorbent Assay + Enzyme Immunoassay + EIA