The linked enzyme immunoabsorption assay technique – in English Enzyme-Linked Immuno Assay – or ELISA test is an immunological test which allows the detection or the assay of molecules in a biological sample. It was conceptualized and developed by two Swedish scientists, Peter Perlmann and Eva Engvall at Stockholm University in 1971.

The molecules assayed by the ELISA method are generally proteins. And sample types include fluid biological materials – plasma, serum, urine, perspirant -, cell culture media, or a recombinant protein – protein produced by a cell whose genetic material has been altered by genetic recombination – purified into solution.

The ELISA test is mainly used in immunology to detect and / or measure the presence of proteins, antibodies or antigens in a sample. This serological test detects in particular the antibodies produced by the body in response to viral contamination.

The use of antibodies for the diagnosis of infectious diseases represents a specific and rapid method. The ELISA technique is an immuno-enzymatic technique which makes it possible to visualize, from a biological sample, the reactions between an antigen – a body considered as foreign by the living organism – and an antibody using a color reaction produced by an enzyme marker – generally alkaline phosphatase and peroxidase – previously attached to the antibody. The color reaction confirms the identification of the isolated bacterium or the presence of the desired virus and the intensity of the color gives an indication of the amount of antigens or antibodies in the given sample.

There are four main types of ELISA test:

• ELISA direct, makes it possible to detect or measure antibodies. It only uses a primary antibody;
• ELISA indirect, the most widely used, also makes it possible to detect or assay antibodies. It uses a secondary antibody which gives it better sensitivity than direct ELISA;
• ELISA in competition, allows the dosage of antigens. Produced by competition for bonds, it does not use an enzyme;
• ELISA «in sandwich», allows the dosage of antigens. This technique is commonly used in research.

The ELISA test is used for:

• Find and measure antibodies in serology for the diagnosis of infectious diseases: virology, parasitology, bacteriology, etc.;
• Dose proteins at low concentrations: specific dosages of certain plasma proteins (immunoglobulin E (IgE), ferritin, protein hormones, etc.), tumor markers, etc. ;
• Dose small molecules: steroid hormones, thyroid hormones, drugs …

The ELISA test is used in particular to diagnose many infectious diseases:

Sexually transmitted diseases (STDs)

Including hepatitis, syphilis, chlamydia and HIV. Recommended by health authorities, it is the main AIDS screening test: it shows the presence of anti-HIV antibodies and the p24 antigen six weeks after infection.

Regional or endemic diseases

Yellow fever, Marburg virus disease (MVM), Ladengue, Lyme disease, Chikungunya, Rift Valley fever, Ebola, Lassa fever, etc.

The Covid-19

To be carried out more than 2 to 3 weeks after the onset of symptoms, the ELISA test makes it possible to identify, in less than an hour, the presence of anti-SARS-CoV-2 antibodies.

Viral pathogens that cause prenatal infections

Toxoplasmosis, rubella, cytomegalovirus, herpes simplex for example.

Other cases

But he has also found applications in detection:

• Pregnancies;
• Autoimmune diseases;
• Food allergens: the quantitative determination of total immunoglobulins E (IgE) helps in the assessment and treatment of allergies;
• Hormonal disturbances;
• Tumor markers;
• Plant viruses;
• And many more

As part of the detection of anti-SARS-CoV-2 antibodies, a pilot study carried out in August 2020 by the Institut Pasteur, the CNRS, Inserm and the University of Paris confirms the reliability of the ELISA test: both ELISA tests tested use the entire N protein of SARS-CoV-2 (ELISA N) or the extracellular domain of the virus spike (S) as target antigens. This technique would make it possible to identify antibodies in more than 90% of cases, with a very low false-positive rate of 1%.

Carried out in analysis laboratories on medical prescription, Elisa tests cost around 10 euros and are reimbursed at 100% by Health Insurance.

Carried out in free information, screening and diagnostic centers (CeGIDD), they can be free for HIV and SARS-CoV-2.