ELISA (enzyme-linked immunosorbent assay) !

ELISA detects and measures antibodies or antigens in a sample using an enzyme that makes colour when the antibody binds its target.

Updated 2026-11-01

Key facts

Antigen or antibody is fixed to a plate; the sample is added and washed; an enzyme-linked secondary antibody (peroxidase, alkaline phosphatase) makes colour with its substrate.
Colour intensity is proportional to the amount of what is being measured (read in a spectrophotometer).
Indirect: looks for the patient's antibodies (e.g. anti-HIV). Sandwich: looks for antigens (e.g. p24 antigen, dengue NS1). Competitive: small molecules.
It is very sensitive: ideal for screening. A reactive result is confirmed with another test.
It is a protein (immunology) technique, not a nucleic acid one.

Indirect ELISA steps: coat the plate with antigen → block → patient serum → wash → enzyme-labelled anti-human IgG → wash → substrate → read.
Sandwich ELISA uses two antibodies against the same antigen: one captures it on the plate and the other, labelled, detects it.
Fourth-generation HIV tests combine antibodies and p24 antigen, which shortens the window period.
Chemiluminescence immunoassays work the same way, but the signal is light instead of colour.
Lateral-flow rapid tests (cassettes) are a simplified version of the same principle.

Screening for HIV, hepatitis B and C and syphilis in blood banks and prenatal care.
In dengue: NS1 antigen in the first days of fever and IgM after day five.
Measuring hormones, tumour markers and autoantibodies in the clinical laboratory.

Think of the answer, then tap the question to see it.

Which type of ELISA detects viral antigens such as dengue NS1?

Sandwich ELISA: one antibody captures the antigen on the plate and another, enzyme-labelled, detects it.

Why does a reactive HIV ELISA need a confirmatory test?

It is very sensitive but can give false positives; it is confirmed with a more specific test.

What creates the measurable signal in an ELISA?

The enzyme on the detection antibody, which turns a colourless substrate into a coloured product.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: ELISA blood test: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Screening and diagnosis for HIV: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Dengue Fever Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.