Note: this site last updated in 2006
ELISA is the Enzyme-Linked Immunoadsorbent Assay. It's a sensitive immunoassay used to detect a specific protein – for example antibodies to HIV borne in a sample of blood.
There are a few methods for performing an ELISA. Here is one common one, and the one relevant to detecting HIV antibodies:
- Put a specific antigen protein (i.e. prepared HIV protein) in a plastic container. The protein molecules bind (adsorb) to the plastic by hydrophobic or other attraction.
- Add the serum sample from the patient. If the patient serum contains HIV antibodies, these will attach to the HIV antigens already present.
- Wash the container gently, so that all the unbound components of the patient serum are washed away.
- Add an antibody which is raised to recognise the human antibody, and of which each molecule is covalently linked with a molecule of an enzyme such as alkaline phosphatase or peroxidase - any enzyme which can catalyse a chromogenic reaction.
- Wash the container gently, so that any unbound test antibody from the previous step is washed away.
- Add the reagents which are the inputs to the chromogenic reaction. These reagents will only react in the presence of the enzyme - that is, where our test antibodies have bound to the plastic container via the antibody/antigen complex.
- The colour produced in the container is an indication of the amount of antibody found. Normally the ELISA is performed in a multi-welled container, and a positive result comes when the amount of wells showing the appropriate colour change is a certain proportion.
False positives can occur - for example antibodies induced by a recent flu jab can cause a positive result - and so although ELISA's an important diagnostic test for HIV, it should be followed up with a Western blot test.
You may also want to read about Tests of HIV progression.