Enzyme-Linked Immunosorbent Assay (ELISA)

• What is an ELISA?
• What is the test used for?
• How does the ELISA test work?
• How to prepare for the test
• What does the test involve?
• Results and what they mean
• What can affect the test?

What is an ELISA?

An ELISA or enzyme-linked immunosorbent assay, is a method used in the laboratory to aid in the diagnosis of a wide range of diseases. This test is performed on blood or urine and is used for measuring the amount of a particular protein or substance in these bodily fluids, such as infectious agents, allergens, hormones or drugs.

This test relies on the interaction between components of the immune system called antigens and antibodies. Antibodies are proteins produced by the body to identify and neutralise any foreign substances that may be encountered, such as viruses and bacteria. The substances to which antibodies are produced are known as the antigens as they stimulate an immune response.

What is the test used for?

ELISAs are used for numerous types of tests in the laboratory which can assist in the diagnosis of many different conditions.

It is most commonly requested if it is suspected you have been exposed to viruses such as HIV and Hepatitis B or C, or bacteria and parasitic infections such as Toxoplasmosis, Lyme disease and Helicobacter pylori.  It can also measure levels of antibodies to see if you have been vaccinated against certain diseases such as mumps and rubella.

Other uses of the ELISA include:

• Measuring certain hormone levels such as HCG in the pregnancy test, thyroid hormones
• Detecting dust and food allergies
• Detection of illicit drugs such as cocaine and methamphetamines.
• Measuring antibodies which are produced in auto-immune conditions such as Lupus and rheumatoid arthritis.

Some kits are also available for the general public to use for example; the home pregnancy test is based on the ELISA principle and detects the presence of a hormone known as human chorionic gonadotrophin (hCG) which is excreted in the urine of a pregnant woman.

How does the ELISA test work?

There are several variations of an ELISA test depending on whether antibodies or antigens are being detected.

If antibodies are being detected for example to HIV, then a portion of the HIV virus is attached to a solid surface such as a tube or plate. This will act as the antigen. Your serum will then be added to the tube and if it contains antibodies to the antigen then it will bind to it. Another antibody which recognises the HIV antibodies is then added and binds to any bound antibody. This second antibody is linked with a chemical known as an enzyme (an enzyme speeds up a chemical reaction) and in the final step a substance which reacts with the enzyme on the antibody is added to produce a coloured product. If the test is positive then a colour reaction will occur. If you don’t have antibodies to that certain antigen then no reaction will occur and no colour change will be seen.

How to prepare for the test

No preparation is required.

What does the test involve?

This test usually requires blood to be taken from a vein or a urine specimen.

Results and what they mean

Most ELISAs give either a positive or a negative result, either your body has the antibody or it hasn’t. Some others are semi-quantitative and a value is given, as is the case for measuring hormone levels.

What can affect the test?

A drawback of the ELISA is the occurrence of false positive or false negative result, so in many cases it is followed up by a confirmatory test. For example, the ELISA for HIV and Lyme disease may often give a false positive result therefore a different type of test is also carried out on all positive samples to verify this. False positives may occur if you have an underlying condition such as Lupus or rheumatoid disease.

A negative result doesn’t always mean there is no infection as some antibodies are not produced immediately following infection. Antibodies to HIV do not appear in blood until 6 weeks after exposure to the virus so it is recommended that if you suspect you have been exposed then the test should be repeated after three months.

References

1. Branson BM. State of the art for diagnosis of HIV infection. 2007. Clin. Infect. Diseases. 45: Supplement 4:S221-5
2. Prescott LM, Harley JP and Klein DA. Microbiology. Wm. C. Brown publishers. Dubuque, IA. 1996
3. Staffan P, Perlmann H, Bermenn P. 2005. Encyclopedia of life sciences. Enzyme-linked immunosorbent assay. Published online 27/1/2006
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