ELISA: An important diagnostic tool
Dr Th Dhabali Singh *
A 96-well microtiter plate being used for ELISA. :: Pix Courtesy- Wikipedia/Jeffrey M. Vinocur
ELISA stands for Enzyme-Linked Immunosorbent Assay, also sometimes called EIA or Enzyme Immunoassay. It 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.
ELISA 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 which excite antibody production are known as the antigens as they stimulate an immune response.
WHAT IS THE USE OF AN ELISA TEST?
ESLISA tests are widely utilised to detect substances that have antigenic properties, primarily, proteins. This test is used to determine if one has antibodies that are related to certain infectious conditions. It is most commonly requested if it is suspected that one has been exposed to viruses such as HIV and Hepatitis B or C, or bacteria and parasitic infections such as Toxoplasmosis, Helicobacter pylori, etc. It can also measure levels of antibodies to see if one has been vaccinated against certain diseases such as mumps and rubella.
Other uses of ELISA include:
o Measuring certain hormonal levels such as human chorionic gonadotropin (hCG), thyroid hormones,etc.
o Detecting dust and food allergies.
o Detection of illicit drugs such as cocaine and methamphetamines.
o Measuring antibodies which are produced in auto-immune conditions such as Lupus and rheumatoid arthritis.
o Detecting various cancer markers, e.g. AFP, CEA.
Some diagnostic kits are also available for the general public to use, e.g. the home pregnancy test kit is based on the ELISA principle and detects the presence of hCG which is excreted in urine of a pregnant woman.
Often ELISA test is used as a screening tool before other, more in-depth tests are done.
HOW DOES IT WORK?
There are variations of the ELISA test depending on whether antibodies or antigens are being detected, but the most basic type consists of an antibody attached to a solid surface.
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. The sample (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 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 one does not have antibodies to that certain antigen then no reaction will occur and no colour change will be seen.
PREPARATION NEEDED FOR THE TEST
No special preparation is needed.
WHAT DOES THE TEST INVOLVE?
This test usually requires blood sample or a urine specimen.
WHAT DO THE RESULTS MEAN?
Most ELISA tests give either a positive or a negative result. Either the body has the antibody or it does not have. Some others are semi-quantitative/quantitative and a value is given, as in the case for measuring hormone levels.
WHAT CAN AFFECT THE TEST?
A drawback of the ELISA test is the occurrence of false-positive or false-negative result, so in many cases, it is followed by a confirmatory test. For example, the ELISA test for HIV may give a false-positive result, therefore, other confirmatory tests are carried out on all positive samples to verify this. False positives may occur if one has an underlying condition such as Lupus or rheumatoid arthritis.
Likewise, a negative result does not always mean there is no infection as some antibodies are not produced immediately following an infection. Antibodies to HIV do not appear in blood until about eight weeks after exposure to the virus. But this also varies from person to person. So, it is recommended that if one suspects that one may have been exposed, then the test should be repeated after a few months.
* Dr Th Dhabali Singh, MD, wrote this article for The Sangai Express
The writer is Senior Consultant Pathologist & Managing Director, BABINA Diagnostics, Imphal.
This article was posted on June 16, 2015.
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