General Studies III

Enzyme-Linked Immunosorbent Assay (ELISA)

What is ELISA test?

  • ELISA (enzyme-linked immunosorbent assay) is a plate-based assay technique designed for detecting and quantifying substances such as peptides, proteins, antibodies and hormones.
  • Other names, such as enzyme immunoassay (EIA), are also used to describe the same technology.
  • In an ELISA, an antigen must be immobilized on a solid surface and then complexed with an antibody that is linked to an enzyme.
  • Detection is accomplished by assessing the conjugated enzyme activity via incubation with a substrate to produce a measurable product.
  • The most crucial element of the detection strategy is a highly specific antibody-antigen interaction.


What are antibodies?

  • An antibody is a large, Y-shaped protein produced mainly by plasma cells that are used by the immune system to neutralize pathogens such as pathogenic bacteria and viruses.
  • There are five immunoglobulin classes (isotypes) of antibody molecules found in serum: IgG, IgM, IgA, IgE and IgD.
  • They are distinguished by the type of heavy chain they contain.


Application of ELISA

  • Presence of antigen or the presence of antibody in a sample can be evaluated
  • Determination of serum antibody concentrations in a virus test
  • Used in the food industry when detecting potential food allergens
  • Applied in disease outbreaks- tracking the spread of disease e.g. HIV, bird flu, common, colds, cholera, STD etc


  • Robust antibody tests are critical for surveillance to understand the proportion of the population exposed to infection.
  • The test will have the advantage of testing 90 samples together in a single run of 2.5 hours.
  • Moreover, ELISA based testing is easily possible even at the district level as the ELISA kit has an inactivated virus.
  • There are also minimal bio-safety and bio-security requirements as compared to the real-time RT-PCR test.
  • The test has the advantage of having much higher sensitivity and specificity as compared to the several rapid test kits which have recently flooded the Indian market.


  • Since the ELISA test is based on the detection of antibodies, it can only help in knowing if the person has been previously infected by a coronavirus.
  • It takes one-three weeks for the antibodies to develop in response to infection.
  • So, if a person who has been recently infected by the virus is tested during the window period (the time taken to develop antibodies) the result will turn out to be negative.
  • But a repeat test after a couple of weeks will indicate the true infection status.

How it is different from the PCR test?

  • While the RT-PCR, which detects the RNA of the coronavirus, enables detection of current infection, it will not be useful if the testing is carried out days after the infection clears as the virus will no longer be present.
  • However, antibodies developed in response to the coronavirus infection will be present in the blood for a longer duration and hence the ELISA test can help detect past infection.
  • The maximum time the antibodies will be present in the body is not known for coronavirus.

 Reverse Transcriptase – Polymerase Chain Reaction (PCR) Test

  • It uses a technique that creates copies of a segment of DNA. ‘Polymerase’ refers to the enzymes that make the copies of DNA.
  • Kary Mullis, the American biochemist who invented the PCR technique, was awarded the Nobel Prize for Chemistry in 1993.
  • The ‘chain reaction’ is how the DNA fragments are copied, exponentially — one is copied into two, the two are copied into four, and so on.
  • However, SARS-COV-2 is a virus made of RNA, which needs to be converted into DNA. For this, the technique includes a process called reverse transcription.
  • A ‘reverse transcriptase’ enzyme converts the RNA into DNA. Copies of the DNA are then made and amplified.
  • A fluorescent DNA binding dye called the “probe” shows the presence of the virus. The test also distinguishes SARS-COV-2 from other viruses.

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