How Covid test detect Corona Virus in human body?


Last Updated on 09/06/2021 by Info idea

Scientists are trying to figure out which coronavirus variants are circulating in India, where a virulent second wave of COVID-19 has wreaked havoc on the country and caught authorities off guard.  Our country recorded nearly 28,900,151 coronavirus cases and about 348,923 deaths on 6th June 2021. (see ‘’).

Evidence is mounting that a variant discovered in India could be more transmissible and stronger at evading immunity than other variants.

What is Covid 19?


Coronavirus disease (COVID-19) is a newly discovered coronavirus that causes an infectious disease.

Coronaviruses (CoV) are a large family of viruses that can cause illnesses ranging from the common cold to more serious illnesses like Middle East Respiratory Syndrome (MERS-CoV) and Extreme Acute Respiratory Syndrome (SARS) (SARS-CoV). Coronaviruses are zoonotic, meaning they can spread from animals to humans.

Is Covid 19 available in more than one strain?

A Chinese analysis of 103 COVID-19 cases discovered two strains, dubbed L and S. Although the S type is older, the L type was more prevalent during the outbreak’s early stages. They believe one is more likely to trigger the disease than the other, but they’re still figuring out what that means.

A virus’s ability to alter or mutate as it infects people is also natural, and this virus has done so. There are many variants named after the regions where they were found, but they have now spread to other areas and nations, with several proving to be more dangerous as well as lethal.

The B.1.617 variant has become the dominant strain in India in just a few weeks, and it has spread to over 40 countries, including the United Kingdom, Fiji, and Singapore.

What are the different types of COVID-19 test?

  • Polymerase chain reaction (PCR) test.
  • Lateral Flow tests (LFTs)
  • Antibody (or serology) tests.

What is PCR testing?

PCR tests are used to screen for the presence of viral RNA in the body, which can be detected before antibodies are formed or signs of the disease appear. This ensures that the tests will detect whether or not anyone is infected with the virus at an early stage of their illness.

In a lab, substances known as reverse transcriptase or DNA polymerase are applied to a nasopharyngeal sample during Covid-19 PCR testing. These substances function by duplicating any viral RNA that may be present. This is to ensure that enough copies of the RNA are present to indicate a positive result, as specially engineered primers and probes bind to sequences of the virus’s genetic code to indicate the presence of a pathogen.

“PCR gives us a clear indication of who is infected,” says Dr Edward Wright, senior lecturer in microbiology at the University of Sussex.

Public health officials can get a better picture of the spread of a disease like Covid-19 by scaling PCR testing to screen large swaths of nasopharyngeal swab samples from within a population.

However, there are some limitations to PCR. With various PCR tests, false negatives can occur up to 30% of the time, making them more effective for verifying the existence of an infection rather than giving a patient the all-clear. They can also produce false positive results because they’re so sensitive that they can detect dead, deactivated virus in the body of someone who has recovered from Covid-19 and signal a positive result.

How about a measure of Lateral flow covid test?

LFTs and PCR tests are also antigen tests, but LFTs are designed to detect active Covid-19 infection rather than antibodies to the disease. A nasopharyngeal sample is placed on a small absorbent pad and drawn along the pad through a capillary line to a strip coated in antibodies that bind to SARS-Cov-2 proteins using a Covid-19 LFT. If these proteins are present, a coloured line on the test will appear, suggesting infection.

The primary advantage of LFTs over PCRs is that they do not need validation and can provide results in 15 to 30 minutes. What they gain in pace, however, they lose in accuracy.

A analysis of 64 studies from Europe and the United States found significant differences in accuracy between different LFT brands. (See ‘Covid-19: Lateral flow tests are better at identifying people with symptoms) The tests were also found to be much more effective at detecting Covid-19 in people who had symptoms than in people who did not. In symptomatic individuals, LFT sensitivity ranged from 34% to 88 percent, with an average accuracy of 72 percent. LFTs correctly classified 58 percent of those who were infected in people who had no symptoms.

What is Antibody testing?

An antibody test is a test that looks for antibodies in your blood. When the body battles an infection, such as COVID-19, it produces these. “An antibody test tells us what percentage of the population has been infected,” Wright explains. Since antibodies are produced after a week or two, the virus should have been cleared from the system, it won’t tell you who is infected. However, it informs you as to who has been infected and who should be immune to the virus.”

People who recover from even mild cases of Covid-19 develop antibodies for at least five to seven months, and possibly much longer, according to a study published in the journal Immunity.(see ‘Orthogonal SARS-CoV-2 Serological Assays Enable Surveillance of Low prevalence communities and reveal durable Humoral Immunity)

Unlike PCR tests, which typically use swabs to detect Covid-19, antibody tests typically use blood samples. This is because, relative to the respiratory tract, there would be a relatively small amount of Covid-19 circulating in the blood, but a strong and detectable antibody presence in the blood following infection.

Antibody experiments are being used to assess immune responses in people who have received the Covid-19 vaccine. Researchers are unsure how long vaccine-induced immunity will last or if booster vaccinations will be necessary. While there has been some evidence that Covid-19 variants are making some vaccines less safe, they still appear to provide adequate protection in most cases.

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