Covid-19 Testing-Genetic vs Antibody

On Wednesday, the Indian Council of Medical Research invited bids for an estimated 10 lakh antibody kits (for serological tests) for the diagnose of COVID-19.

The kits have to be supplied to 6 locations– Dibrugarh, Mumbai, Chennai, Hyderabad, Bhopal, Delhi, and need to be FDA/CE IVD/NIV Pune certified.

What are serological tests?

Viral infections are mainly identified by two kinds of tests– genetic and serological.

Genetic tests can identify infections that are active, but cannot be used to detect past infections.

To trace how infections like the novel coronavirus have spread so far, it is important to detect people who contracted the disease in the past and have recovered. This is what serological tests seek to determine.

How are the two different?

The genetic test is conducted on a swab collected from the back of the throat, a liquid sample from the lower respiratory tract, or a simple saliva sample.

For SARS-COV-2, the virus’s RNA is first converted into DNA. By a process called polymerase chain reaction (PCR), DNA fragments in the sample are copied exponentially — one is copied into two, the two are copied into four, and so on.

Unlike genetic tests, which look for RNA in swab samples, serological tests work on antibodies in blood samples. Hence, they are also called ‘antibody tests’.

Antibodies, or protective proteins produced by the immune system to neutralise pathogens such as bacteria and viruses, are present in one’s bloodstream for a considerable period of time after the infection has gone.

To disable a pathogen, the antibody latches to a unique protein molecule on pathogen’s surface, called an antigen.

Serological tests use antigen molecules to detect the presence of antibodies relevant to the infection.

Generally, a blood sample is placed in a test tube that is lined with antigens on the inside. If the relevant antibodies are present, they latch on to the antigens.Such tests are relatively inexpensive, and can display results within a few minutes

In Scientific Terms, these two methids are classified as:

There are two methods of testing available to find out if a person is infected with the novel coronavirus (SARS-CoV-2) or not. These two methods are: Molecular assays and immunoassays.

India currently uses the molecular assays method of testing.

Molecular assays can either be manual or automated.

  • In this method, an oral swab sample is taken. Viral Ribonucleic Acid (RNA) material is isolated from the swab using several chemicals, since the novel coronavirus is an RNA virus.
  • The amount taken, however, is little, making it impossible to find the pathogen directly from such a sample.
  • A technique called the polymerase chain reaction technique (PCR) is used to increase the material.
  • An additional step of treating the sample with an enzyme, called reverse transcriptase, is needed to use the PCR technique.
  • The RNA is then converted into a complementary strand of Deoxyribonucleic Acid (DNA). This DNA is then usually replicated around 40 times over.
  • Enzymes and nucleotides, fluorescent probes and other primers — specific to the viral genetic material — are added during the replication process.
  • The fluorescent probes provide a visual signal and are released from each DNA strand, when replication of the strand is complete.
  • Replication would occur and the visual signal would be seen if the sample contained the novel coronavirus RNA.
  • India currently uses imported RT-PCR test kits to carry out testing using this method.
  • The Indian Council of Medical Research approved two test kits on March 23, 2020 after finding them to be 100 per cent sensitive to true positive and true negative samples.

Immunoassays — also called rapid tests — follow the same principle used in pregnancy tests.

  • They identify Immunoglobulin M and Immunoglobulin G antibodies developed against the SARS-CoV-2.
  • Blood, serum and plasma can be used for the test. Antibodies, if present in the sample, bind to the antigen immobilised on the test strip and give a coloured reaction.
  • These test kits are easy to use, provide quick results and are also effective in identifying asymptomatic patients.
  • The results from this method, however, need to be confirmed using a more advanced test as there is a risk of getting false positives.
  • Many such tests are available globally, but not in India.
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