Decoding COVID-19 tests: How are they different, who needs them and when to get them done?

By Shriya Roy

In the past few months, all of us have heard about the multiple tests that have been doing the rounds to detect the SARS-CoV-2 virus. ‘Rapid test’, ‘antigen test’ and ‘antibody test’ have become common terms in conversations today, but are only adding to the confusion, as information about these is incomplete and the options for testing keep changing rapidly. So what are these tests and how are they different from each other? Who needs them and when do you need to get them done? We clear the air.

Tests for Covid-19 fall into two categories: diagnostic or molecular tests (such as RT-PCR) and antigen tests, which detect parts of the SARS-CoV-2 virus. Then there comes the antibody test that senses molecules that people produce when they have been infected by the virus. Antibodies can take several days to develop after an infection and often stay in the blood for weeks after recovery. So antibody tests have limited use in diagnosis.

The diagnostic PCR test can not only detect whether someone is infectious, but it also detects people who have the virus, but are not likely to spread it. Then come rapid tests. According to health experts, any of the above tests that can be done in 30-60 minutes without using any automated machine can be classified as rapid tests. They are simple, do not require great expertise to perform and are easy to interpret. These can be performed in the field or at primary healthcare centres. The test uses a mucus sample from the nose or throat and can be analysed at a doctor’s clinic. The result can be available within a few hours. The test is called ‘rapid’ since the results can be furnished in just about 15-30 minutes.

With such fast results, though, certain questions regarding its accuracy might arise in people’s minds. “As per the guidelines, while a positive rapid result is reported as confirmed positive, a negative report needs further clinical and diagnostic evaluation for final confirmation. The sensitivity of the test, as per kit literature, is 84.38%,” says Ami Varaiya, laboratory medicine coordinator and consultant microbiologist at Nanavati Hospital in Mumbai.

Laxman Jessani, consultant doctor of infectious diseases at Apollo Hospital, Mumbai, adds, “Rapid antigen tests have a high percentage of around 30% false negative. The positive results are usually highly accurate, but negative results need to be confirmed with a molecular test.”

The next question that needs addressing is how is one to know which test they are supposed to take? In India, according to the guidelines laid down by the Indian Council of Medical Research (ICMR), any person with history of contact with a Covid-19-positive patient (but asymptomatic) can undergo the antigen test.

If the test is negative, it can be confirmed with a diagnostic molecular test. Any person who is symptomatic with Covid-19-like symptoms should definitely undergo a molecular test, say experts. “The objectives of opting for an antigen or antibody test are different. At the same time, one should opt for the tests after considering the symptoms, existing comorbidities, status of the infection and exposure to infected individuals,” says Varaiya.

There are certain guidelines for people who need to get the tests done, says Sweta Shah, consultant of microbiology and infection prevention at Kokilaben Dhirubhai Ambani Hospital, Mumbai. She segregates people who need the antigen test and those who need the antibody test into two categories. “Those who need the antigen test include patients with fever, dry cough, body ache and breathlessness or anyone in contact with a Covid-positive patient. The test can also be done before travelling, admission to any healthcare facility or surgery,” she says. On the other hand, the antibody test, she says, can mainly be done before travelling, admission to a healthcare facility, surgery or for testing a person for plasma donation.

There has also been a growing demand for rapid test kits among people and organisations to maintain precautions. However, one must remember that very often the rapid antigen tests are less reliable, with an accuracy rate in some cases being as low as 50% (in India). In the UK, the most common type of rapid test has an error margin of 20% for giving false negative results and was originally meant to be used in virus hotspots and healthcare settings. A person taking the test might need to take it a couple of times for it to give the correct result. Furthermore, the test can only detect if an individual is infected at the time of taking the test and has no relation with past infection.

The next question that arises is are follow-up tests required? And if so, which ones and how many? Health experts believe that it is always advisable to do a follow-up antigen test despite of the result just to be doubly sure. One can also do a molecular RT-PCR test after the antigen test to confirm or rule out the presence of the SARS-CoV-2 virus.

As for the antibody test, Shah says, “Rapid antibody tests can be followed up by eCLIA, CMIA or ELISA, which are automated technologies for detection of antibodies and can provide semi-quantitative value. The value can determine the approximate quantity of antibodies present in blood that can be extremely useful for plasma donation.”

The ICMR has also put down some further guidelines and an advisory, allowing on-demand Covid-19 testing. The medical body has also left it to individual states to modify the approach as per their discretion.

In terms of cost, the antigen test kit used in India, which has been developed by South Korean biotechnology firm SD Biosenso, costs about Rs 450. For antibody tests, most state governments have fixed the cost between Rs 250 and Rs 300.

One must, however, remember that Covid tests are evolving every day. New tests are being put to use by different countries to get the most accurate results, but an absolute accurate test is yet to be discovered.

Who should get which test done

Rapid antigen test/RT-PCR test

• People with fever, dry cough, body ache and breathlessness. Some may have vomiting and diarrhoea only
• People with any of the above symptoms plus if they have had contact with a Covid-positive patient in the past seven days
• People working in high-risk areas like hospitals, etc
• People travelling, before surgery or those going to be admitted to a healthcare facility, etc

Antibody test

• To determine if a person has antibodies without any previous history of infection or contact with known positive patient
• For testing a person for plasma donation
• To give health officials a better idea of how common the virus is
• To find out how sick it makes most people