The Indian Council of Medical Research (ICMR) recently approved ELISA test kits for Covid-19 developed by two companies. These were the first ELISA test kits approved other than those that use the ICMR’s own technology, and add to the basket of choices for testing for Covid-19. A look at these choices:
Developed in 1974, ELISA stands for enzyme-linked immuno-sorbent assay. It detects whether a person’s immune system has produced antibodies against a particular infection — such as HIV. The test is called “enzyme-linked” because it uses enzymes to detect presence of antibodies in a blood sample.
An ELISA test is of two types depending on the antibodies tested for — immunoglobulin G (IgG) and immunoglobulin M (IgM). “IgG detects antibodies developed in later stage of infection, and IgM detects antibodies produced in early stages of infection,” said Dr Sujata Baveja, microbiologist with Sion Hospital. Currently only IgG testing kits have been approved in India.
In India, the ELISA test for Covid-19 is only approved for serosurveys— which estimate the proportion of the population exposed to infection— and for surveys in high-risk areas and segments like containment zones, immunocompromised individuals, and frontline and health workers. “Depending upon the level of seroprevalence of infection, matching public health interventions can be implemented for prevention and control of the disease,” the ICMR says.
In May, ICMR designed a Covid Kavach ELISA IgG test through the National Institute of Virology in Pune. Seven companies are manufacturing kits using this technology in India — Zydus Cadila, Meril Diagnostics, Voxtur Bio, Trivitron Healthcare, J Mitra & Co, Karwa Enterprise, and Avecon Healthcare.
Earlier this month, ICMR approved ELISA kits for Covid-19 testing designed by Transasia Bio Medicals (based in Mumbai) and Euroimmun US Inc. “We can produce 3 crore kits in a month,” said Suresh Vazirani, Transasia managing director.
While ELISA is expected to relatively inexpensive and is fast, its use is limited to making population-based estimates that can inform policy decisions. For individual diagnosis and treatment of Covid-19, the test used worldwide is RT-PCR (reverse-polymerase chain reaction). Earlier it was also used for Ebola and Zika diagnosis.
In India, RT-PCR remains the final confirmatory test for Covid-19. The test involves taking swabs from the nasal and oral tracts, extracting the viral RNA in a printer-like machine and amplifying it to detect SARS-CoV-2, the virus that causes Covid-19.
RT-PCR is expensive, while free in government labs. Until May end, the ICMR had capped the cost of a test at Rs 4,500 for private laboratories, but has since removed the cap for private labs, allowing states to fix their own prices.
ICMR has evaluated 97 kits of various manufacturers for RT-PCR testing, of which hem 40 have so far been approved.
Apart from nasal or oral swab, another option for RT-PCR is the bronchoalveolar lavage (BAL) method, which a bronchoscope is passed to obtain fluid from lungs or sputum. Sputum or BAL has a higher viral load, so there is a higher chance of virus detection than nasal or oral swab.
Rapid antibody test
This, too, looks for antibodies in the blood, takes hardly 20-30 minutes, and is the cheapest. But a rapid test involves a high risk of false results — it may detect antibodies against of some other infection and show that the sample is positive for Covid-19. Hence this test is only used for population surveys. If a person tests positive through a rapid test, he has to undergo a confirmatory RT-PCR test before treatment. ELISA is more accurate than a rapid test.
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Rapid antibody test involves taking a blood sample from the finger and putting it in a testing template. It cost Rs 600. Plasma or serum can also be used to test instead of blood.
The ICMR has evaluated 46 rapid testing kits, and approved 14 so far. Eleven of the manufacturers are based in India. According to ICMR, a person may test positive for antibodies 7-10 days after contracting Covid-19 infection and may continue to show positive results for several weeks. While a positive result indicates the person has been exposed to the coronavirus, a negative result may not entirely rule out Covid-19
“Rapid test kits are useful for giving results within 30 mins but the results are not 100% reliable as they can give false positive results. Though the antibody tests on ELISA… take a little more time than rapid tests to give results, they have better specificities as the reagent formulation is different… Hence these are more reliable in diagnosis than rapid tests. Probably this is the reason ICMR has not yet approved use of rapid tests in diagnosis yet,” said Dr K Krishnan, Technical Expert, Ortho Clinical Diagnostics, which recently got ICMR approval.
This is a privately designed test that works on the same principle as RT-PCR, but with a smaller kit and with faster results. TrueNat, designed by MolBio Diagnostics Pvt Ltd, Goa, is commonly used for tuberculosis and HIV testing. Recently, the ICMR approved TrueNat for screening and confirmation for Covid-19. If a sample test negative, it has to be treated as negative; if it tests positive, a second test called RdRp gene confirmatory assay has to be performed.
The TrueNat machine is small and portable, mostly running on batteries, and provides result within 60 minutes. It involves taking nasal or oral swabs. Across India there are over 800 machines to test for TB; hence the government will not have to invest further in machines.
When to use which
To understand which test to use, the purpose has to be defined. A person may test positive in these tests at different point of time during an infection. After the person is exposed, the viral load may be high in the respiratory tract within a few days, and an RT-PCR or TrueNat test may return positive. But if the person has not developed antibodies, both a rapid test and ELISA will return negative. In a few days, say over a week later, antibodies start getting produced, at which point RT-PCR may show negative but ELISA and Rapid will show positive.
To diagnose and treat, doctors rely on RT-PCR, which implies active infection. Once confirmed the person has to be isolated, and treated if symptoms emerge.
A positive result from ELISA or rapid antibody test may not mean the person needs isolation or is infectious; it may simply mean the person was exposed to the virus and has developed antibodies. These two tests, being cheaper than RT-PCR, are employed in large-scale population surveys. According to Dr Archana Patil, Additional Director in the Directorate of Health Services, Maharashtra, rapid or ELISA tests are tools that only show how widespread the infection is.
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