Explained: Why rapid tests are off, again
On Monday, the Indian Council of Medical Research (ICMR) put an end to the question of rapid tests when it told the states that the newly procured kits for such tests had failed quality checks and should be returned to the suppliers.
This effectively brings the curtains down on a long conversation on rapid testing between the states and the ICMR, at least for the time being. A look at the tests, how the conversation started and where it stands internationally.
What is a rapid test?
A rapid test essentially is a test that takes less time than the 8-9 hours that the standard RT-PCR test result takes. In the Indian context, a rapid test means the serological tests with kits that India had imported from China, which have now failed quality tests.
The difference between the two procedures is that the RT-PCR test looks to find the virus in nasal or throat swabs of the patient, while the serological test usually looks for antibodies in the blood, which the body develops to fight the virus. Blood has two components, a matrix called plasma and the blood cells. The plasma minus the components of whole blood that cause it to clot, known as serum, is examined in a serological test.
According to the Johns Hopkins Bloomberg School of Public Health, “Serology tests are blood-based tests that can be used to identify whether people have been exposed to a particular pathogen. Serology-based tests analyze the serum component of whole blood. The serum includes antibodies to specific components of pathogens, called antigens. These antigens are recognized by the immune system as foreign and are targeted by the immune response.”
The serological test is faster, taking less than 30 minutes, while the PCR-based test takes up to nine hours. The long wait time is also because the nasal or throat swab is first subjected to a screening test to decide the family of the virus; if that test confirms that it is a coronavirus, then a second test is done to ascertain if it is indeed the novel coronavirus SARS-CoV2.
The Health Ministry as well as the ICMR have reiterated several times that the “gold standard” for diagnosis is RT PCR, and the serological tests should be used only for the purpose of generating epidemiological data or for surveillance in hotspots.
Are there different kinds of rapid tests?
While these commonly look for antibodies developed against the virus, rapid tests (also known as point of care immunodiagnostic tests) can also seek to detect the presence of the virus itself. More specifically, the latter kind of rapid test looks for viral proteins expressed by the SARS-CoV2 virus in a sample taken from the respiratory tract of a person. If the presence of the protein is detected, it is inferred that the virus is present too.
According to the World Health Organization (WHO): “There is another, more common type of rapid diagnostic test marketed for COVID-19; a test that detects the presence of antibodies in the blood of people believed to have been infected with COVID-19. Antibodies are produced over days to weeks after infection with the virus. The strength of antibody response depends on several factors, including age, nutritional status, severity of disease, and certain medications or infections like HIV that suppress the immune system. In some people with COVID-19, disease confirmed by molecular testing (e.g. reverse transcription polymerase chain reaction: RT-PCR), weak, late or absent antibody responses have been reported.”
This is the test kit that India had imported.
What has India’s stand on rapid tests been, and how has it evolved?
Kerala was one of the earliest states to announce rapid tests. States like Tamil Nadu and Chhattisgarh too pushed the ICMR for a direction to follow the “South Korea-China model” of rapid tests, until the ICMR gave in earlier this month. On April 4, the ICMR said rapid tests could be used in areas reporting clusters (containment zones) and in large migration gatherings/evacuees centres.
However, after the kits rolled in from China, complaints started pouring in from the states. On April 21, while asking the states to suspend the tests for a few days pending field testing, Dr R R Gangakhedkar, head of epidemiology and infectious diseases at the ICMR, said: “We have been getting complaints that when the serological tests are being done on the blood samples of people who have already tested positive in the RT-PCR test, the accuracy ranges from 6% to 71%. This needs to be investigated.”
He added: “This is a first generation test developed in just three-and-a-half months and needs refinement, yet the variations cannot be ignored. We will send personnel from eight institutes to the field to validate the kits. Meanwhile, we have advised the states not to use the tests for the next two days till we come out with an advisory.”
What has been the global experience for rapid tests?
Singapore was the first country to use rapid tests, while South Korea has been the standout example. In its containment strategy, South Korea aggressively used mass testing, including with serological kits, to reduce the number of cases. Experts say they used their experience of SARS and MERS to take quick and decisive action. While it is testing that has earned it global praise, it is also a fact that South Korea’s success was as much because of its hospital infrastructure that was already ramped up during the earlier outbreaks. Even in Daegu, where the country’s 31st patient became an index for some 1,100 contacts, hospitals were clogged but hospitalisation rates remained high.
The WHO has been cautious about rapid testing. In a scientific brief on April 8, it said: “However, before these tests can be recommended, they must be validated in the appropriate populations and settings. Inadequate tests may miss patients with active infection or falsely categorize patients as having the disease when they do not, further hampering disease control efforts. At present, based on current evidence, WHO recommends the use of these new point-of-care immunodiagnostic tests only in research settings. They should not be used in any other setting, including for clinical decision-making, until evidence supporting use for specific indications is available.”
The WHO brief covers rapid tests that detect both the virus and the antibodies developed against it.
