Until last year, no one had even heard of them. But suddenly after the Covid-19 pandemic struck, RT-PCR and Rapid Antigen Tests were terms almost everyone worldwide was talking about.
Even today, not many people know what the RT-PCR stands for. How is it different from the Rapid Antigen test? And also what is meant by false-negative tests?
The Rapid Antigen Test (RAT)
The Rapid Antigen Test (RAT) means exactly that - results are available quick and fast in just 15 minutes. The test which can be done by any healthcare worker, detects viral proteins and reveals patients at the peak of the infection when the body has the highest concentration of these proteins. During the test, any SARS-CoV-2 antigens contained in the sample react with the antibody-coated nanoparticles contained in the reactive test strip. As a result, the blend migrates chromatographically by capillary action along the reactive strip. If the specimen contains SARS-CoV-2 antigens, these will bind to SARS-CoV-2 antibodies contained in the (T) Test line region and generate a coloured line on the test strip, indicating a positive result. Though RAT is much cheaper, it will only indicate the presence of SARS-CoV-2 antigens in the specimen and should not be used as the sole criterion for the diagnosis of SARS-CoV-2 infection.
The more effective Covid19 testing method recognised by the World Health Organisation (WHO) and the US Centers for Disease Control and Prevention is the real time Reverse Transcription Polymerase Chain Reaction Test (RT-PCR test). This test is an extremely powerful diagnostic tool as it allows for a very small sample of DNA to be amplified to a large enough amount to make a detailed study. This requires a professional lab set up and hence is more expensive. The results also take two to three days.
How does RT-PCR work?
Since the coronavirus is an RNA virus, a sample swab, either a nasopharyngeal or oropharyngeal, must be sent to a laboratory to go through several steps to convert the RNA into viral DNA (reverse transcription).
Then the PCR test takes a small amount of the DNA and amplifies specific sequences of the genetic material to diagnose an acute infection. The test is very specific for the virus. If a patient tests positive, it’s highly likely that the patient has Covid-19.
However, many patients are getting false-negative results – that is they are testing negative even though they are showing the symptoms. This is because the test results are dependent on various factors. Some experts feel that inadequate sampling, improper transport and late swabs may miss the SARS-CoV-2 virus in RT-PCR tests.
For example, the more the time gap between when the patient had symptoms and when he is tested, the more likely the test result will be false-negative. If a patient is >10 days since they have had symptoms, the false-negative rate goes way up.
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The accuracy of the test can also depend on where the sample is taken - whether it is from the naospharynx (nasal) or oropharynx (oral) cavity - the deep nasopharynx swap appears to be the best location.
Some are of the opinion that the virus did not colonise the nasal or throat cavity due to which swab samples taken from these areas didn't yield a positive result.
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What about the new mutants of the virus?
Earlier this year the US Food and Drug Administration (FDA) had alerted clinical laboratory staff and health care providers that false negative results may occur with any molecular test for the detection of SARS-CoV-2 if a mutation occurs in the part of the virus’ genome assessed by that test.
The FDA recommended clinical laboratory staff and health care providers who use molecular tests for the detection of SARS-CoV-2 to:
• Be aware that genetic variants of SARS-CoV-2 arise regularly and false negative test results can occur.
• Be aware that tests that use multiple genetic targets to determine a final result are less likely to be impacted by increased prevalence of genetic variants.
• Consider negative results in combination with clinical observations, patient history, and epidemiological information.
• Consider repeat testing with a different test (with different genetic targets) if Covid-19 is still suspected after receiving a negative test result.
To find the Indian Council of Medical Research (ICMR) list of Covid-19 testing labs click here.