COVID-19 testing includes analyzing patient samples to determine current or past infections with SARS-CoV-2. Read the article to know more.
As of now, the tests that are commonly done to diagnose COVID-19, the infection with the new Coronavirus SARS-CoV-2, are polymerase chain reaction (PCR) and antibody testing. Both these methods have their advantages and limitations and use different kinds of samples to search for different markers of the SARS-CoV-2 virus. Sadly, neither of them are perfect.
With such a high demand for testing and quick diagnosis of COVID-19, scientists are trying hard to find alternative ways to screen patients with the deadly disease.
Ideally, during any infectious disease outbreak, public health officials begin testing for it early. This leads to the quick identification of cases, prompt treatment, and immediate isolation to prevent spread. Testing also helps to identify people who came in contact with infected people early so that they do not infect others unknowingly.
At present, we are not in this ideal situation with COVID-19, as testing remains critical. In most countries, testing has been insufficient for optimal early containment. In countries like South Korea and China, as tests were readily available when the first few cases were being reported, they have managed to drastically lower the death rate compared to other countries, such as the UK and the US. A lack of early testing has resulted in a rapid rise in hospitalizations, overburdening the healthcare system. These countries are now bracing for more and more critically ill patients in the coming months.
Apart from diagnosing an active COVID-19 infection, the other important test is the one that determines previous COVID-19 infection. When we get infected with a virus, our immune system fights the virus and protects us by producing antibodies. These antibodies can be detected with the help of blood tests. Even though the individual might not be infectious any longer, the information about past infections can be extremely valuable. Information about immunity to the new Coronavirus can be obtained from this. Also, plasma from such recovered patients can be used to treat others with an active infection (theoretical as of now).
1) RT-PCR Testing:
Presently, the majority of COVID-19 reports are coming from RT-PCR (reverse transcription-polymerase chain reaction). PCR (polymerase chain reaction) amplifies or replicates a small segment of the DNA several thousands of times, making it easy to be detected. RT-PCR first converts viral RNA into DNA, then amplifies that DNA. As SARS-CoV-2 is an RNA virus and contains RNA, RT-PCR can help detect them. This test requires a few hours. The detection of viral RNA is only possible when the virus is present in the body, and someone is actively infected. Viral RNA is present in the body before antibodies get formed or signs and symptoms develop. As a result, this test will help diagnose a person much early.
The likelihood of this test detecting the virus depends on the collection method and the time passed since infection. Throat swabs are only recommended in the first week, as the virus moves deeper into the respiratory tract and multiplies in the lungs, and abandons the throat. During the second week, sputum collection is preferred. The samples can be obtained by throat swabs, nasopharyngeal swabs, sputum, or saliva. Nasal and throat swabs are 63 % sensitive to RT-PCR, the pharyngeal swab is 32 %, and sputum is almost 75 %.
PCR is useful when it comes to identifying individuals early in the course of infection. Such individuals can be isolated, and their contacts (people they came in contact with) can be quarantined too. It helps in breaking the transmission chain.
RT-PCR can be labor-intensive, and as is it such an extensive process, errors can happen anywhere from sampling to analysis. 30 % of the time, the reports can be a false negative.
2) Antigen Test:
The part of the virus that makes the body produce antibodies is called an antigen. SARS-CoV-2 antigen test looks for antigen proteins in the virus’s surface spikes. These antigens can be detected before the onset of symptoms, but this test is less sensitive than the PCR test. But, as the PCR test takes up so much time and is expensive, the antigen test is the way to scale up testing to a much greater level.
Here, samples are collected through nasopharyngeal swabs or saliva. The collected sample is then exposed to strips that contain artificial antibodies. The antigen present in the sample will bind to the coronavirus antigens on the strip and give a visual result.
This takes less than 30 minutes. No expensive equipment or training is required.
Throat and nasal swabs of respiratory viruses usually lack detectable viral antigen. This is more commonly seen in asymptomatic patients, as they have little to no nasal discharge. As the viral antigen is not replicated in antigen testing, the available antigen might be too little to get detected.
As the antigen test is only reliable if the viral load is high in the body, various doctors do not recommend getting it. At the same time, others argue that people with high viral load are highly infectious, and by mass screening, such individuals can be identified.
3) Antibody Tests:
This test detects the antibodies produced by our body to the viral antigen. Serology or blood tests can detect such antibodies. Antibody tests might not tell if the person has an active infection, but it can be used to assess the percentage of a population that has already been infected. As scientists still do not know the potency of antibodies produced against SARS-CoV-2, a positive antibody test might not imply immunity to reinfections. It is also not clear if mild or asymptomatic patients produce sufficient detectable antibodies. The types of antibodies test that can be used to identify antibodies against SARS-CoV-2 are-
Rapid diagnostic test (RDT).
Enzyme-linked immunosorbent assay (ELISA).
Antibody testing will help identify the number of people in a population who are immune to the virus. Such immunity will stop frequent outbreaks of the new Coronavirus within the population. This is called herd immunity. In case someone gets infected, the virus will not be able to spread to others as long as others are immune.
More testing is needed to identify who is infected and who needs to be isolated. Countries like Singapore, South Korea, and Germany seem to have had a better course of the pandemic as compared to other countries that do not have such a high testing capability. Testing, both PCR and antibodies, is vital and will provide more information to tackle this pandemic.
Last reviewed at:
06 Nov 2020 - 4 min read
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