The worldwide death toll by Coronavirus has already crossed two lacs, still, researchers are trying hard to develop a vaccine that completely eradicate the virus. The rate of current test capacities is not rapid enough to diagnose every patient’s data. During this dark phase doctors as well as testing laboratories are trying hard to reach each infectious person. Heres, how coronavirus test works.
Presently the tests that are being used to identify coronavirus infection are known as PCR (polymerase chain reaction) tests. PCR tests have been used since the 1980s and have a range of applications including the diagnosis of infectious diseases. Hence these testing method is not new. PCR tests allow the copying of a small amount of DNA millions of times over so that there is enough for detection and confirmation of infection.
How the PCR test works
For testing a small sample of the swab is collected from a person’s back of the throat or nose and then it is stored properly for further laboratory analysis. It is essential to complete analysis within a few days of the sample taken. This swab test that the Government currently uses can only tell whether a person has the virus, not if they have had it and recovered. These swab tests takes longer time to show result. However new test methods are developing which can show the result within a few minutes.
The virus which causes COVID-19, SARS-CoV-19, does not contain double-stranded DNA, instead, it contains single-stranded RNA. As the PCR tests can only make copies of DNA, we need to convert the RNA into DNA first. The RNA of the virus is extracted from the patients’ swab than it needs to be purified from the human cells and enzymes which may interfere with the PCR test. The purified RNA virus is then mixed with reverse transcriptase enzyme which converts it into double-stranded DNA for the PCR test. The virus DNA is then added to a test tube to which the following are also added:
Primers: these are short sections of DNA designed to bind to characteristic parts of the virus DNA. Because of this they won’t bind to DNA which isn’t from the virus.
Nucleotides: these are the building blocks that makeup DNA.
A DNA-building enzyme: this makes copies of the DNA.
A sort of mixture is obtained which is then heated. After cooling the primer can bind with the DNA. This is how PCR amplifies the virus’ genetic code.
How the virus is detected
A type of fluorescent dyes are added to the test tube containing virus DNA. These dyes bind with the DNA which boosts their fluorescence, making them give off more light. This light shows confirmation of the SARS-CoV-19 virus. If the fluorescence crosses a certain limit above expected background levels then the test is positive. If the virus wasn’t present in the sample, the PCR test won’t have made copies, so the fluorescence threshold isn’t reached which means the test is negative.
Limitations of the PCR test
PCR tests seem to be a complex test however they are reliable and widely used to test COVID-19. Based on a lab’s facility and capacity the time for the test to show results may vary. It can take a few hours or days. Another limitation is the availability of reagents needed to run the tests. Some companies sell RNA extraction kits for this step of the process. Global demand for these tests in the wake of the pandemic has led to a shortage, limiting the number of tests carried out. This type of testing can only tell if someone has the virus at the time of testing. It can’t tell us whether they’ve had the virus but have subsequently recovered before testing.
What is an antibody or serological test?
Recently a new test kit has been developed that detects the power of the immune system of a person through blood analysis, it’s called antibody test. An antibody test detects if a person has had coronavirus before and has since recovered. The test, carried out by a device that pricks your finger for blood, works this out by testing your blood for coronavirus antibodies to see if they have already beaten the virus and gained some immunity to it. The test just takes 15 minutes to show the result.
What is an antigen test?
The antigen test detects the presence or absence of an antigen, in the body. An antigen is a structure within a virus that triggers the immune system’s response to fight off the infection. An antigen test is effective because it can take a few days for the immune system to build enough antibodies to be detected in a test, however, antigens can be detected almost immediately after infection. Hence this test can tell much faster whether someone has the virus. Antigen tests are also used to diagnose HIV, malaria, and flu.
Less number of testing means difficulty to track the spread of the virus. Hence people are advised to stay self-isolated until the testing capacity increase. Staying home until the situation is stabilized seems like a small price to pay for saving lives.