Difference between Nucleic Acid and Serology tests for detection of COVID-19
The world is suffering from a COVID-19 pandemic which is caused by the SARS-CoV-2 virus and the lock-down and social distancing has caused serious disruptions in the living and working conditions of people globally. As of the beginning of June 2020, there have been more than 6 million confirmed cases and more than 380,000 deaths. The impact on the world economy due to shutdowns and restrictions due to travel bans has been palpable. The need for rapid and effective means of testing to indentify and stop the spread of the disease is more important than ever. Many universities, research labs and companies are trying hard to come up with new methods of testing and effective test kits to rapidly identify the virus in infected individuals and help stop the spread of virus by making the affected individuals to self isolate.
Figure 1 shows one of these rapid test methods that was deployed in South Korea and was very effective in helping to flatten the curve in that country
Figure 1: COVID-19 rapid testing in South Korea
When it comes to testing for COVID-19, there are two basic methods of testing as follows:
· Nucleic acid based tests
· Serological tests
The nucleic test is the gold standard for testing COVID-19 and detects the virus directly (Reference 1). The serological test on the other hand, tracks the spread of the virus in the population by detecting the antibodies produced against the virus. This could help in understanding how the virus has spread in the population and assesses the true extent of the spread of COVID-19. Up until a vaccine is found, rapid and accurate testing is the most effective way of stopping the SARS-CoV-2 virus to spread any further.
The article attempts to describe the details of each method of testing along with different kinds of tests which exist in each category
Nucleic Acid based Test:
An assay is an investigative procedure in laboratory medicine, pharmacology, environmental biology and molecular biology for qualitatively assessing or quantitatively measuring the presence amount or functional activity of a target entity. Molecular assays for detection of viral nucleic acids include the following tests:
Reverse Transcription –Polymerase Chain Reaction (RT-PCR)(Reference 2). The technique relies on amplifying a small amount of viral genetic material in a sample and is considered to be a gold standard for detection of SARS-CoV-2 virus. The samples can be collected from serum (Yellow plasma in the blood), stool and eye discharges.
Although the RT-PCR is considered to be the most widely accepted method of detection, it has a number of disadvantages. These are listed below:
· The need for very expensive instrumentation
· The need for highly skilled personnel to do the testing
· It will take a few days for the results to get ready
Due to these factors, many labs and research institutes around the world are working hard to improve the efficiency of this test and come up with new methods for faster and cheaper testing. Figure 2 shows the basics of the RT-PCR.
Figure 2: The basics of RT-PCR (Reference 3
As depicted in figure 2, in the first step, a sample swab is taken from the upper respiratory tracts of an individual. The single stranded RNA extracted from the sample is then purified and converted into double stranded DNA. The produced DNA is then amplified through several thermal cycles and is mixed with a fluorescent dye. If the result is positive, the fluorescence from the amplified DNA will be visible and confirms the presence of SARS-CoV-2.
Isothermal nucleic acid amplification
The RT-PCR method requires thermal cycling and sophisticated thermal cycling equipment. Therefore the results of the test could take days to be ready. For this reason another test called the isothermal nucleic acid amplification test has been designed that does not require the thermal cycling and can produce the results faster. Several methods based on this principle have been developed and one of these methods called Reverse Transcription Loop-Mediated Isothermal Amplification (RT-LAMP) eliminates thermal cycling and does the amplification of DNA at constant temperature. The technique only relies on heating and visual inspection and makes it a promising method for virus detection (Reference 2)
Serological tests as mentioned at the beginning of the article, detect the anti-body and can be used to monitor the progress of the disease through different stages. It can be used to broaden the identification of past infections and immunity. The samples can be collected from saliva, sputum and other biological fluids (Reference 2). There are two types of anti-bodies which are called IgM and IgG. IgM becomes detectable at the initial stages of the disease and last for a couple of weeks. It will then turn into IgG. IgM can be an indication of early stage infection and IgG can be an indicator of current or prior infection. The most important serology test is Enzyme-Linked Immunosorbent Assay (ELISA) and is described below.
Enzyme-Linked ImmunoSorbent Assay (ELISA)
One can describe ELISA as a microwell plate based method designed for qualitative assessment and quantitative measurement of several substances such as proteins, hormones and anti-bodies (Reference 2). It takes between 1 to 5 hours to obtain the results with this method. Figure 3 shows a microwell which is a flat plate with several wells used as small test tubes. It has become a standard tool in analytical research and clinical diagnostic testing laboratories
Figure 3: Microwell used for clinical diagnostic testing
When ELISA is used to detect SARS-CoV-2 the following steps has to be followed.
· The plate wells are coated with a protein which acts as an antigen. Antigens are parts of a pathogen that alert the body to an infection.
· Patient samples taken from the body’s liquid discharges are then applied to these microwells. If antiviral antibodies are present in the sample, they will bind the protein coating.
· The bound anti-body- Protein combination can be detected with a tracer antibody using a colorimetric or fluorescent based observation. In colorimetric methods a color change is observed whereas in fluorescent methods a fluorescent emission is detected.
All these steps are shown in Figure 4.
Figure 4: ELISA methodology (Reference 2)
ELISA is a fast method and several samples can be tested at the same time. It can be automated for increased efficiency and can be used for bedside testing.
Allied Scientific Pro offers test kits based on nucleic acid test and the following link contains all the required information.
2- Assay techniques and test development for COVID-19 diagnosis, Linda, J. Carter et.al, American Chemical Society, 2020,6, 591-605.