If you experience cough or fever with breathing discomfort or any other symptoms, the only way to find out whether you are infected by Covid-19 is to get tested. Several diagnostic test manufacturers have developed rapid and easy-to-use devices for Covid-19 testing. The sampling procedures include –
- Swab test – A special swab is used to take a sample from the patient’s nose or throat
- Nasal aspirate – A saline solution is injected in the nose, and a sample taken with light suction
- Tracheal aspirate – A thin tube with a torch (bronchoscope) is inserted into the mouth reaching the lungs to collect a sample
- Sputum test – Sputum (a thick mucus accumulated in the lungs) is collected in a special cup, or a swab is used to take the sample from the nose
- Blood test – A blood sample is taken from a vein in the arm.
Covid-19 testing methods
The novel Covid-19 diagnostics methods fall into two main categories – a confirmatory test for the infection, or an antibody test for the past presence of the virus. The first category includes molecular assays for detection of SARS-CoV-2 viral RNA using polymerase chain reaction (PCR)-based techniques.
The second category comprises serological and immunological assays to detect antibodies produced by infected individuals. These two testing methods serve overlapping purposes in the management of the Covid-19 pandemic. The FDA has issued Emergency Use Authorization to several devices to diagnose, treat, and prevent the spread of Covid-19.
Confirmatory tests for Covid-19
Polymerase chain reaction (PCR) – A polymerase chain reaction is an amplification technique for cloning the targeted parts of a DNA sequence to generate thousands to millions of copies of DNA of interest. Abbott Diagnostics, Atila BioSystems, Cepheid Sherlock Biosciences, Roche Molecular Systems, and others have developed molecular diagnostic tests used to detect viral genetic material in SARS-CoV-2.
Bosch has developed a fully automated rapid test for Covid-19 that can help medical facilities make fast diagnoses. The rapid molecular diagnostic test runs on the vivalytic analysis device from Bosch Healthcare Solutions.
Reverse transcription-polymerase chain reaction (RT-PCR) – This technique uses reverse transcription to convert RNA into DNA and then uses PCR to amplify the specific DNA, creating enough to be analyzed. RT-PCR tests for Covid-19 generally use samples collected from the upper respiratory system using swabs. Although RT-PCR is one of the widely used methods for detecting Covid-19 infections, it requires expensive laboratory instrumentation, skilled technicians, and more time to generate results. Several companies and laboratories worldwide are working to improve the efficiency and timeliness of RT-PCR technologies and developing various other techniques.
Isothermal amplification assays – Isothermal nucleic acid amplification tests also amplify the Covid-19 Virus’s genome. This test is faster than PCR. The DNA detected using fluorescent tags is read out with specialized machines. Mammoth Biosciences and Sherlock Biosciences are exploring the possibility of using CRISPR gene-editing technology for the detection of SARS-CoV-2. They are expecting the resulting test to be cheap and easy to use.
Antigen tests – Antigen tests detect antigen proteins from the viral surface. A nasopharyngeal swab is exposed to paper strips containing artificial antibodies designed to bind to coronavirus antigens. The test takes less than 30 minutes to deliver results and does not require expensive equipment or extensive training.
Many scientists raise questions on the reliability of antigen tests because respiratory viruses often lack enough antigen material to be detectable. According to WHO, half or more of infected subjects might be overlooked by such tests. The FDA also states that the positive results from antigen tests are highly accurate, but there are chances of false negatives.
Antibody tests for Covid-19
Antibody tests can be used to estimate the once-infected people who have recovered and are now assumed to be immune. The novel Covid-19 antibodies’ potency and protective period are still unclear.
Serological tests analyze blood serum or plasma, saliva, sputum, and other biological fluids to detect the presence of immunoglobulin M (IgM) and immunoglobulin G (IgG) antibodies. This test plays a vital role in epidemiology and vaccine development, as it provides an assessment of trajectories of antibody response, abundance, and diversity.
According to the FDA, Covid-19 IgM antibodies are detectable several days after the initial infection. IgG antibodies become detectable between 10 to 14 days after infection, or even earlier, and typically peak around 28 days after infection onset. Thus, IgM can be an indicator of early-stage infection and IgG of current or prior infection.
Abbott Core Laboratory, Advagen Biotech, BioMedomics, Creative Diagnostics, SD Biosensor, Epitope Diagnostics, and others have developed serological and immunological tests to detect viral protein or antibodies to SARS-CoV-2 virus.
Rapid diagnostic tests (RDT) – RDTs typically use a small, portable, positive, or negative lateral flow assay that may be carried out at the point of care. RDTs may process blood samples, saliva samples, or nasal swab fluids and produce colored lines to indicate positive or negative results.
Enzyme-linked immunosorbent assay (ELISA) – ELISA is a lab-based qualitative or quantitative test performed on whole blood, plasma, or serum samples. A plate is coated with a viral protein, such as the SARS-CoV-2 spike protein. Samples are incubated with the protein, allowing any antibodies to bind to the protein. The antibody-protein complex can then be detected with another wash of antibodies that produce a color or fluorescent readout.
Neutralization assay – This lab test observes whether the sample (blood, plasma, or serum) antibodies prevent viral infection in test cells. The test culture cells allow viral reproduction. By varying antibody concentrations, researchers can visualize and quantify how many test antibodies block virus replication.
Chemiluminescent immunoassay – This is a quantitative lab test. The test blood, plasma, or serum sample is mixed with a known viral protein, buffer reagents, and specific, enzyme-labeled antibodies. The resulting chemical reaction produces light and that radiance (wavelength) is used to calculate the number of antibodies such as IgG, IgM, and IgA.
Currently, the urgent need for accurate and rapid diagnosis of Covid-19 infection remains critical. Serological and immunological testing, particularly of infected asymptomatic and symptomatic individuals, is in high demand. However, questions remain to be addressed, including the degree of sensitivity, specificity, and correlation with immunity to the virus of serological tests.
Significant progress continues in the development of diagnostic tests. Several institutions and companies are working on more accurate and faster diagnostic solutions. With the help of collaborative work and technological advancements, Covid-19 diagnostic methods are likely to develop further.