No gold-standard test approved to diagnose COVID-19
Antigen test is a cheaper and quicker method to diagnose COVID-19
Antigen test has high false-negative rates, low sensitivity, and low accuracy.
COVID-19 or Coronavirus Disease is a highly contagious disease caused by the SARS-CoV-2 virus. The drug regulator has not approved a vaccine yet. So, isolating the suspected or confirmed cases is an effective measure to prevent the disease’s spread. Nevertheless, the majority of SARS-CoV-2 positive patients remain asymptomatic or develop mild symptoms that are often mistaken as flu or a common cold. These patients who do not get tested are the potential carriers of viral transmission. Therefore, it is crucial to develop a diagnostic test with maximum accuracy to diagnose the presence of an infection in an individual. The U.S. Food and Drug Administration (FDA) did not approve any gold-standard test to diagnose the SARS-CoV-2 infection. However, the FDA has permitted the Emergency Use Authorization (EUA) of a few tests to diagnose the novel coronavirus.
Covid-19 Test: Rapid Antigen Test
Currently, there are three types of tests to diagnose SARS-CoV-2: Molecular test, rapid antigen test, and serology, or antibody test.
The rapid antigen test detects the protein fragments (antigen) from the virus in biological samples, such as saliva or tissue sample swabbed from the nasal cavity. The technique involved is similar to the pregnancy test and the rapid strep test. The results come within minutes. Health experts use the rapid antigen test to detect active infection. Much as it is quicker, cheaper, and easier than a molecular test, its accuracy is very low than the molecular test.
How is it used? A sample is collected using a nasal swab and mixed with a buffer solution, which enables it to move across the test strip. A few drops of the liquid mixture are spotted at one end of the test strip. A positive test generates a signal in the form of a color change.
There are several debates on the use of the rapid antigen test as a diagnostic tool for COVID-19. Indeed, there are mixed reactions from different researchers on its use, as they are skeptical due to its lesser efficacy when compared to molecular RT-PCR test. Studies reported that the false-negative results with the rapid antigen test were as high as 50%. So, the FDA disapproved of it as a single test for diagnosing active infection. Some researchers recommend repeating the test in those with negative results. The rapid antigen test has a sensitivity between 50-90%, which means that 1 in 2 infected people might be misdiagnosed as not having the infection.
Why is the Antigen Test less accurate than the RT-PCR test?
The antigen test does not involve amplifying the viral protein when compared to PCR.
The protein signal is diluted when samples are mixed with a liquid needed to make the sample flow across the test stripes.
Specimen collection and storage can also affect the accuracy of a test.
How can we improve the accuracy of the Rapid Antigen Test?
Protocols for the sample preparation must be revised to improve the rapid antigen test’s accuracy. A method that does not require dilation of the swab sample can enhance the sensitivity to around 90%.
According to the U.S. Food and Drugs Administration, a positive antigen test indicates the presence of SARS-CoV-2 infection. However, a negative test has a higher probability of missing an active infection. So, a negative antigen test should always be confirmed with a molecular test to rule out the disease.