Testing is a critical element to the overall prevention and control strategy for COVID-19. As of July 2021, the United States has carried out the third most COVID-19 tests per 1 million population among the countries most severely impacted by the pandemic. The U.S. has performed roughly 526 million SARS-CoV-2 tests in total, according to the latest Statista reports.
Screening and diagnostic testing for SARS-CoV-2 are critical components to the overall prevention and control strategy for COVID-19. Understanding the differences between tests in terms of their intended use and performance is paramount to achieve positive health outcomes for the individual and the population as a whole. Here is a quick guide on how to distinguish screening from diagnostic testing.
COVID-19 Screening tests
According to the FDA fact sheet “Screening for COVID-19”, COVID-19 screening tests are used to identify individual infections in a group, even if there is no suspicion of a COVID-19 infection.
Based on the recent testing guidelines from the Centers for Disease Control and Prevention, screening tests are recommended for unvaccinated people to identify those who are asymptomatic and do not have known or suspected exposure to COVID-19. Screening involves the identification of unknown cases so that measures can be implemented to prevent further transmission.
Since every COVID test features a different turnaround time, their performance characteristics may vary. Based on preliminary BMJ reports, the accuracy of the tests is found by comparing their results with a gold standard, often using viral RNA detection with PCR testing when the symptoms occur. However, the sensitivity in PCR testing could be as low as 70%. The maximum sensitivity for combined Immunoglobulin tests (IgG & IgM) was 96% at 22 to 35 days after the onset of COVID symptoms.
However, according to Harvard Health Publishing, antibody tests have a 20% rate of false negatives. Test results may differ based on the time when the test is carried out. It takes several days for the human body to generate antibodies against the virus. Furthermore, the level of antibodies fluctuates in the next few months post-infection. Therefore, the range of false negatives can be anywhere from 0-30%. Although testing positive signifies a person was exposed to the pathogen, it may not indicate long-term protective immunity or lack of contagiousness.
COVID-19 screening tests are not used to diagnose the illness. They are meant to detect it early on and encourage those affected to change their lifestyle, explains the American Society for Microbiology.
COVID-19 Diagnostic tests
COVID-19 diagnostic tests spot the current infection. They are performed on patients with symptoms or signs of an infection. COVID-19 diagnostic tests or viral tests can also be proposed for asymptomatic patients that were exposed to the virus.
- NAATs (nucleic acid amplification tests) – featuring 80% sensitivity and 99% specificity, these tests are very accurate, research shows. If coronavirus prevalence is 20%, the false negatives rates increase with prevalence. These tests take 1 to 3 days for results. The turnaround time can take less than an hour to over a day.
- Antigen tests – with a specificity quite similar to NAATs, antigen tests are less sensitive and more affordable than NAATs. Their turnaround time is 15-30 min. However, their results are presumptive, stated the CDC. Roughly 70 out of 100 positive tests can be false positives, explains the FDA.
The US Centers for Disease Control and Prevention does not recommend the use of antibody tests to diagnose active infections. Antibody tests can detect antibodies to the COVID-19 virus following an individual’s infection or vaccination.
Comparison of Diagnostic & Screening Tests
COVID-19 diagnostic tests are employed to establish an absence (or a presence) of the pathogen. COVID-19 screening tests, on the other hand, are meant to detect the possible risk factors or predisposition for that pathogen. They can also be used to monitor the early stages of the disease, experts explain.
COVID-19 screening tests target a broader population. People are broadly selected and get tested, whereas diagnostic testing focuses on those who had a positive screening test. Diagnostic tests use invasive maneuvers but for a justifiable cause. Their results are definite and come at a higher price.
The screening is simpler and tailored towards the general public. However, it warrants further confirmation. So, even if a patient tests positive, they will need to get a definite diagnosis. The idea of a screening test is to spot potential cases. This can aid patients in avoiding or managing the illness.
For large-scale screening, antigen testing is the most suitable solution. Antigen tests can be utilized in sensitive areas for rapid screening, such as airports, schools, and workplaces. Thanks to their short turnaround time (approximately 15 minutes), rapid antigen tests can detect infected individuals, preventing the further spreading of the virus.
COVID testing is now more critical than ever. With the unpredictable development of COVID-19 epidemic “waves”, it is crucial to dispose of all the testing methods that best suit the individual’s specific scenario to stop and prevent the spread of the virus.
Each type of test has something to offer, but all the various testing strategies share a common goal of curbing the spread of COVID-19. The information listed here above reflects the state of research on the SARS-CoV-2 epidemic at the time of writing. In addition to rigorous testing and prevention measures, the advancement of scientific knowledge and research is equally as important to control the spread of COVID-19.
Post time: Aug-25-2021