Coronavirus Disease 2019 (COVID-19) is a contagious respiratory virus caused by the novel (new) strain of severe acute respiratory syndrome coronavirus (SARS-CoV-2). Symptoms may appear 2-14 days after exposure to the SARS-CoV-2 virus. Since signs and symptoms of COVID-19 are difficult to distinguish from the flu and other viruses, testing is required to diagnose a current or past infection.1 There are two testing categories – diagnostic (molecular and antigen tests) and antibody tests. 2
Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) is a gold standard molecular diagnostic test that detects the virus’s genetic material (RNA) from the respiratory tract. RT-PCR is a highly complex laboratory method that allows the viral RNA from a small sample to be converted into DNA and amplifies a specific target to measure the amount of virus present in the sample.3 The specimen sources that may be collected from patients experiencing symptoms include nasopharyngeal and/or oropharyngeal swabs, saliva, sputum, or bronchoalveolar lavage. Nasopharyngeal or oropharyngeal specimens maybe collected from asymptomatic patients. The turnaround time for results to become available will vary based on the testing location but can take 1-3 days. A positive result indicates that RNA from SARS-CoV-2 virus is present and the patient is likely to have COVID-19 and presumed to be contagious. These results do not predict the severity of the symptoms or indicate a past infection. A negative PCR result can either indicate the absence of COVID-19 infection or a low viral load below detectable limit. Poor specimen collection or an early infection can produce false negative PCR results.4 Molecular tests are highly accurate and do not need to be repeated.
Antigen tests are rapid point-of-care (POC) that are used to detect the SARS-CoV-2 viral protein in a respiratory sample. The specimen source includes nasopharyngeal or oropharyngeal swabs and results are usually available within 15-20 minutes. An antigen test has 100% specificity and 84.0-97.6% sensitivity, which means that there may be false negative results.5 A positive result indicates an active presence of SARS-CoV-2 protein in the sample. However, a negative antigen test result, does not rule out an active COVID-19 infection and outcomes should be interpreted in conjunction with clinical symptoms. A confirmatory test (molecular test) is recommended prior to making a medical diagnosis. 2,6
Finally, antibody tests detect antibodies (IgM and IgG) produced by the individual’s immune system in response to SARS-CoV-2 virus 7-14 days after COVID-19 infection.7 The antibody test is not used to diagnose an active COVID-19 infection. A blood sample is collected from a vein or a fingerstick for testing and results are reported within 1-3 days.2 A positive antibody test indicates that the individual was previously infected with SARS-CoV-2 and that there is immunity to the virus. However, it does not provide information on the duration of the immunity. A negative antibody test result can mean one of three things: (1) there was not exposure to the SARS-CoV-2 virus; (2) there was exposure to the virus, but the immune system has not yet produced antibodies; or (3) there was exposure to the virus, but the level of antibodies is below the detection limit.8
Because this is a novel virus, much about its infectivity, its effects, and the immune system’s response to it remain unknown. Additionally, many of the rapid tests on the market were “fast-tracked” via the Emergency Use Authorization (EUA) process at the Food & Drug Administration (FDA) and are being found to have poor sensitivity and specificity. RT-PCR remains the gold standard in testing for SARS-CoV-2, but is highly complex and time-consuming to perform; many labs have also been challenged in acquiring the necessary reagents for it as well. Needless to say, clinical laboratories will have an intimate role to play as the world continues to battle this virus.
References:Preview (opens in a new tab)
- Centers for Disease Control and Prevention. Symptoms of Coronavirus. CDC.2020. Available at: https://www.cdc.gov/coronavirus/2019-ncov/symptoms-testing/symptoms.html
- S. Food and Drug Administration. Coronavirus Testing Basics. 2020. Available at: https://www.fda.gov/consumers/consumer-updates/coronavirus-testing-basics
- S. Food and Drug Administration. LabCorp COVID-19 RT-PCR Test EUA Summary. 2020. Available at: https://www.fda.gov/media/136151/download
- S. Food and Drug Administration. Fact Sheet for Healthcare Providers. 2020. Available at: https://www.fda.gov/media/134920/download
- Centers for Disease Control and Prevention. Interim Guidance for Rapid Antigen Testing for SARS-CoV-2. CDC. 2020. Available at: https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antigen-tests-guidelines.html
- COVID Antigen Testing: An Imperfect but Practical Strategy. AACC.2020. Available at: https://www.aacc.org/publications/cln/cln-stat/2020/september/3/covid-antigen-testing-an-imperfect-but-practical-strategy
- Farnsworth CW, Anderson NW. SARS-CoV-2 Serology: Much Hype, Little Data. Clinical Chemistry.2020;66:7. Available at: https://academic.oup.com/clinchem/article/66/7/875/5826351
- Larman HB. Coronavirus Testing: What is an Antibody Test?. Johns Hopkins. 2020. Available at: https://www.hopkinsmedicine.org/health/conditions-and-diseases/coronavirus/coronavirus-testing-what-is-an-antibody-test