COVID-19 testing has been on the rise – with more and more tests being developed and more testing sites available. Many are drive-thru and walk-in test sites, and anyone is able to show up and get tested. There are so many different types of COVID-19 testing (PCR testing, antigen testing, and antibody testing) via different routes (nasal, nasopharyngeal, oral, saliva, venous blood sample, and fingerpricks), that patients often don’t know which one to have done and what a positive or negative test result means. Although COVID-19 testing to identify individuals who are infected is crucial in containing the spread of the virus as well as understanding this pandemic, knowing the differences between all these different tests is equally important to ensure the correct test is done in each situation.
There are two main types of COVID-19 testing – viral tests (to test for active infection) and antibody tests (to test for the immune systems response to an infection).
Viral testing tests for the active infection. There are two main types of viral tests – polymerase chain reaction (PCR) and antigen testing.
PCR is a technique that looks for the presence of viral RNA by amplifying the virus’s genetic material, making it easier to detect. Most of these tests involve a nasal, nasopharyngeal, or oral swab. These tests are highly accurate, with sensitivity and specificity rates of almost 100%. PCR tests, however, require specialized lab equipment and reagents, and trained personnel. As such, these tests are costly to perform and time-consuming. Currently, many labs are backlogged, and results could take up to 12 days.
Antigen testing, which detects specific proteins on the virus’ surface, is another technique to test for the active infection. Antigen testing requires the sample to have high enough amounts of viral proteins to yield a positive test, which makes these tests less accurate than PCR. Although antigen testing is highly specific, it is less sensitive than PCR. This means, if a patient tests positive on the antigen test, you can be almost certain the patient is infected. If the patient tests negative, however, the possibility of a false negative must be considered. There are numerous benefits of antigen testing: They are easy to perform, can be done with a nasal swab, yield test results in approximately 20 minutes, and is much cheaper than PCR.
Antibody testing, also called serological testing, identifies exposure to SARS-CoV-2 (the virus which causes COVID-19) by looking for antibodies generated by the immune response. Most SARS-CoV-2 antibody tests detect IgM and/or IgG. IgM is the first antibody the body builds when fighting a new infection and may indicate you are still infected or recently recovered. IgG, on the other hand, take 7 to 10 days to develop, and indicate you have previously been infected and recovered. While the venous blood draws will often measure specific antibody titers and are much more accurate, rapid fingerprick antibody tests are also available. Since antibody testing detects the immune response to an infection, there are limitations to the effectiveness of antibody testing in diagnosing COVID-19. At the start of an infection, especially in the first day or two, the body may not have developed sufficient IgM antibodies to be detectable in peripheral blood. Individuals who exhibit very mild to no symptoms, including asymptomatic carriers, may not mount a sufficient immune response to generate detectable levels of antibodies. Furthermore, some individuals who are immunocompromised may not build enough antibodies and therefore lead to false negatives. Lastly, there are so many different rapid tests available. Accuracy varies widely.
When to use each test?
Viral testing, including PCR and antigen testing, is required for identifying individuals who currently have active infection and can spread the virus to other people. Viral testing is required when a patient is symptomatic and wants to know whether or not their symptoms are due to COVID-19. When there is high suspicion for COVID-19, PCR testing should be done, as it is both highly sensitive and specific for the virus. Antigen testing is an alternative method to testing for the virus, and is ideally used when PCR testing becomes backlogged, or there is a need for rapid and frequent testing, such as in-school classrooms.
Antibody testing is done when you want to know if you had been infected in the past. Antibody testing is required if a patient was sick in the past and wants to know if it was COVID-19. It is especially useful from a public health perspective, in identifying the prevalence and exposure rates of COVID-19 among a population. It also helps identify individuals who have recovered from the virus and may be eligible for convalescent plasma donation, which is blood donation to donate antibodies which may be useful in treating severely ill COVID-19 patients.
There are numerous different COVID-19 tests available, and understanding the differences among each type of test is crucial in utilizing the correct test in the correct situation. All these tests, including PCR, antigen, and antibody testing, are important and play different roles in fighting this pandemic.
Christine Lau is a physician.
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