Why should I test and what test options are available?
Early diagnosis of coronavirus disease 2019 (COVID19) is important for both infection prevention and patient management.
There are two broad types of tests available, each described in more detail below. The first test type is based on detecting part of the genetic material of the virus (PCR) or proteins associated with the virus (Antigen Tests) and the second test type aimed at detecting the body’s immune response to the virus (Antibody Tests). These different detection methods impact how soon after infection the test can become positive and how accurate these results are.
The graph below (IOGP-IPIECA Health Committee) shows the different detection methods and timeframes.
Testing for the SARS-Cov2 virus by detecting RNA (PCR)
PCR tests (to detect SARS-CoV2-RNA) are used to verify whether a suspected case carries the virus, even while asymptomatic, and has high specificity for COVID-19.
The window period for a PCR test ranges from 3 to 17 days post exposure and infection. Evidence suggests that testing tends to be less accurate within three days of exposure when virus levels are still below the detection limit or not in the sample even when taken correctly. It can take almost a week after exposure to COVID-19 to register a positive test result. The best time to get tested is five to seven days after you were exposed.
• PCR tests are the best diagnostic method, particularly in the early phase of a viral infection. Symptoms may start at an average of 5 days after exposure (for those that develop symptoms) with 97.5 % of people who develop symptoms that will have developed them within 11.5 days. A patients can be infectious two days before symptoms show.
• PCR testing is a valuable tool to test close contacts of confirmed cases in the workplace and can be used to shorten quarantine times prior to starting work from the regular 10 to 14 days to less than a week without significantly increasing the risk of missing an infected person when compared to the risk at the end of a 14-day quarantine period. According to the CDC, if you got tested on the fifth day after exposure or later and the result was negative, you can stop isolation after seven days.
Antigen Tests detect the presence of viral proteins (antigens) expressed by the COVID-19 virus in a sample from the respiratory tract of a person. The antigen(s) detected are expressed only when the virus is actively replicating; therefore, these tests are best used to identify acute or early infection (up to 5 days from symptom onset). Antigen tests are highly specific, but not as sensitive as RT-PCR. This means there is a higher chance of false negatives. Antigen test performance also depends on factors such as the time from onset of illness, the concentration of virus in the specimen, the quality of the specimen collected from a person, how it is processed, and the precise formulation of the reagents in the test kits.
Overall, in alignment with WHO, use of virus antigen tests is not recommended for diagnosis of SARS-CoV-2 infection. Antigen tests with >95 % specificity and >80 % sensitivity may be used for diagnosing infection with SARS-CoV-2, where no nucleic acid amplification tests are available or have prolonged turnaround times.
Antibody tests to detect IgM and IgG are quick and easy to perform and scalable. Limitations apply to the antibody tests as they may have high rates of false negative results due to the immune response only appearing days after someone already has high levels of virus in their body. Additionally, antibody tests are also unable to differentiate between active disease and the post disease stage. Due to variable test quality (specificity and sensitivity) and a lack of understanding of the extent of immunity (if any) to COVID-19 after having recovered from the illness makes plans to use these tests to identify those who are immune to COVID-19 premature. The test window to detect IgM antibody is typically day 7 to 21 and IgG after day 14.