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What's the difference between PCR and antigen testing?
PCR looks for the RNA of the virus, consequently, fragments of the RNA can be detected even after the virus is dead. Antigen tests look for the proteins of the virus.
Currently, all rapid Ag tests look for the nucleocapsid protein, which can be detected after the antigen (virus) has been broken down. The new BioSure test looks for the surface spike protein. This means the BioSure test will only detect LIVE VIRUS i.e. the person is currently infectious.
Why can a person test antigen positive but antibody negative before the 8 week antibody "drop-off"?
People who are asymptomatic may not produce detectable levels of antibodies due to very low levels being produced. A PCR test will give a positive result for fragments of the virus and it is possible that these fragments are present in the nasopharyngeal passage, where the swab is taken. The nasopharyngeal passage is the body’s first line of defence when presented with the virus, and it is possible the person fights off the infection without producing antibodies.
Is a person infectious if they test for IgA and/or IgM without the presence of IgG?
When can an antigen be detected?
PCR – can detect the RNA of the virus as soon as 24h after infection and provides a true positive up to approximately 5 days after symptoms. Viral DNA shedding (detectable by a PCR test) can exist for up to 30 days. BioSure Antigen test – detects only live virus and can detect from 2 days after infection until approximately 7 days (while a person is infectious).
What is the earliest antibodies can be detected?
This depends on the individual immune response and the level of infection
IgA 4 days
IgM 4 days
IgG 7 days
There are irregularities in the seroconversion across the population – some people can product IgG antibodies as part of their initial immune response (which is unusual). The test can only detect antibodies if they are present in sufficient quantities.
What is the point of testing for antibodies and not just antigens?
There is a limited window period to correctly detect antigen/virus. It is also critical that the swab sample is taken correctly, especially nasopharyngeal for PCR testing. With antibody testing it is possible to tell if a person is still infectious, even if they have received a negative PCR test (predominantly due to incorrect swabbing).
Antibody testing is the only way to tell if you have had COVID-19 infection. The drop off in antibodies after a number of weeks (which is unusual with coronaviruses) happens because your body initially is stimulated to make antibodies because of the presence of the virus.
Once the virus has been defeated, there is nothing to keep the ongoing stimulation of antibody production and therefore they decline after a period of time. This will differ from person to person – Ag window. For the purposes of track and trace, it is not only important to know who you could have transmitted to but also the source of the infection. The only way to define this is by antibody detection and triangulation of the results.
Why do some people take longer to develop antibodies than others?
This varies from person to person, just as all human bodies are different. Also, the amount of antigen/virus contracted and how long that takes to replicate before an immune response is initiated has an impact. All people are different.
What is the ideal testing regime?
Regular testing is important. Every time you are in a situation with other people, there is a risk of exposure. There are no guidelines to how often for PCR/antigen testing, other than if you think you have symptoms or know you have definitely been exposed. Fortnightly antibody testing means you can create an ongoing picture of your status to COVID-19 and if you have already had the virus, how long you retain detectable antibodies.
How long after symptoms should you test for antibodies?
You can test for antibodies from 7 days after infection. Because everyone’s immune response varies, if you get a negative result at 7 days and think you may have been infected, you should test again after 14 days.
Can an asymptomatic person test positive with an antibody test?
Yes an asymptomatic person can test positive for antibodies, but because of lack of symptoms (your body’s response whilst it is making the antibodies), you are likely to have lower levels of antibodies. The triple antibody test was independently tested in Liverpool against PCR positive/asymptomatic samples and correctly detected 83% of samples.
What difference does IgA make to the result of triple antibody test?
IgA antibodies form the body’s first specific line of defence. These antibodies latch onto the virus and help ‘killer’ white blood cells to kill the virus. Essentially since it was first demonstrated with HIV/AIDS, it is now generally recognised that in many virus infections, IgA class antibodies in the blood can be positive, even when IgG class antibodies cannot be detected. Tests that do not include IgA detection will miss those individuals who respond in this way that is, by making IgA antibodies and not IgG antibodies (studies show @7% of cases). If you get a positive IgA COVID-19 test result it indicates that you have been very recently infected and that your immune system is responding to the virus. It is likely you will still be infectious. IgA antibodies CANNOT tell you anything about immunity.