Dr James Gill, Locum GP & Honorary Clinical Lecturer, Warwick Medical School, said:
“There has been a lot of swirl about the timelines with regard to antibody and PCR tests. Let’s clear up what is being looked for. As with everything about coronavirus and science in general, the more you simplify, the more get’s missed. So let’s try to add a little detail back in, so people can understand the PCR and antibody investigations as more than black and white tests.
“With PCR testing – here you are look for DNA or RNA evidence of an infectious agent – in the case of Coronavirus, we’re looking for RNA. If you take a test-tube filed with a virus – no patient here – and run a PCR test on it, the test will come back indicating the RNA sequence of what is in the tube. We match that sequence to known sequences of viruses, which then tells us “Yes you’ve found a virus and the sequence matches to this virus”. That is obviously a pure lab test example. When used in practice, we are taking a swab from a patient, hoping that if they have the corona virus infection, that enough of the virus is picked up on the swab for the PCR test to be able to detect the virus. As the swab needs to pick up enough of the virus to work, this is one of the reasons the coronavirus the PCR test can have a higher false negative test rate – i.e. missing the virus even though someone has the infection.
“On the other side of the fence we have the newer antibody blood test. The antibody test needs to be run on a sample of blood from a patient. If you were to put a sample of the virus directly on the antibody test you will get no result. The antibody test is not looking for the virus, but evidence of the virus being in the body, and the body reacting to it by producing antibodies to help fight it off. Not all antibodies are the same, nor created at the same time over the course of an infection – and here is the complicated bit with the antibody test – WHEN do you do the test? The antibody IgM is the bodies first response to an infection – normally within 5-10 days of an infection taking hold, peaking at 21 days after the infection. That time frame is crucial, if you have just developed symptoms you think are coronavirus, it will take approximately a week for your body to raise IgM antibodies to the virus. Now whilst with coronavirus we do have evidence of IgM being presenting the blood within 1 day of symptoms, but that isn’t going to be a reliable test at that stage, as the likely won’t be large amounts of IgM for use to detect. The best test for an early infection is combining the antibody test AND the PCR swab taken from the patient. Then we have a 98.6% detection rate within the first 5.5 days of infection – https://www.jwatch.org/na51255/2020/03/31/serologic-tests-sars-cov-2-first-steps-long-road
“When tested alone, the PCR test has a 66.7% detection rate within the first week, whilst the antibody test has a lower 38.3% detection rate.
“But what about the other antibody, IgG, also detected by the new Coronavirus antibody test? IgG shows that someone has had the virus and is now protected from the virus, this can be detected in a patients blood ~10-14 days after infection. Perhaps one way of looking at this is the IgM is the fast reaction force, whereas IgG is the main, slower, but more powerful army, which acts to keep the body safe afterwards as well. What if a patient were to be exposed to coronavirus again – not infected? Once a patient has cleared the infection, the IgG, the army now knows it’s target and can be quickly mobilised again. A huge volume of IgG is raised to combat the virus within 24-48hrs of reexposure and will then hopefully prevent a new infection. Thus if a patient is found to have IgM, that suggests that they may be within the first week of an infection. If they have BOTH IgM and IgG, that suggests they will be within the first month of infection, and should hopefully be protected from repeat infection.”