Antibody testing update 6th August 2020
When an individual has COVID-19, their immune system responds by producing proteins called antibodies that can attack the virus. Detection of these antibodies in the blood can therefore indicate that they have had COVID-19. This differs to the PCR swab tests for antigens (part of the virus) which are used to detect current COVID-19 infection.
Scientists still don’t know what an ideal immune system response to COVID-19 looks like. As we know, it’s possible that merely having antibodies isn’t enough to prevent reinfection by the virus. Plus, the antibodies may also gradually disappear from someone’s system over time leaving that person vulnerable to COVID-19 again.
Our immunity to seasonal flu tends to last for about a year, however, it’s still unknown whether COVID-19 will be the same, longer, or shorter. We cannot currently say when we’ll be able to confirm the strength or duration of any potential immunity, as detailed clinical studies are required in order to provide the necessary data.
The latest Cochrane review
A most recent Cochrane review (a British organisation internationally recognised as the highest standard in evidence based healthcare), which brought together the best available evidence, gathered data up to April 2020 on COVID-19 antibody testing.
A key message from the findings was that timing was important for detection of antibodies -testing too soon could miss cases. When tests of the antibodies were done between 1 and 7 days from when symptoms started, they correctly identified only 30% of people who had COVID-19. This rose to 70% at 8 to 14 days and 90% at 15 to 35 days. They reported that there was not yet enough evidence to determine the accuracy of the tests beyond 35 days.
The review also determined that a small percentage of tests may have incorrectly diagnosed individuals as having had COVID-19 and also missed some cases. The review authors gave the following examples. If 1000 people had antibody tests, and 50 (5%) of them really had COVID‐19 (as might be expected in a national screening survey):
- 58 people would test positive for COVID‐19. Of these, 12 people (21%) would not have COVID‐19 at all (a false positive result)
- 942 people would test negative for COVID‐19. Of these, 4 people (0.4%) would actually have had COVID‐19 (a false negative result)
The limitations of COVID-19 testing
When looking at any data or review, limitations need to be considered. The review authors were not able to compare the accuracy of different tests and the data represented only a small proportion of the available antibody tests. Most of the studies in this review evaluated seriously ill patients in hospital, so the researchers could not determine whether the tests can detect lower antibody levels associated with milder and/or asymptomatic COVID-19 disease.
Reasons for COVID-19 antibody testing
From a public health perspective, antibody testing is useful for tracing patterns of disease spread and for evaluating public health measures.
On a personal level, those with a positive antibody test should still consider themselves at risk of infection and still take the usual precautions, as we do not yet know that recovery from COVID-19 gives any protection from being infected again.
In time, we will learn whether having previously had the infection provides individuals with immunity to future infection, and this is likely to be the key to the personal value of having this test. Our reviews will be updated as we gather more patient data from well reported studies, and views on the accuracy and value of antibody testing may change and become clearer.
If you are considering having a COVID-19 antibody test, please contact one of our GPs for a consultation to discuss your individual circumstances and considerations. We do not endorse the provision of any medical tests without personal clinical consultation and oversight, which many services are currently providing.
Do also read our post about the currently available evidence about the “Abbott test”, which we provide and is approved by Public Health England.
Dr. Sidra Malik BMBS MRCGP DRCOG DFSRH
