In an April 5th memo, Professor Sir John Bell noting that no tests to date have performed well. The UK government testing strategy is intended to manage all aspects of Covid-19, including PCR tests on swabs from the nose or throat to detect the virus as well blood testing to detect people who might have had the virus and hence be protected with immunity.
These people would be able to go back to work without the fear of being infected again because they are now immune to
the disease. This combination provides citizens the information they and the health system need to manage those with the disease by giving access to rapid disease diagnosis and manage the population as a whole to ensure we will not put ourselves or others at risk while the virus is circulating.
Large scale testing will be crucial for getting us back to our normal lives in the coming months. One strand of the government strategy has been to use home testing kits to allow people to test and see whether they have long term immunity and hence can confidently go back to work.
Creating home test kits is, however, not easy. They detect antibodies in blood by capturing these antibodies on virus components held in the test kit. Blood flows from one end to the other, the anti virus antibodies are captured and a signal appears as a line on the test device. This can be read by the person at home or on their mobile phone camera. If there are no antibodies no line appears.
There are many challenges to creating accurate tests; hence these tests need to be validated carefully. There are 100 or more such tests kits from different suppliers available for identifying Covid-19 antibodies.
Crucially it is essential that the test does not tell you are positive for anti-Covid serology when you are not; otherwise you might return to work when you are not immune. Similarly, it is important that the test does not tell you that you are not immune when you actually are.
To validate these tests you need to obtain a range of tools:
- You need a gold standard test so you know the correct answer and you need sera from patients who have recovered from the virus infection they had approximately 28 days before.
- You also need blood from people who donated before the epidemic so you know whether you falsely see positive tests when there is no Covid-19 in the sample.
For example, there are a number of other coronaviruses circulating that might stimulate antibodies that cross react to Covid-19 proteins.
Multiple tests have been provided for evaluation, and a range of convalescent sera has been used to determine whether the tests can identify both low and high levels of antibodies. But the tests we have looked at to date have not performed well.
We see many false negatives (tests where no antibody is detected despite the fact we know it is there) and we also see false positives.
Everyone is having difficulty identifying commercial tests that work in a home test kit format. The Spanish apparently returned test kits that were not working, and the Germans who are developing their own sensitive kits believe they are three months away from getting these available and validated.
We will of course continue to look for a test that meets the criteria of an acceptable test.
Professor Sir John Bell, GBE, FRS is Regius Professor of Medicine Oxford University. Government Advisor on Life Sciences