Uncertainty remains in the connection between SARS antibodies and immunity. Our recent research findings – which concur with a REACT-2 study by Imperial College London – demonstrate antibody levels are falling in individuals who had COVID-19. Individuals may be incorrectly informed they are negative for SARS-CoV-2 antibodies due to the high cut off levels for some antibody testing kits.
We have grown increasingly concerned about antibody tests with arbitrary cut-off values. These tend to group people into those ‘with’ versus those ‘with no’ antibodies and can therefore indicate incorrect results.
If antibody presence is no longer detected with an antibody test, such as the Fortress lateral flow immunoassay used in the REACT-2 study, it does not mean that people have lost immunity nor that they are susceptible to reinfection. The possibility of reinfection remains uncertain. It may be possible that the signal cut-off for this kit is too high. Consequently, people are being told they do not have antibodies when they do – just at lower levels. We are concerned that individuals who were asymptomatic and were not diagnosed with COVID-19, may have subsequent issues receiving appropriate healthcare.
At Richmond Research Institute, we recently tested people with a commonly used qualitative antibody test. We found some individuals known to have been infected with SARS-CoV-2 who had signal cut-off values below 1.4. According to the manufacturer guidelines, these patients would be declared negative for SARS-CoV-2 antibodies. Because of the lower-than-cut-off results, these individuals are told that they do not have any antibodies to the virus. However, there are many people who fall between 0.01 and 1.4.
The reason the threshold signal cut-off is too high could be that the assay was validated in patients with highly symptomatic COVID-19. The existing threshold is therefore unsuitable for all. This seems to be a common issue with many COVID-19 antibody tests.
To avoid false negatives, these tests should be fully validated alongside quantitative methods in patients with diverse symptoms. It is inappropriate to label individuals negative based on these tests alone. Any predictions regarding long-term immunity to COVID-19 are not possible unless proper quantitative tests are used.
Testing for COVID-19 is crucial to understand who is infected and therefore a risk to others by spreading the infection. We continue to perform polymerase chain reaction (PCR) and antibody tests for staff and visitors to the clinic. View our findings on the levels of antibodies over time.
Our results suggest that individuals who have previously been infected with SARS-CoV-2 achieve markedly higher antibody titres than those who have yet to show signs of infection.
We are delighted to extend the testing opportunity to support the wider community.
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