The authors of a 3 September editorial in the British Medical Journal offer a number of reasons why covid antibody tests, which focus on detecting IgC antibodies in the blood, might underestimate the percentage of people who have been infected:
- The test isn’t sufficiently sensitive to detect immune response to mild infection.
- The test fails to detect immune response to recent infection/recovery.
- IgC antibodies decline rapidly over time in recovered individuals, so the test might be administered too late to detect prior infection.
- IgA antibodies, the primary immune response on mucosal surfaces that the coronavirus attacks, are not measured in current testing methods.
- Saliva tests may be more sensitive than blood tests for detecting all varieties of covid antibody.
The authors don’t speculate as to the magnitude of the possible undercount. A fairly straightforward research project could throw light on the question:
- Administer diagnostic tests to a stratified random sample of the population, or to 100 percent of some well-defined subset of the population (e.g., incoming students at a university).
- Over subsequent weeks/months, administer antibody tests to those who came up positive on the diagnostic test.