I have continued to dig into the COVID-19 antibody testing issue as this will likely remain of concern for the foreseeable future.
A few weeks ago I posted about concepts you should consider when getting any test in general and the COVID-19 antibody test in particular.
I did not want to get too deep in the weeds about the statistical concepts of “sensitivity” and “specificity” so I lumped them together in the examples I used in my screencast and assumed that the sensitivity and specificity were the same for the example test. They can be the same, but in reality they are usually different.
Briefly a tests sensitivity is related to a tests “false negative” rate and “specificity” is related to a tests false positive rate. The more sensitive, the less false negatives. The more specific, the less false positives.
Forget sensitivity for a moment. Focus on specificity. For an antibody test you want to minimize false positives because that would give the patients a false sense of security: “yea! I have antbodies, I’m free to frolic!”
In my screencast I showed that tests accuracy AND the prevalence of the thing you re looking for with the test (antibodies) both matter when assessing whether a test will give you good information or not.
Low prevalence of antibody in a population and low test specificity stack the odds against you. High prevalence of antibody and high test specificity stack the odds in your favor.
I reviewed a paper that examined the effectiveness of many COVID-19 antibody tests. There are now many of these antibody tests on the market.
Encouragingly the authors discovered that a number of the tests came in with specificity over 95% and even 99%. Another one I saw claimed to have specificity of 99.75%. Pretty damn good.
Running the numbers I found that if your test specificity is 99% and the antibody prevalence in your population is about 5% then your odds of false positive is about 17%. Not perfect. But it improves as you get closer to 100% specificity and a higher population antibody prevalence.
Bottom line: There are highly specific tests now on the market. If you are considering getting the COVID-19 antibody test ask about their test specificity.
I would personally reconsider getting an antibody test if the lab could assure that specificity of their test ran 99% or higher (preferably higher).