On Twitter I’ve seen lots of discussion on the sensitivity and specificity of tests for SARS-CoV2 (both diagnostic and antibody tests) but not a lot of discussion around the positive and negative predictive value of these tests (PPV and NPV). Previous discussions about sensitivity/specificity vs PPV on this site have argued that PPV and continuous risk measures should be prioritized for clinical decision making. With this in mind, why is there not more discussion about the PPV of these tests?
My initial thought is that we currently do not know prevalence (lots of testing studies underway), so it would be hard to calculate PPV. If so, is there a time when we should start thinking about the PPV of these tests and how PPV statistics factor into clinical and policy decisions?