This dynamic is a large part of why there have been so many allegedly asymptomatic carriers of the virus: 1) a “confirmed case” was defined by the CDC as anyone who tested positive (CDC Interim Case Definition 2020); 2) however, with highly inaccurate tests and widespread testing of asymptomatic individuals, the large majority of “cases” seem to have been false positives (Braunstein et al. 2021 makes a similar point).
Figure 3 summarizes the false results for the two scenarios already discussed, but in a range from 1-20% pre-test probability. False positives remain very high through 5% and higher background prevalence and false negatives remain low.
Figure 3. Summary of results: a) antigen test sensitivity of 58% and specificity of 99%; b) PCR test sensitivity of 80% and specificity of 70%. False positive results dominate in real-world low prevalence conditions.
a)