diagram of swab test significance
1% Prevalence 35% Prevalence

1% Prevalence

Showing a low level of current infection in the general community and how this allows the nearly perfect (but not quite perfect) specificity score to produce enough false positives such that they outnumber true positives by 6 to 4.

35% Prevalence

This might be typical of a bad outbreak in a hospital or care home. Here the false negatives from the poor sensitivity contribute to a worsening of the true negative probability to approximately 86%. A repeat test might be required to provide reassurance to a concerned healthcare worker that they really are negative for Covid-19.