ROC curves showing the ability to determine infectivity (as assayed by viral culture in Vero E6-TMPRSS2 cells) using the CT values of WHO-E, Wölfel-sgE, and Mills-sgE for each of the samples for which these values were determined. For each test, the true-positive rate (chance of correctly predicting samples to be culturable at a given or lower CT) was plotted against the false-positive rate (chance of incorrectly predicting samples to be culturable at a given or lower CT) as a step function line. The lines show performance of the tests as the (CT) threshold for a positive result is increased; the lines are solid for detectable values and dashed when a primer set returned an undetectable value. Each line is labelled with the lowest CT value that surpassed a 95% sensitivity threshold (the horizontal dotted line) for that marker. 