Since I have written so much about the issue of the utility of PCR testing recently, I thought I would include a particularly useful chart from one of the studies. This chart depicts the number of cycles versus the percent of culture positivity. Culture positivity is the gold standard for determining viability of virus being shed. You can see the rapid dropoff around 30 cycles. From what I have been told, Minnesota for example, may be using a cycle number as high as 38. People could reasonably disagree about trying to pick up every possible infectious case versus being more cautious, given the consequences, for example in regard to closing a school.