There are concerns that the common RNA test may not correlate with presence of the active virus. From this disease course data, we can see the trend and calculate the correlation.
The virus RNA PCR test can detect on RNA segment from a billion others. This is done by cyclic amplification. After 35 to 37 cycles and there is no measurable specific RNA, the test is deemed negative.
When researchers compared this data to the actual measurement of viable virus, the data compared very well. A perfect correlation is R sqr. =1.0. In this case the R sqr. was 0.8899. This is very good considering measure live virus is rather difficult and fraught with error.
TAKE HOME: A negative PCR test means no viable virus are likely present. When a person tests positive, then negative and then positive again, it is not likely that virus are present once again. It may be just detecting viral RNA debris being coughed up. We need more research.