My symptomatic, boosted SO received 4 negative nasal-swab PCR tests (1 day apart each), given by a nurse, before finally testing positive on the 5th test. Seems totally unreliable
When my son tested positive, they refused to let us re-test to see if it may have been a false positive. He's 2 and was completely asymptomatic.
She also thought it would be helpful to know if exposure = any additional immunity down the road, since all of the facts constantly change
There are other possibilities like that she had a cold and the 5th test was a false positive or she had a cold and got covid several days later.