The PCR test amplifies RNA fragments. If after a certain amount of replication cycles the amount of detected RNA crosses a threshold, it is considered a positive.

A positive PCR test does not imply infectiousness. Many people who have a positive test are not just asymptomatic, they are not really infected to begin with.

A negative PCR test does not rule out disease. Some people who have a clinical diagnosis of COVID may produce a negative PCR result.