According to a review of existing studies, COVID-19 PCR tests have a false negative rate of at least 20%. This means that 20% of people who have the new coronavirus receive a negative test result. 

The research team — from Johns Hopkins University in Baltimore, MD — analyzed seven studies that included data from 1,330 patient samples. The team found that the rate of false negative results depended on the time since infection.
The authors estimate that 4 days after acquiring the virus, a staggering 67% of people will have a false negative result. This rate is similar at 3 weeks.
Around 8 days post infection, which is approximately 3 days after the average time that symptoms start, is the most accurate time to test. However, around 20% of people will still have a false negative result.

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