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As specified in this link, COVID-19 tests start with taking samples from the nose or back or throat of people.

Based on growth rate of this disease, I thought what if those samples are not positive (showing infection with COVID-19), but that person has that virus in other parts of his body, because he's in the early stages of the infection.

So is it possible to be infected with COVID-19 and have no virus in nose and throat, and if it's possible, how long would it take to be certain that a sample shows the reality and a negative answer can be trusted?

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You are searching for the false-negative rate of these tests. A test resulting in a negative outcome while it should have been a positive outcome.

A recent article stated that most of the false-negatives of COVID-19 tests are actually caused by not properly taking the samples from nose and throat. While I cannot find the original article I've got this information from, I've added two other relevant quotes. So yes false-negatives are possible but are likely caused by samples that are not properly collected.

The rollout of COVID-19 testing is accelerating as more kits become available. However, a leading pathologist who was director of virology at Stanford, points out that the results are not 100% accurate. "It's not that these tests can't detect virus," said Dr. Bruce Patterson. "My concern is that the sampling involved in detecting the virus can lead to clinical false negatives." - https://abc7news.com/6053940/

Also, the false-negative rate can vary based on the test method used and its quality and accuracy.

COVID-19 tests are new, and assessing their accuracy is challenging. PCR tests may produce false negatives, failing to identify evidence of SARS-CoV-2. Sometimes false negatives result from human error or problems with the procedure. Giving the test too early or late, for example, can lead to a false negative. - https://www.medicalnewstoday.com/articles/coronavirus-testing#accuracy

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  • "actually caused by not properly taking the samples from nose and throat." Because that's the normal area of entry for the virus?
    – Don Branson
    Mar 30, 2020 at 18:47
  • @DonBranson That's a whole other question. According to the WHO: Current evidence suggests that a person can be infected when aerosols or droplets containing the virus are inhaled or come directly into contact with the eyes, nose, or mouth.
    – Bob Ortiz
    Jul 22, 2021 at 13:59
  • Thanks. I'd forgotten I had asked this question - it's been a while. It's not a question for me at this point, though.
    – Don Branson
    Jul 22, 2021 at 14:40
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The SARS-CoV-2 virus infects cells that possess the ACE2 receptor but this is described as being present mainly in pneumocytes lining the alveoli, vascular endothelium, cardiac tissue, and small bowel luminal cells. The virus appears to preferentially attack well differentiated cells. There are also some ACE2 containing cells in the respiratory epithelium. Nevertheless virus can be detected from nasal and oropharyngeal swabs indicating infection there. The tests themselves are only about 75% sensitive which suggests there is not a high concentration of virus in the upper respiratory tract，though it may also relate to the time the swab is taken as greatest viral shedding in the upper airways occurs in the pre-symptomatic phase.

So a positive test may relate to when it was taken, and test sensitivity.

A deep sputum test is best but dangerous to obtain as large amounts of virus are exhaled or coughed into the environment endangering medical staff.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1287568/

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