PCR tests detect traces of the coronavirus in mucus samples collected from a person. But, many factors impact whether or not test successfully detects the virus:
First, it depends on how much virus the person is shedding whether through sneezing, coughing or other secretions.
Second, the way the sample was collected is important as it should be done appropriately by someone who is used to collecting these swabs.
Additionally, the test results are affected by how long the sample sat in transport. A major problem lies in locating the virus within the body as its area of highest concentration shifts with time.
The main nasal swab tests examine the nasopharynx. This is where the back of the nose meets the top of the throat. Swabbing requires a trained hand to successfully perform. A portion of the false negatives arises from improper swabbing procedures.
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But, even if swabbing is done correctly by an expert the swab might produce a false negative. That is because the coronavirus passes from the upper to the lower respiratory system as the disease progresses with time.
Studies about PCR test reliability are still considered preliminary as the new coronavirus has only been spreading among humans for 4 months. Reports from China suggest that the current PCR test's sensitivity is around 60 and 70%.
Different companies around the world are now producing slightly different tests which make the task of having a precise figure even harder. The risk of getting false negative remains high as the number of people tested is growing day after day.
Experts say that with 40 million people, even if 1% of the population was tested, 20,000 false-negatives should be expected.