The tests are not perfect: sometimes, people who have the disease will be wrongly told that they do not
Some people require more than one test because of false-negative outcomes
The number of COVID-19 tests carried out will be similar to the number of people tested, but they won’t be quite the same, because some people may need to be tested multiple times. The reason for this is that there are “false-negative” test outcomes.
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A “false-negative” outcome is when someone is tested and found to be clear of the disease, but when tested again, are found to have it. The WHO’s guidelines for laboratory testing of COVID-19 say that negative results “do not rule out the possibility of COVID-19 virus infection.”
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This means that even in countries that have done lots of tests, the true number of COVID-19 cases is still uncertain, although of course more testing still means more certainty.
There haven’t been many studies into how common false negatives are, so it’s hard to know how big an impact they have on our understanding – but research is going on.
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Why might COVID-19 tests fail?
There are several reasons why someone infected with COVID-19 may produce a false-negative result when tested:
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- They may be in the early stage of the disease with a viral load that is too low to be detected.
- They may have no major respiratory symptoms, so there could be little detectable virus in the patient’s throat and nose.
- There may have been a problem with sample collection, meaning there was very little sample to test.
- There may have been poor handling and shipping of samples and test materials.
- There may have been technical issues inherent in the test, e.g. virus mutation.
The WHO suggests that these issues should be taken into account and that for some people, tests should be carried out several times.
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