COVID-19: What is a false positive

Testing is important as it allows Hunter New England Health to identify individuals with the disease, and for that person to then isolate. This approach minimises the chance of a person with COVID-19 passing it on to other people.

In the last week we were told that a local person had tested 'positive'. This naturally resulted in concerns about a new case in our area, but then we found out about a day later that it was a 'false positive'.

Not surprisingly, people then started to ask how this can happen - surely we can tell if someone has the COVID-19 or not?

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Unfortunately, diagnostic testing is rarely 100 per cent accurate, and particularly in circumstances where it is desirable to get a quick outcome in order to prevent the spread of an infectious disease.

The common test for COVID-19 is the PCR test which uses a sample taken from a swab of either the back of your throat or nose. This sample is then checked to see if there is genetic material from the virus.

On face value, this seems very straightforward. However, if someone with COVID-19 is tested just after they become infected, or if their swab doesn't collect enough genetic material, it is possible that their sample may not register as positive.

This is called a 'false negative', where the test reports a negative result in a person who is actually positive.

There is also the chance that something may go wrong with the test itself. This could be where there is a positive reaction to other genetic material in the sample not from the COVID-19 virus, or when there is an error in the testing process, such as cross-contamination or even just a labelling mistake.

This could mean that person who does not have COVID-19 receives a positive test. This is called a 'false positive'.

Both false positives and false negatives are rare, but, as we have seen this week, they will occur when there are hundreds of thousands of samples being tested every day.

Testing positive to COVID-19 would be very stressful, and ideally no-one will end up in that situation if they don't actually have the disease.

However, in the context of COVID-19 it is better to have false positives than false negatives. The alternative is very undesirable - we do not want an individual with COVID-19 moving around our local area without even realising they are potentially spreading the disease.

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