News last night that two of Melbourne’s most concerning cases of COVID-19 were actually false positives caused Victoria’s overall case numbers to go backwards yesterday.
False positives in COVID-19 tests are rare — and experts say their discovery is proof Australia’s testing system is robust — but they can happen when tens of thousands of tests are being taken on a given day.
Here’s how they happen, and why.
What is a false positive test?
The vast majority of COVID-19 tests conducted in Australia return a negative result.
But very occasionally a test returns a positive result when it shouldn’t, because the person tested was not infectious with COVID-19.
That can lead to cases being reported when the person in question has not in fact been infected.
“False positives and false negatives are inevitable. They do happen and they will continue to happen,” Dean Whiting from Pathology Technology Australia said.
“But we rely on the expertise of our laboratorians and on the technology to be able to protect these areas and prevent them from happening too often.”
The two false positive cases in Victoria were of particular concern because they had been described as “fleeting contact”, suggesting a high level of infectiousness in the variant currently active in Melbourne.
How does a false positive test happen?
Mr Whiting, a trained clinical biochemist who heads Australia’s peak pathology technology body, said PCR tests — which usually involve a swab of the nose and throat — were the most common tests in Australia.
“In the case of PCR testing for COVID, there’s two general buckets into which false positives fall,” he said.
“The first is the lab error, a mix-up of samples or a contamination of samples somewhere between the collection of the sample and through to the measurement.
“The second bucket is a little more complex.
“PCR is extremely sensitive and will pick up remnant RNA from the virus many days, or even months, after the virus has resolved and is no longer infectious.
He said there were different methods of administering tests — including testing saliva, the back of the throat, and the nostrils, each of which had different positivity rates.
How frequent are false positives?
The rate of false positives is low for PCR tests, and Mr Whiting said the fact these false positives were detected meant the system was working.
“Australians can have utmost confidence in the testing technology and the laboratories doing the testing,” he said.
“In fact, it’s because of the quality systems in the laboratories that we probably found these two false positives.”
Acting Victorian Premier James Merlino said the cases were identified through “a lot of testing and retesting and examination of the cases, of their circumstances and making sure we have got the right information”.
“There is the rare occasion where you get a false positive or get a false negative. That has been the case all the way through.”
Don’t expect it to change Melbourne’s lockdown
Despite the announcement of the false positives, the news has not impacted the timeline of Victoria’s lockdown, which is set to last until next week.
The health advice for Victoria still required a lockdown, Mr Merlino said, noting that other “fleeting contact” cases remained genuine, and that the infection threat was real.
“There have been a number of other cases where we have seen fleeting contact and transmission,” he said.
“The proposition put forward by public health was that we needed this further seven-day period for Greater Melbourne to absolutely run this thing to the ground. That remains the case.”
Authorities are also concerned about the emergence of the Indian variant known as the Delta strain, which has not previously been noted in the Australian community.
“It has obviously had very high transmission potential, because it spread extremely rapidly across India to become the predominant variant, almost the exclusive variant there,” Chief Health Officer Brett Sutton said.