For demonstration purposes, an employee of the Centogene company takes a throat swab from a colleague at the airport in Frankfurt, Germany, Monday, June 29, 2020. In the future, people will be able to be tested for the corona virus within a few hours at Germany's first "Airport Corona Test" centre. (AP Photo/Michael Probst)
gslade@medicinehatnews.com@MHNGillianSlade
There is potential for a positive COVID-19 test to in fact be wrong – a false-positive – and there is a higher chance of that in people without symptoms.
At the end of May the health authority in Grey Bruce County Ontario reported five false positives out of a total of 3,000 tests done in long-term-care residences. All the false positives were from people who had no symptoms. They reported no false positives in people that had symptoms.
“False positives become apparent when testing large numbers of people that do not have symptoms,” says an online report from that health authority. They evaluate every test result and if a positive result is in question the original sample is retested along with two additional samples. If most of those come back negative “a false positive is declared.”
A couple of months ago there were a number of false positive tests linked to a laboratory in Ottawa, The Canadian Press reported at the time.
In South Korea a study suggested initially that some people were being “re-infected” with COVID-19 before discovering there were in fact false positive lab results.
Even a test that has an accuracy of 99.9 per cent, which is considered exceptionally good, there is still some potential for a false test result.
It has not been possible to get a number from Alberta Health Services, which is in charge of COVID testing, on how often there is false positive.
“We do not have a precise measure of the rates of false-negative and false-positive results, but expect that these are very rare. This is based on the evaluations that lab officials have carried out,” said Kerry Williamson executive director issues management for AHS.
He says the performance of the province’s public health laboratory equals or exceeds that of many other COVID-19 tests used in Canada and those that have received regulatory agency approval. No laboratory test is 100 per cent accurate.
“There are multiple factors involved in achieving accurate results within the lab including the quality of the sample collected or stage of disease when sampling occurred,” said Williamson. “If rare errors do occur and we are made aware of them, results are corrected, the root causes investigated, and corrective actions are taken to further mitigate the risk of such errors recurring.”
Initially in the pandemic Alberta was only testing people who had symptoms. When the number of positive cases in Brooks escalated AHS did some community wide testing that included people without symptoms. That was later done in Calgary as well.
On May 29, Dr. Deena Hinshaw announced anyone in Alberta qualified for a COVID test whether or not they had any symptoms. This continues.
If someone tests positive for COVID even if they have no symptoms they are not retested.
“Once someone tests positive, they are not normally re-tested. You are legally required to isolate for a minimum of 10 days if you have tested positive for COVID-19. This is the most effective use of our lab system and the best way to protect the health of Albertans,” said Williamson. “In addition, if someone tests negative, but has symptoms, they should also isolate for a minimum of 10 days.”
