COVID-19 testing is solid despite Blackwater bungle
AFTER the death of a Central Queenslander who had falsely tested positive to COVID-19 post-mortem, much speculation has arisen surrounding the accuracy of testing and the fallout from a false positive result has rocked the small Central Queensland town of Blackwater.
However, a CQUniversity immunology expert says while there will be instances of false positives and negatives (the latter rarer), the COVID-19 testing program remains robust.
CQUniversity immunology lecturer Dr Jason Steel said the test for COVID-19 was based on a molecular technique called RT-PCR to measure the presence of the SARS-CoV-2 virus' RNA from various swabs such as nasal.
"It is a very sensitive technique that can detect a very small amount of the virus," Dr Steel said.
"When we push this technique to measure smaller and smaller concentrations of the viral RNA the accuracy is reduced.
"So, when there are lots of virus present it is both very sensitive and accurate, but as you reduce the amount of virus you lessen the accuracy."
"The technique has a cut-off point where the sample is positive. With a test for an infectious agent you want to be as sensitive as you can, so this cut-off point is close to the detection limit."
Dr Steel said with testing such as this, it was better to get a false positive and isolate someone, rather than a false negative and not isolate an infected person.
He said as pathologists looked for smaller and smaller amounts of the virus, the likelihood of a false positive reading was higher, thus why testing was done more than once.
Perhaps more reassuringly, Dr Steel explained while this was the case for false positives, the likelihood of getting a false negative, when the swab has virus on it, was very low.
"Failing to get the sample on the swab can be a reason why there are false negatives," he said.
"If we look at the testing for (Nathan) Turner, the initial test would have been conducted on a swab post-mortem.
"It might have been a false positive because it was at that borderline between positive and negative or it might have been contaminated in some way.
"The second test (which is done for all positives) would have been on autopsy where they have much better access to the back of the throat and lungs, so it would be more likely to be more accurate."
While the accuracy rate for COVID-19 testing is high, Dr Steel said false tests were very rare but inevitable.
"If the test is 99.9 per cent accurate that means that 0.1 per cent of tests are not accurate.
"In Queensland we have tested over 200,000 people, so just on this number alone, over 200 people would test incorrectly even in the most ideal situations."
Dr Steel said that Mr Turner's initial false positive result spoke highly of the success of the testing program.
"Positive tests are almost always retested so that we can catch these false positives. This is the system working."
He said rather than casting doubt on the positive numbers he believed the result probably reaffirmed the testing.