How a COVID false positive happens

By | August 5, 2020

On a quiet Brisbane street stands one of the most important buildings in Queensland’s defence against COVID-19.

Microbiologists, scientists, sample collectors, data entry personnel and clinicians are all part of the machine at Sullivan Nicolaides Pathology at Bowen Hills, processing thousands of COVID-19 tests a day.

As fears of community transmission ripple through the state, clinical microbiologist Dr Evan Bursle and principal scientist Katrina Collins are urging people to be patient.

More than 50,000 Queenslanders rushed to get tested over the past week after two women, who allegedly returned from Melbourne without properly declaring their whereabouts, were out in the community while infectious.

It prompted Queensland Health to release dozens of contact tracing details, resulting in massive queues for respiratory clinics and pathology labs.

But the time it takes for a test result to be returned varies. Dr Bursle says it is important people understand just how COVID-19 testing is undertaken.

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Dr Bursle said at peak capacity, thousands of samples were being processed a day.

“We have a huge team of collectors … that are really highly trained in collecting samples properly, which is really important to ensure we have the best chance of getting a good result,” he said.

“They take the specimens and make sure it’s all labelled and entered correctly, and then those samples from all over the state come here.

“There’s a huge amount of data and pre-process that has to happen, that takes a lot of time and effort … If you make a mistake at the data entry, and someone is assigned someone else’s swab accidentally and that ends up being positive for someone that’s negative, or negative for someone who’s positive, that’s a huge problem.”

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Before the samples reach Ms Collins in the lab, they have already been allocated numbers. There is no personal identification in pathology labs, but numbers flagged as critical cases — including known contacts of confirmed cases, essential workers such as those in healthcare, and aged care residents — are prioritised.

“We then have to extract a specimen out of the swab that’s been sampled, which is a timely process,” she said.

“A scientist will then get that specimen and then the testing procedure begins. Your viral DNA is extracted and then it is copied, and then repeatedly copied over and over again through a machine until we can see whether there is a positive result or not.

“This process is just shy of two hours long.

“Once you’ve done all of this, the result then needs to be read by a scientist who will determine whether it’s a true positive or whether there are questions surrounding it.”

In the case of a questionable positive, public health officials are notified, ensuring contact tracers are on standby just in case a second test confirms it as a true positive.

In some instances, the person behind the questionable positive will need to be re-swabbed, but Dr Bursle said in most cases the machine can pick up a true result.

“Those cases can get two or three tests … but sometimes you will need another swab or extraction,” he said.

“Some of the vagaries of testing for patients who might have low levels of the virus can fluctuate around the ‘cut-off’ that determines positive from negative, so sometimes retesting the same sample can give us more information, sometimes we’ll need to engage public health to request a retest.”

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After the machines determine positive from negative, the results are analysed by scientists and potentially reviewed by pathologists, before the results are given to the requesting clinician and public health if need be.

“It’s hours of process,” Ms Collins said.

“In a normal, non-pandemic scenario, you could potentially have results by the end of the day if you get the extraction in the morning … but these are thousands of swabs a day.”

Ms Collins and Dr Bursle said due to the increased demand for testing, labs had to prioritise samples.

“We prioritise known contacts as we’ve been informed by public health. They come through and sometimes we can even do rapid tests, of which we have a limited number, for a quick result,” Dr Bursle said.

“As you cascade down from there, the next priority is essential workers, like healthcare workers, that need their results more critically.

“Samples from high risk areas like nursing homes need a results turnaround as quickly as possible, because if you get a case there that can be catastrophic.

“And then there’s Joe Blow that has symptoms but can stay home and isolate … we ask people in that category to be a little more patient because we need to focus our efforts on those really important results back first.”

Ms Collins urged those frustrated by lengthy waits in getting their results to consider the alternative.

“We don’t want to say you’re negative if you’re really positive because we’ve skipped a step to get you the results quicker,” she said.

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“We know people will be frustrated … the average turnaround for someone not on the priority list may be extended during really peak periods but this has to happen.

“If you keep everyone’s average turnaround time the same in those scenarios we could have big problems in the community where people at higher risk are really positive and are out in the community.

“That’s a massive challenge to public health if that person hasn’t stayed in isolation.”

Dr Bursle said he wanted sample collectors, right through to clinical administration staff to be treated with respect during this unprecedented time.

“People might be frustrated that they can’t get back to work for a day or so, but in the scheme of things it’s far more important for us to catch those high risk ones,” he said.

“We’re working really hard, everyone is. We want people to remember we’re not just a respiratory and COVID lab, we do a lot of other important testing here too.

“Our sample collectors have a lot of empathy to the situation … but they’re on the front line.”

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