Cross-contamination at lab causes 28 false positive results: Yuen Kwok-yung

Local | 22 Apr 2021 9:03 pm

Cross-contamination at private laboratory BGI have led to “false positive” results in 28 Covid-19 samples, which have been polluted by two samples from actually infected cases, government adviser Yuen Kwok-yung said.

He was invited to inspect the Beijing-founded genome sequencing firm's SAR lab in Tai Po Industrial Estate, after a Centre for Health Protection official said it is "a bit strange" that 30 preliminary positive cases were identified there on the same day.

Treating preliminary positive cases as confirmed ones, the 30 people –  nine from quarantine hotel Ramada Hong Kong Grand in Tsim Sha Tsui and 21 from test stations across various districts – have been sent to hospital isolation wards and their close contacts to quarantine camps.

But as of 8pm, at least a dozen of them had their samples taken at hospitals tested negative while only two of them tested positive.

After inspecting the lab, Yuen said the 30 preliminary positive samples were tested in the same batch, and the two positive ones were the coincidentally the first two technicians handled.

“We believe some of the positive samples were accidentally brought to the 28 other samples, causing all of them to test positive but with a low viral load,” he said.

He explained technicians had to add an “extraction buffer” solution to the samples, which he described as “something like detergent that generates foams.”

“And when they open the lid of the sampling bottles, the foam could spill to their gloves, the pipeter (a tool to suck the samples up from the bottles) or the environment, then subsequently brought to the other samples,” he said.

Yuen said the lab had to go through deep cleaning immediately, followed by tests on its environmental samples to make sure all contamination has been eliminated.

He suggested the lab to use centrifuges – to keep the foam at the bottom of bottles and prevent spilling – before opening them.

He said he also observed technicians putting worksheets into bio-safety cabinets and reuse plastic trays for different batches of samples, which could have also led to cross-contamination.

He said these habits should be changed and reminded staffers to change their disposable gloves after clearing each batch of specimens.

For the 28 “false positive” cases sent to hospitals, Yuen said to play safe, the Hospital Authority would collect their nose and throat swab, as well as feces samples for more rounds of tests.

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