Contamination at private lab causes 28 false positives

Cross-contamination at private laboratory BGI has led to "false positive" results in 28 Covid-19 samples that have been tainted by two samples that are actually infected cases, says government adviser Yuen Kwok-yung. Yuen was invited to inspect the genome sequencing firm's lab in Tai Po...

Jane Cheung

Friday, April 23, 2021

Cross-contamination at private laboratory BGI has led to "false positive" results in 28 Covid-19 samples that have been tainted by two samples that are actually infected cases, says government adviser Yuen Kwok-yung.

Yuen was invited to inspect the genome sequencing firm's lab in Tai Po Industrial Estate after a Centre for Health Protection official said it was "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 yesterday, at least a dozen who had their samples taken at hospitals tested negative and only two tested positive.

After inspecting the lab, Yuen said: "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."

Yuen said technicians had to add an "extraction buffer" solution to the samples, due to which "when they open the lid of the sampling bottles, the foam [generated by the solution] could spill to their gloves, the pipette or the environment, then subsequently be brought to the other samples," he said.

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

He suggested the lab use centrifuges to keep the foam at the bottom of bottles and prevent spillage.

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

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

For the 28 "false positive" cases, Yuen said to play safe, the Hospital Authority would collect their nose and throat swabs, as well as feces samples for more tests.

"If these come back negative, as well as their antibodies test, they can go home and their close contacts can be freed from quarantine centers," he said.

This came as Hong Kong yesterday reported 14 cases, including two from unknown local sources, just one day after the city recorded zero local infections.

Centre for Health Protection's Albert Au Ka-wing said one of the unknown-source cases was a 31-year-old female teacher at Greenfield English (International) Kindergarten in Lohas Park.

The woman, who lives in Block Four of La Cite Noble in Hang Hau, developed a fever and coughs on Tuesday. On the same day, she taught two classes for Kindergarten Three children for 15 to 20 minutes.

Around 30 pupils and several of her colleagues will be sent to quarantine as a prudent measure, Au said.

"One parent will be allowed to stay with their children at quarantine centers," he said, adding if no carer can accompany a child, he will be sent to the isolation camp at Lei Yue Mun Park and Holiday Village to be looked after by the Social Welfare Department.

The other unknown-source infection was a 19-year-old man working with Wang Kee at Kwai Tsing Container Terminals, who lives in San Pik House of San Wai Court in Tuen Mun.

He undergoes regular tests due to his job, but his samples taken on Tuesday came back positive. Around 40 colleagues who worked the same shift as him will be sent to quarantine.

The other 12 cases were imported, including eight from India, two from Nepal, and one each from Turkey and Pakistan.

Hong Kong's tally yesterday was 11,719, including 207 deaths.

jane.cheung@singtaonewscorp.com