False positive is better than false negative results: TAG 

Younten Tshedup

Last week, rumours were rife that there would be another lockdown in the country. The health ministry, however, was quick enough to debunk any such claims.

While the reason behind the rumour still remains unknown, many believe that it could have surfaced after an individual tested positive on the rapid antigen test at national referral hospital’s emergency ward. The individuals however, tested negative on confirmatory RT-PCR test.

The case at the emergency ward was one of the many stray cases health officials have come across. However, there is little to worry about such incidences according to experts.

Clinical microbiologist with the national referral hospital, Dr Tshokey, said that although there were a few incidences where people had tested positive during the rapid antigen test, none of these individuals were positive on the RT-PCR.

He said that every antigen test analyser machine had a cut-off index (COI) set where any reading above 1 on the COI gave positive results. “Anything below one, reads negative on the machine.”

He explained that those individuals testing positive on the antigen test had readings of 1.05, 2 and sometimes 3. However, these readings, he said, were low and borderline readings. Only those who had readings above 20 and 30 on the antigen analyser machine have tested positive on the confirmatory RT-PCR test so far.

“We have had these kinds of cases in most of the hospitals and none of the individuals tested positive so far, even after repeating the RT-PCR test after 24 hours,” said Dr Tshokey.

 

The antigen test  

This latest mechanism for testing Covid-19 (SARS-CoV-2) is specifically designed to detect and identify antigens. Antigens are any foreign body (proteins) that enters the body.

The human immune system works on a simple idea: Any protein in your body that isn’t encoded by your own genes is probably from a pathogen and should be captured and destroyed. For this, when the immune system detects a foreign protein, the white blood cells create antibodies to trap and destroy these proteins.

People with no risk factors or any recent travel histories to high-risk areas have tested positive on the antigen test.

Royal Centre for Disease Control’s (RCDC) head, Dr Sonam Wangchuk, said the health ministry’s team was in contact with the manufacturers of the test kits in South Korea.

During the recent discussions with the manufactures, it was learnt that there could be some field-related factors causing the issue. Among others, the manufacturers had recommended that people collecting and shipping samples from the field needed to be extra careful.

Dr Sonam Wangchuk said that initially when antigen testing first started in the country, samples used to be collected and transported to the RCDC laboratory manually by health workers.

“The manufacturers don’t recommend shipping samples from one place to another. When you collect samples and keep it for a longer time, there is some lysis (breaking down of cells) happening in the cells.”

He said that ideally, antigen tests needed to be conducted on the spot from where the samples were collected. Today, all the 54 flu clinics have individual antigen testing facilities, conducting tests on the spot.

Dr Tshokey said that because the antigen test was a fairly new method, large quantities of the test kits were being constantly produced and supplied across the world.

He said that the manufacturers in Korea have indicated that certain batches of the test kits that had been circulated had these issues. The company has asked countries not to use a certain lot of the kits.

“The company has also asked us to collect the data from the machine and send it to them so that they can analyse how the samples were read by the machine,” he said. “They have communicated that what we have read as 1.05 and 2 on the machines all seems to be false positive.”

The number of such cases has gone down recently.

 

Nothing to worry 

Both the members of the TAG said that detection of false positive results among the population was better than having false negative results.

With the country going all out on testing the population, Dr Tshokey said that the more tests the ministry conducts, there is more likelihood of getting false positives.

He explained that a highly specific test with about 99 percent specificity would still produce one percent false positive results. “If we test 100 people, one would test false positive. But when we test 1,000 people, we would get 10 false positive individuals.”

The government has recently started to perform Covid-19 tests on all individuals coming for admission at the hospitals including their attendants. All health staff are tested fortnightly.

Dr Tshokey said, “What is important for us is that we are taking no chances. Anyone testing positive on the antigen test, we confirm it by doing a RT-PCR test. And if needed, we repeat the RT-PCR in 24 hours.”

Amidst the global shortage and increasing demand for testing kits, Bhutan has been using all the three available testing methods. The rapid antigen and antibody tests are used as screening methods.

The confirmation if an individual is infected with Covid-19 is done through the RT-PCR test, the gold standard, according to the World Health Organisation.

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