The accuracy of the rapid COVID-19 antigen tests being used in South Korea may be considerably lower than previously suggested, according to a recent analysis, prompting concerns that widespread use could undermine disease control efforts.

Korea has deployed rapid antigen tests at testing centers around the country since last week following President Moon Jae-in’s order on Dec. 7.

The country now offers three choices for people wanting to get tested for COVID-19: a rapid antigen test using a nasopharyngeal smear, or PCR using either nasopharyngeal smear or saliva. 

While PCR is still highly recommended, it is up to individuals to choose which type of test they get.

The rapid tests are capable of returning results within 30 minutes, at the cost of compromised accuracy. It takes around six hours for PCR tests to produce results.

Before this point, the country only used PCR, which is considered the “gold standard” for COVID-19 testing for its high sensitivity and specificity. Sensitivity refers to a test’s ability to correctly identify patients with a disease, and specificity is its ability to correctly identify people without the disease.

SD Biosensor’s Standard Q COVID-19 Ag test, the only rapid antigen test kit to be approved for use here, has a 90 percent sensitivity and 96 percent specificity, according to the data provided by the company. This compares with the PCR’s over 98 percent sensitivity and 100 percent specificity.

But new findings suggest that the test may deliver even lower levels of accuracy.

The Korean Society for Laboratory Medicine said in a statement Tuesday evening that SD Biosensor’s rapid antigen test showed only about 29 percent sensitivity based on its analysis, meaning that more than 7 out of 10 infected individuals could be wrongly told they don’t have the virus.

The society ran the rapid antigen tests on 680 samples -- 380 positive and 300 negative as confirmed with PCR -- to derive the results.

According to the company, the rapid antigen test can detect cycle threshold or Ct value in the ranges of up to 23.37. The laboratory medicine society says the majority of Koreans tested with PCR exhibited a Ct value of between 25 and 30 -- a level that the rapid test is unable to detect.

A Ct value indicates the amount of the virus present in an infected person. A Ct value of 20 is typically considered a high amount of virus, while 35 is lower. A high Ct is associated with a lower risk of infectivity.

“Korea has been applauded for its aggressive testing and contact tracing during the early phase of the pandemic. But recently, the number of patients being confirmed per day has surged past 1,000, with tracing efforts failing for an increasing proportion of them. This calls for an expansive testing to sift out hidden infections in the community,” the laboratory medicine society said.

“And in doing so, accuracy of the tests is paramount. Tests that lack sensitivity and specificity will only lead to more chaos with false positives and false negatives,” it said. “The sensitivity of testing using saliva samples is also relatively low at 91 to 94 percent compared to that using nasopharyngeal samples.”

Laboratory medicine specialist Dr. Lee Hyuk-min at Severance Hospital, said on the effects of rapid antigen testing, “There will be a significant proportion of cases going undetected, but what’s even riskier is that it could give people who test negative a false assurance that they don’t have the virus.”

He added that the rapid antigen tests are also less reliable in detecting asymptomatic patients.

In a Facebook statement posted Dec. 11, infectious disease specialist Dr. Kim Tark said the decision to use rapid antigen tests was “almost equivalent to declaring Korea would be giving up on its strategy known as the 3Ts -- test, trace and treat.”

He said the mass testing plan involving the new types of tests “does not seem to be based on strategic thinking.”

“The rapid antigen tests are going to lead to a cascade of missed cases. Ramping up testing without these considerations is not just unhelpful but a waste of resources,” he said.

Laboratory medicine specialist Dr. Sung Heung-sup of Asan Medical Center said the use of rapid antigen tests could translate to heavier workload for the testing workers.

“If a person is positive, then we have to follow up with a PCR test to see he or she is truly positive. If a person is negative, then we cannot for certain know that he or she is truly free of the virus,” he said.

According to the laboratory medicine society, Korea has the capacity to conduct 130,000 molecular tests a day. Since the third wave, a daily average of between 60,000 and 80,000 tests has been performed here -- which includes rapid antigen tests. The society argues that there is still room for around 50,000 and 80,000 more tests that can be done, just using the more reliable PCR.

The society said the biggest hindrance in introducing the kind of mass testing being executed here was a shortage of workers. There needed to be more laboratory scientists as well as workers at testing sites to collect specimens from people.

As for self-testing kits for home use, which have been floated by some politicians, experts are skeptical.

The Democratic Party of Korea’s chairperson Rep. Lee Nak-yon said in a Dec. 14 meeting of the party’s supreme council, “Now may be time to discuss having people easily test themselves using rapid test kits, and seek additional testing based on the results.”

“How a clinical specimen is collected and handled can sway test results. Which is why tests meant for self-collection and testing at home are fraught with the potential to go wrong in so many ways,” said Sung of Asan Medical Center.

“Obtaining a nasopharyngeal specimen requires training. When done by untrained hands, the nasal passages could get hurt in the process. This is not the kind of examination that one can do by oneself,” said an infectious disease expert who wished to remain anonymous.

Experts say rapid antigen tests could be useful in certain settings, but generally agree their use as a tool for diagnosis has limits.

Sung recommends they could be used for patients who are already diagnosed, when frequent PCR testing is unavailable.

The National Medical Center said in a press conference last month that rapid antigen tests could have some merits at the emergency care units.

“When patients with suspected COVID-19 symptoms arrive at emergency care services, they can be given the rapid antigen tests for quick screening. If they test positive, they can be treated at isolation wards. If they test negative, their viral loads are likely to be at a level where transmissibility is low,” said the center’s scientist.

“But then again, to know for sure that they don’t have the disease, they should be tested again with PCR,” he said.

By Kim Arin (arin@heraldcorp.com)