An authorized rapid antigen testing device, currently used at Covid-19 test facilities, has only 29 percent sensitivity, a group of health experts said. This means that test results of 71 out of 100 people who had tested negative with the rapid antigen test could be false-negative.

The Korean Society for Laboratory Medicine’s expert panel verified SD Biosensor’s Standard Q Covid-19 Ag Test, the nation’s only rapid antigen test device, and released the results on Wednesday.

The KSLM tested its performance using 380 positives and 300 negative samples already confirmed by real-time reverse transcription-polymerase chain reaction (rRT-PCR, qRT-PCR), the standard test for Covid-19. The selection of Covid-19 positive specimens was based on distributing the results of 33,294 PCR tests previously done in Korea and the detection limit of the rapid antigen diagnosis kit presented by SD Biosensor.

The expert panel found that the rapid antigen testing device had 29 percent sensitivity and 100 percent specificity in 680 samples. The results were in contrast to SD Biosensor’s explanation that its device had 90 percent sensitivity. Trusting SD Biosensor’s data, the Ministry of Food and Drug Safety granted the nod for the rapid antigen diagnostic kit.

Sensitivity refers to the probability of determining positive as positive. Specificity refers to the likelihood of identifying negative as negative. If a testing device has a high sensitivity, it has a low probability of producing a false-negative result.

The KSLM further analyzed the device’s sensitivity based on a cycle threshold (Ct) value of 23.37. The rapid antigen diagnosis kit can detect at maximum, according to the data presented by SD Biosensor.

The Korean Society for Laboratory Medicine’s expert panel verified SD Biosensor’s Standard Q Covid-19 Ag Test, the nation’s only rapid antigen test device.
The Korean Society for Laboratory Medicine’s expert panel verified SD Biosensor’s Standard Q Covid-19 Ag Test, the nation’s only rapid antigen test device.

The results showed that the rapid antigen testing device had 81 percent sensitivity in samples with a high viral load and Ct value lower than 23.37.

In contrast, in samples with a low viral load and Ct value exceeding 23.37, the sensitivity was as low as 11 percent.

The false-negative results started to appear from Ct values at 17.7 and up. In samples with Ct value at around 23.37, 50 percent of the results turned negative. In specimens with Ct values higher than 25, over 80 percent were negative.

These results were similar to inaccurate results of rapid antigen tests in Italy and Hong Kong, the KSLM said.

In Korea, it is not desirable to use rapid antigen testing to detect Covid-19 cases, the society said. “In an urgent situation where a rapid result is needed, we could try an antigen test. But in this case, we have to conduct the PCR test to screen out the possibility of false-positive and false-negative,” the KSLM said.

Simply increasing the number of tests without guaranteeing accuracy could cause more social confusion with false-positive and false-negative results, it warned, adding that the government should expand the number of accurate tests.

To expand Covid-19 tests, the KSLM proposed testing on mixed specimens, rapid PCR testing, and automated molecular testing. It also said that the government should secure more manpower for testing and sample collecting.

The Korea Disease Control and Prevention Agency (KDCA) said the nation ran 6,171 rapid antigen tests and conducted secondary nasal/throat PCR tests on 21 people who tested positive. Of them, 11 were positive, seven, negative, and three, waiting for results.

The KDCA did not say whether any of the 6,150 negative cases were false-negative.

Copyright © KBR Unauthorized reproduction, redistribution prohibited