Rapid antigen failed to detect 3.7% asymptomatic patients: PGI study

Naina Mishra

Tribune News Service

Chandigarh, July 26

Around 3.7 per cent asymptomatic patients who tested Covid negative in the rapid antigen test were found positive in the RT-PCR test, a study conducted by researchers at the PGI has found. The study was published in the Indian Journal of Medical Microbiology on July 20.

PGI study

Antigen and RT-PCR methods were used for 484 samples to determine the sensitivity and specificity of the tests. The SARS-CoV-2 antigen was found positive in 28 per cent patients while the RT-PCR report was positive for 39 per cent patients. The RT-PCR could detect the 11 per cent, who were reported negative in the antigen test, resulting in a sensitivity of 71.96% for RAT.

The PGI studied the utility of rapid antigen detection tests in a tertiary care hospital in both symptomatic subjects and their asymptomatic contacts.

The nasopharyngeal swab samples were collected from symptomatic patients and their contacts. The diagnostic accuracy of the antigen kit was determined in comparison to RT-PCR tests.

Antigen and RT-PCR detection was performed simultaneously for 484 samples to determine the sensitivity and specificity of the test. The SARS-CoV-2 antigen was found positive in 28 per cent of the patients while RT-PCR report was positive for 39 per cent patients. The RT-PCR could detect an additional 11 per cent of patients, who were reported negative by antigen test, resulting in a sensitivity of 71.96 per cent for RAT. Sensitivity is the ability of a test to correctly identify patients with a disease.

According to the study, 60.5 per cent (293/484) patients tested negative by both methods. Therefore, the specificity (the ability to detect those not affected by a disease) of RAT was 99.32 per cent. However, the RAT was unable to detect 3.7 per cent of the asymptomatic patients who tested positive by RT-PCR.

The PGI found that in comparison to the RT-PCR, the antigen test has a low sensitivity. The RT-PCR can amplify low viral load. Testing with RAT in patients suspected of SARS-CoV-2 infection can produce false negative results, researchers noted in the study.

The study concluded: “The rapid antigen test was sensitive in symptomatic cases. Since a majority of the SARS-CoV-2 patients are asymptomatic and considering the huge population, the testing strategy formulated by the Indian Council of Medical Research at the national level was cost-effective. It can’t be used as a standalone test for diagnosis. Despite limitations, RAT has the ability to play a significant role in guiding patient management and surveillance during the pandemic.”

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