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Antigen test good to rule-in patients within 5 days of illness: Study

The overall sensitivity and specificity of the test was 81.8% and 99.6% respectively, and the test accuracy was 95.4%. Sensitivity of the test was higher (85.9%) in participants with duration of illness lesser than five days.

Written by Astha Saxena | New Delhi | Updated: November 2, 2020 6:48:33 am
delhi coronavirus latest update, delhi covid cases, delhi covid testing, rapid antigen test, covid rapid antigen test, delhi city newsA health worker sorts samples for COVID-19 test. (Express photo by Amit Mehra/File)

A cross-sectional study conducted by researchers from AIIMS on 330 patients concluded that the rapid antigen test showed an excellent specificity to ‘rule-in’ Covid-19 patients within the first five days of illness with a moderate sensitivity.

The overall sensitivity and specificity of the test was 81.8% and 99.6% respectively, and the test accuracy was 95.4%. Sensitivity of the test was higher (85.9%) in participants with duration of illness lesser than five days.

The study, published in the Indian Journal of Medical Research, was done on patients admitted at AIIMS, Delhi, between May 31 and July 24. Researchers included symptomatic patients and asymptomatic/pre-symptomatic contacts of laboratory-confirmed cases with exposure between 5 and 10 days.

“The analytical performance…depends on mixing of NP swab with buffer and the viral load in the sample, but clinical performance of the test may be variable which depends on the technique of sample collection and duration of illness of patients. Hence, sensitivity will be average in asymptomatic patients because it is difficult to analyse pre-test probability in asymptomatic patients,” stated the paper.

Of the 330 participants, 77 were RT-PCR positive for Covid-19. Sixty four of them also tested positive in the rapid antigen test. The most commonly presented symptoms among screened participants were fever (31.5%), cough (25.4%), fatigue/malaise (11.8%), headache (3.3%) and runny nose (3.3%). As many as 57 participants presented with sore throat but only two of them (3.5%) had Covid-19.

“This is not easy to estimate in asymptomatic cases. Therefore, this test may not be good for surveillance purposes. This test can perform as well as RT-PCR in high-prevalence areas with high pre-test probability. Moderate sensitivity of this test leads to false-negative results…,” it added.

However, the study had several limitations. Since participants were recruited from a screening OPD, they were either asymptomatic/pre-symptomatic or mildly symptomatic. Testing was not done in moderate to severe cases.

“Secondly, this test can miss up to 25-30% of Covid-19 cases as sensitivity was 81.8% as compared to a standard reference (RT-PCR). Another limitation is that a positive rapid detection test is not 100% specific to SARS-CoV-2, as it shows cross-reactivity with SARS-CoV based on analytical performance of the test provided by the kit manufacturer, but not evaluated further in the clinical setup,” said the study.

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