Rapid COVID-19 Antigen Test Provides Less Sensitivity Than PCR Assay

A rapid antigen assay developed for COVID-19 detection was associated with an approximate 81% sensitivity and 100% specificity compared to standard PCR testing.

In new study findings presented at the American Society of Microbiology (ASM) 2021 World Microbe Forum, a team of Virginia-based investigators reported the capability of the Abbott BinaxNOW COVID-19 antigen test—marketed for its ability to provide assay results in approximately 15 minutes for SARS-CoV-2 infection.

Led by Alexandra L. Bryson, PhD, of the Department of Pathology at the Virginia Commonwealth University Health System, the real-world comparison of PCR versus rapid antigen testing showed the latter missed an approximate 18% of COVID-19 cases among those who tested positive on the former assay.

The US Food and Drug Administration (FDA) originally granted Emergency Use Authorization (EUA) to a litany of COVID-19 diagnostic tests based on what Bryson and colleagues described as “limited validation data,” a factor which is particularly concern for use of antigen-based assays.

“Rapid, antigen-based assays present as an enticing test method because of their quick turnaround time, low cost, and ease of use,” investigators wrote, “however, antigen-based assays are known to have inferior sensitivity and specificity compared to PCR-based assays.”

The team conducted a prospective, paired-specimen trial to assess the sensitivity and specificity of the BinaxNOW antigen test versus 2 different PCR tests: the TaqPath COVID-19 Combo Kit from ThermoFisher, or the Cobas SARS-CoV-2 test from Roche.

Their evaluation included 283 patients from the academic medical center, who consented to 2 specimen assays. Nasal swabs collected by a care provider were tested with the BinaxNOW system, then nasopharyngeal swabs from both nostrils were tested by PCR within 24 hours.

Bryson and colleagues observed 60 PCR positive results among the 283 tested patients, versus 233 PCR negative tests. Nearly all (n = 250) patients reported COVID-19 symptoms; just 12 were asymptomatic, and 21 had unknown symptom status.

The rapid COVID-19 antigen test demonstrated 81.7% sensitivity, with 11 false negatives, and 100% specificity. The BinaxNOW system failed to detect all 4 PCR-positive samples with average Ct value greater than 28. It additionally missed 3 strong PCR-positive samples with average Cts of 12, 16, and 18.

The 4 remaining false negatives observed in the antigen samples had Cts ranging from 20 – 26, indicating an observed inaccuracy of the rapid antigen assay in detecting SARS-CoV-2 in patients with greater viral loads.

“This study demonstrates that the BinaxNOW COVID-19 Ag Card misses (approximately) 18% of COVID-19 cases across a wide range of PCR positive Ct values including patients presumed to have high viral loads,” they concluded.