This story has been updated to add a statement from Abbott on the study.
NEW YORK – The US Centers for Disease Control and Prevention published on Tuesday an evaluation of Abbott's SARS-CoV-2 BinaxNow rapid antigen test, finding that the test is less sensitive than PCR but could be particularly useful when PCR tests aren't available or when quick turnaround times are needed.
The analysis, published in the CDC's Morbidity and Mortality Weekly Report, analyzed 3,419 specimens collected from people between ages 10 and 95 at two testing sites in Pima County, Arizona, between Nov. 13 and Nov 17, 2020. The researchers attempted to perform viral culture on 274 of the RT-PCR specimens that had received positive results from either test. Abbott's BinaxNow test received Emergency Use Authorization from the US Food and Drug Administration in August and received EUA for at-home use last month.
According to the study, compared to real-time RT-PCR testing, the BinaxNow had a sensitivity of 64 percent, a positive predictive value of 100 percent, and a negative predictive value of 91 percent for people with symptoms. Meanwhile, sensitivity for asymptomatic people was nearly 34 percent, with PPV of 92 percent and NPV of 97 percent. Specificity was near 100 percent for specimens from either group.
For patients who were within a week of symptom onset, BinaxNow sensitivity was 71 percent, with specificity and PPV of 100 percent and NPV of 93 percent. There were four false positives for all of the asymptomatic participants, while 142, or nearly 48 percent of PCR positive results, were false negatives from the BinaxNow.
Among the 96 specimens where the virus was able to be cultured, sensitivity was nearly 93 percent for symptomatic people and nearly 79 percent for asymptomatic people.
To perform testing, an anterior nasal swab was collected from patients with the BinaxNow and tested at the testing site and immediately followed by a nasopharyngeal swab for PCR testing, which was analyzed within two days using either the CDC's 2019-nCoV Real-Time RT-PCR diagnostic panel or the Fosun COVID-19 RT-PCR detection kit.
When the samples were tested, 24 percent of the participants reported at least one COVID-19 symptom, while 76 percent were asymptomatic. Among the symptomatic participants, nearly 14 percent received positive BinaxNow results and 21 percent received positive PCR results. For asymptomatic participants, 2 percent received a positive BinaxNow result and nearly 5 percent received a positive PCR result.
Some of the samples with negative antigen results and positive PCR results "could represent noninfectious viral particles, but some might also represent infectious virus not detected by the antigen test," the researchers wrote. Negative antigen test results should be confirmed by nucleic acid amplification tests, they emphasized.
However, they also said that in spite of the lower accuracy, the test could have a place in current strategies to contain the spread of SARS-CoV-2.
"Despite a lower ability to detect infection, the quick turnaround time of the BinaxNOW antigen test can help in rapidly identifying people with COVID-19 so they can be isolated, particularly when used as part of repeated testing strategies," Jessica Prince-Guerra, one of the study's authors, said in a statement. "However, everyone, regardless of test results, should wear a mask, avoid gatherings, and stay at least six feet from others."
They added that there were at least five limitations to the study: the difference in types of swab for each type of test could have contributed to the increased detection for the PCR test; participants could have reported common nonspecific symptoms as COVID-19 symptoms; the results can't be generalized to other SARS-CoV-2 antigen tests authorized by the FDA; the antigen test characteristics might differ depending on if a person had previously tested positive; and many factors can inhibit culturing a virus from a specimen and the lack of culture shouldn't be interpreted as a lack of infectiousness.
Although the antigen test was less sensitive than PCR testing, the researchers emphasized a test such as BinaxNow could still be useful for screening and serial testing.
"Because estimates suggest that over 50 percent of transmission occurs from persons who are presymptomatic or asymptomatic, expanded screening testing, potentially in serial fashion for reducing transmission in specific venues (e.g., institutions of higher education, schools, and congregate housing settings), is essential to interrupting transmission," the researchers wrote.
"Persons who know their positive test result within 15–30 minutes can isolate sooner and contact tracing can be initiated sooner and be more effective than if a test result is returned days later," the analysis concluded. "Serial antigen testing can improve detection, but consideration should be given to the logistical and personnel resources needed."
In a statement, Abbott said the ability of the test to detect highly infectious patients was key. "During a pandemic, having a rapid antigen test like BinaxNOW gives the patient critical information at a point in the infection cycle when they’re at the highest risk of spreading disease – making it a powerful tool in preventing transmission compared to a 24-hour turnaround PCR test," the company said.