In the setting of a nonhealthcare workplace outbreak of COVID-19 with high attack rate (62.3%), we found that BinaxNOW was a useful adjunct to rRT-PCR testing. BinaxNOW showed NPA and PPV of 100%. A total of 55 participants were concordantly identified as positive by BinaxNOW and rRT-PCR, and no false-positive BinaxNOW results were noted. This low false-positive rate is consistent with results from Pilarowski et al. that established the updated BinaxNOW card-reading technique used by the racetrack physician in this outbreak. Results of BinaxNOW testing were available the same day, which enabled more rapid identification of infected workers for isolation than reliance on rRT-PCR alone.
Negative BinaxNOW results were less concordant with rRT-PCR results. The PPA of BinaxNOW was 43.0% and the NPV was 89.9%. Real-time RT-PCR confirmation of BinaxNOW negative results identified 72 additional positive specimens. The median time between rRT-PCR specimen collection date and results reported date for these BinaxNOW false-negative specimens was 5 days (range 1–7 days).
Although Ct cannot be used to define viral load or infectivity of a given person, Ct is inversely related to the amount of target genetic material present in the specimen. Therefore, the significantly lower mean Ct for true-positive BinaxNOW specimens (17.8) compared with false-negative BinaxNOW specimens (28.5) indicates that more viral genetic material was present in those specimens. BinaxNOW demonstrated better concordance with positive viral culture results (88.2%) than with positive rRT-PCR results (43.3%). Positive viral culture is further evidence of the presence of infectious virus, so these findings might indicate that some BinaxNOW false-negative participants were not infectious at the time of specimen collection (i.e., they had low viral RNA load at the beginning or end of their infection trajectory). Numerous factors can affect the outcome of a viral culture; therefore, negative culture results do not necessarily mean that no viable virus was present in those specimens, nor that the participants from whom those specimens were collected were not infectious at the time of specimen collection.
With serial BinaxNOW testing, some of the persons with discordant paired results could have tested positive with subsequent BinaxNOW testing. Further studies are needed to determine whether serial rapid antigen testing alone can identify infectious persons as efficiently as rRT-PCR alone or a combination of rRT-PCR and rapid antigen testing.
The first limitation of our study is that, although other studies have demonstrated differential BinaxNOW test performance in symptomatic and asymptomatic persons,[3,6–8] we were unable to examine test performance by symptom status, because symptom reporting might not have been reliable. At the time of specimen collection, only 11 persons reported symptoms to the facility administrative employee registering them for testing. This number conflicts with data previously collected from the racetrack physician as part of a prospective cohort drug trial on this same population which, out of an enrolled cohort of 113 BinaxNOW-positive staff, identified 60 (53%) persons who were symptomatic at the time of testing. This discrepancy might have resulted from staff feeling less comfortable discussing symptoms with the administrative employee versus the racetrack physician or it could be associated with the incomplete list of COVID-19 symptoms in the administrative employee's question. It might also reflect a language barrier, because the question about symptoms was asked only in English by the administrative employee. According to onsite interactions with staff and reports from racetrack leadership, many staff were native Spanish speakers, although this language difference was not quantified.
Second, the BinaxNOW tests may have been performed in ambient temperatures below the manufacturer's recommended range. The BinaxNOW test kit instructions recommend that all test components be at room temperature (15°C–30°C) before use; the mean daily minimum and maximum air temperature recordings from a nearby National Oceanic and Atmospheric Administration weather station in Richmond, CA, on testing days were 7.9°C and 15.1°C. Performing BinaxNOW tests in the recommended temperature range might have improved performance.
Third, some missing data limit this analysis from encompassing the entire outbreak. The first mass testing dates (round 0) only used rRT-PCR testing, so no comparison with BinaxNOW was possible. Furthermore, each round of testing was intended to capture all staff who had not yet tested positive; however, participant attrition occurred between testing rounds. We attribute this attrition to the logistical obstacles of staff getting to the testing site or to staff leaving their jobs during the outbreak. More complete paired-testing data could have provided better insight as to the usefulness of rapid antigen testing when used for the entire duration of an outbreak.
Our results support considering BinaxNOW-positive employees as infectious without waiting for rRT-PCR confirmation. The rapid turnaround time and high PPV of BinaxNOW enabled some SARS-CoV-2–positive employees to be identified and isolated faster than if rRT-PCR had been used alone. In outbreak situations in which access to laboratory rRT-PCR services is limited, it might be reasonable to act on BinaxNOW-positive results and forgo rRT-PCR confirmation. In contrast, our findings suggest that BinaxNOW negative results in an outbreak investigation should be confirmed with rRT-PCR, because false negatives do occur.
Our results indicate that BinaxNOW performs better at identifying rRT-PCR–positive specimens with lower Ct (suggestive of higher viral loads) and positive viral cultures, although these factors are not precise proxies for infectiousness. Real-time RT-PCR remains a more sensitive test for identifying persons that might be infectious, and our results support the current recommendation that rRT-PCR (or another nucleic acid amplification test) should be used in outbreak situations to confirm BinaxNOW-negative results. Clinical discretion informed by COVID-19 incidence in the relevant population, as well as individual exposure history and symptoms, should be used to determine whether to quarantine persons who test negative for SARS-CoV-2 by BinaxNOW but are awaiting results of rRT-PCR testing.
We thank Lisa B. Hernandez, James Allard, Beimnet Taye, the California Department of Public Health Viral and Rickettsial Disease Laboratory staff, and the racetrack staff described in this report.
Emerging Infectious Diseases. 2021;27(11):2761-2767. © 2021 Centers for Disease Control and Prevention (CDC)