We evaluated the analytical performance of the novel point-of-care BIOSYNEX COVID-19 Ag BSS Ag-RDT compared to multiplex rtRT-PCR for detecting SARS-CoV-2 RNA as the gold standard in a real-life setting. In this study, the sensitivity of the BIOSYNEX COVID-19 Ag BSS Ag-RDT was lower among specimens from asymptomatic persons (79.4%) than among specimens from symptomatic persons (95.0%). It was exceptionally high in patients with suspected COVID-19 (96.8%). Specificity (> 99.0%) was high in specimens from both asymptomatic and symptomatic groups. The prevalence of SARS-CoV-2 RNA-positive rt-RT-PCR results in this population was relatively high (15.3% overall, 9.4% for asymptomatic participants, and 32.6% for symptomatic participants). The estimated PPVs and NPVs of the BIOSYNEX COVID-19 Ag BSS Ag-RDT were elevated in all groups of participants. However, administering the Ag-RDT in lower prevalence settings will likely result in lower predictive values. In the event of significant viral excretion (i.e., N gene Ct values below 33 based on reference rtRT-PCR), the BIOSYNEX COVID-19 Ag BSS Ag-RDT showed high sensitivity (from 83.3–100.0%) and specificity (> 99.0%) for SARS-CoV-2 RNA detection. Concordance, reliability, as well as accuracy were great with the reference assay and PPVs and NPVs above 97.0%. However, the sensitivity of the study Ag-RDT dropped to 55.2% with low or very low viral shedding (Ct> 33). Together, these observations demonstrated the high analytical performance of the BIOSYNEX COVID-19 Ag BSS Ag-RDT. This performance made it suitable for use as point-of-care Ag-RDT in various hospital and non-hospital settings where a rapid diagnosis of SARS-CoV-2 is necessary. Although less sensitive than RT-PCR, the BIOSYNEX COVID-19 Ag BSS Ag-RDT could be beneficial due to its quick results, ease of use, and independence from existing laboratory structures. Testing criteria focusing on patients during the early onset of symptoms could further increase its diagnostic value.
The sensitivity of the BIOSYNEX COVID-19 Ag BSS Ag-RDT was 81.8% overall, and the positive detection rate was comparable to the rtRT-PCR in the majority (88.2%) of patients with Ct ≤ 33. Twelve of 14 (85.7%) false-negative subjects with significant viral excretion (Ct ≤ 33) were asymptomatic, although conflicting evidence exists about the relationship between symptom severity and viral shedding . False-positive test results were rarely observed, providing 99.6%-specificity, exceeding the performance recommended by the World Health Organization (WHO) . False-positive results have been reported in other antigen tests [14–16]. False positivity could be associated with high viscosity of tested specimen samples as well as interference with mucosal antibodies .
Finally, the BIOSYNEX COVID-19 Ag BSS Ag-RDT meets the current WHO criteria which stipulate that Ag-RDTs for SARS-CoV-2 antigens detection must have a sensitivity greater than 80% and a specificity greater than 97% (97–100%) . Furthermore, analytical performances comparable to those in our study Ag-RDT were previously reported for some Ag-RDTs in lateral flow immunoassay format [7, 9, 14, 18–28], while several studies have reported much lower sensitivity levels contrasting with consistently high specificity [3, 29–34]. In addition, the BIOSYNEX COVID-19 Ag BSS Ag-RDT also fulfilled the current recommendations of the French High Authority of Health (Haute Autorité de santé, Saint-Denis, France) for a screening Ag-RTD stating that, at minimum, Ag-RDTs would need to correctly identify significant proportions of symptomatic patients (sensitivity ≥ 80%) as well as asymptomatic individuals (sensitivity ≥ 50%) and have very high specificity (≥ 90%) .
We analyzed our results based on the estimated viral load in SARS-CoV-2 in the samples. There is an ongoing debate about the Ct value corresponding to the threshold of infectivity (i.e., patient considered as contagious) [7, 36, 37]. La Scola et al. found that patients with Ct values > 33 are not infectious because of the low number of positive cultures . The Centers for Disease Control and Prevention (CDC), Atlanta, USA, propose a Ct cut-off value of 33 as a marker for contagiousness , and stress that Ct values ≤ 20 correspond to very high viral excretion [7, 36, 40, 41]. Our results demonstrate that the analytical performances of the BIOSYNEX COVID-19 Ag BSS Ag-RDT were much better in the event of a high viral load, i.e., in the case of significant viral excretion. These observations demonstrate the interest in the BIOSYNEX COVID-19 Ag BSS Ag-RDT as a rapid rule-in test for COVID-19 with samples at high viral load in symptomatic patients, for example, and raise caution about its use as a singular rule-out test, especially in samples with lower viral loads.
Our study has several strengths. All samples were collected from one nasopharynx with flocked swabs, optimal for evaluating Ag-RDT clinical performances in our study. The Ag-RDT and reference rtRT-PCR were carried out in parallel. The study population included various situations outside the hospital setting, with most young adults without comorbidities, who mostly had typical and mild COVID-19 symptoms when being symptomatic. This finding describes most individuals infected with SARS-CoV-2 and represents an essential group for limiting transmission in the community.
The study presents some limitations. Participants may have inadvertently reported general, non-specific symptoms as COVID-19 compatible symptoms. This investigation evaluated the BIOSYNEX COVID-19 Ag BSS antigen test; the results presented here cannot be generalized to other agencies-authorized SARS-CoV-2 antigen tests. The BIOSYNEX COVID-19 Ag BSS antigen test characteristics might differ depending on whether an individual had previously tested positive. Finally, the CDC clarified that Ct values using the rtRT-PCR platform is not a quantitative measure of viral burden in clinical samples and cannot be used to assess when a person is no longer infectious . Consequently, our stratification of samples according to Ct values of the N gene does not necessarily reflect the actual infectivity of the participants.