The Elecsys® SARS-CoV-2 Antigen assay uses monoclonal antibodies directed against the SARS-CoV-2 N protein in a double-antibody sandwich assay format for the qualitative detection of SARS-CoV-2 in upper respiratory tract specimens.31
Immunoassay for the qualitative detection of the SARS-CoV-2 nucleocapsid antigen
SARS-CoV-2, the causative agent of COVID-19, is an enveloped, single-stranded RNA Betacoronavirus.1-3 7 coronaviruses have been identified as agents of human infection, causing disease ranging from mild common cold to severe respiratory failure.4 Coronaviruses share the 4 structural proteins spike (S), envelope (E), membrane (M), and nucleocapsid (N), the latter being the most abundant.5-8
SARS-CoV-2 is transmitted primarily from person-to-person through respiratory droplets and aerosols.9,10 The incubation period from infection to detectable viral load in the host commonly ranges from 2 to 14 days.11,12 Detection of viral load can be associated with the onset of clinical signs and symptoms, although a considerable proportion of individuals remains asymptomatic or mildly symptomatic.13-15 The interval during which an individual with COVID-19 is infectious has not yet been clearly established, however, transmission from symptomatic, asymptomatic, and pre-symptomatic individuals has been well described.16-18
An effective strategy for controlling the COVID-19 pandemic is to develop highly accurate methods for the identification and isolation of SARS-CoV-2 infected patients.19 Diagnostic confirmation of acute SARS-CoV-2 infection can be based on the detection of unique sequences in the viral RNA or detection of viral proteins in respiratory tract samples from infected individuals.20 Viral antigens are only expressed when the virus is actively replicating, thus making antigen tests clinically useful for identification of acute or early infection.21,22 Current research suggests active replication of SARS-CoV-2 in the throat with high viral shedding in the first 5 days of infection, and infectious virus could be isolated from respiratory samples up to the first 7 – 9 days post symptom onset, indicating potential feasibility of antigen detection using throat swabs.23-25 This time period also coincides with the time when the highest viral load is generally observed in infected individuals.14,26-28 Therefore, the best performance of antigen tests is seen around symptom onset in symptomatic individuals and the initial phase of infection.20 Testing of mildly symptomatic or asymptomatic individuals can be considered in the assessment of contacts of confirmed infected persons.20 Antigen tests can also become part of regular testing regimens for identifying, isolating, and thus filtering out currently infected persons, including those who are asymptomatic.29,30
* prepared according to CDC SOP DSR-052-05; ** between runs; # coefficient of variation
The limit of detection (LoD) in different transport media was determined by limiting dilution studies using an inactivated viral lysate (USA-WA1/2020). LoD is defined as the lowest detectable concentration of SARS-CoV-2 at which a minimum of 19 from 20 replicates per concentration generate a reactive test result (≥1.0 COI), and expressed as TCID50*/mL.
|COPAN Universal Transport Medium (UTM-RT)||22.5|
|CDC Viral Transport Medium||22.5|
|Sterile Saline (0.9 % NaCl)||37.5|
Relative sensitivity was evaluated using 232 nasopharyngeal and 158 oropharyngeal swab specimens, collected from individuals with signs and symptoms suggestive of COVID-19, with known or suspected exposure to SARS-CoV-2, and from individuals undergoing pre-admission screening before hospitalization for surgical intervention unrelated to an infectious disease.
All subjects included in the analysis were tested positive in the cobas® SARS-CoV-2 RT-PCR test32. RT-PCR-positive samples were further stratified using Target 2 (structural protein envelope E-gene/ pan-Sarbecovirus detection) cycle threshold (Ct) values.
The figure below correlates the performance of the Elecsys® SARS-CoV-2 Antigen assay in all RT-PCR-positive samples from symptomatic and asymptomatic individuals to the cobas® SARS-CoV-2 Ct values.
* structural protein envelope E-gene/pan-Sarbecovirus detection; ** N (cumulative): reactive in the Elecsys® SARS-CoV-2 Antigen assay/total
The table below shows additional analyses based on days post-symptom onset (DPSO) and stratification by a cobas® SARS-CoV-2 Ct value of 30. The resulting overall relative sensitivity in symptomatic individuals with a cobas® SARS-CoV-2 Target 2 Ct value <30 was 94.5 % (95 %, CI two-sided: 90.4 – 97.2 % [189/200]).
|Cohort||cobas® SARS-CoV-2 Ct <30||cobas® SARS-CoV-2 Ct ≥30|
|N||Non-reactive||Sensitivity (95 % CI§)||N||Non-reactive||Sensitivity (95% CI§)|
(92.8 – 99.5 %)
(12.3 – 45.9 %)
(90.3 – 97.8 %)
(14.3 – 34.0 %)
(19.4 – 99.4 %)
(3.6 – 41.4 %)
(82.3 – 99.4 %)
(6.8 – 49.9 %)
(49.2 – 95.3 %)
(0.1 – 15.3 %)
(42.8 – 94.5 %)
(0.2 – 28.7 %)
The following table summarizes the relative sensitivity of the Elecsys® SARS-CoV-2 Antigen assay in RT-PCR-positive samples from symptomatic patients, stratified by days post symptom onset, and a cobas® SARS-CoV-2 Target 2 Ct value of 30.
Relative specificity of the Elecsys® SARS-CoV-2 Antigen assay was evaluated using 2,747 RT-PCR negative naso-/oropharyngeal swab specimens, collected from individuals with signs and symptoms suggestive of COVID-19, with known or suspected exposure to SARS-CoV-2, and from individuals undergoing pre-admission screening before hospitalization for surgical intervention unrelated to an infectious disease.
|Cohort||N||Reactive||Specificity (95 % CI)|
|Symptomatic||548*||0||100% (99.3 - 100%)|
|Known/suspected exposure and screening||2199**||4||99.8% (99.5 - 100%)|
|Overall||2747||4||99.9% (99.6 - 100%)|