Antigen Testing for COVID-19: Principle, Procedure, Results

The SARS-CoV-2 viral antigen is generally detectable in nasopharyngeal swab specimens during the acute phase of infection. Most of the currently available antigen testing devices for the diagnosis of COVID-19 relies on qualitative detection of nucleocapsid protein antigen of SARS-CoV-2 in nasopharyngeal swab specimens.  One of the test devices i.e. Sampinute COVID-19 Antigen MIA detects receptor binding domains (RBDs) spike proteins.

Sample

Nasopharyngeal swab specimens directly collected or collected in viral transport medium from individuals who are suspected of COVID-19 (a person has signs or symptoms consistent with COVID-19, or when a person is asymptomatic but has recently known or suspected exposure to SARS-CoV-2) by their healthcare provider within the 5-12 days (varies according to device manufacturer) of symptoms onset.

Lists of FDA EUA antigen testing devices currently in use

Principle

Most of the currently availale and authorized antigen-testing devices are detecting the nucelocapsid protein antigen of SARS-CoV-2 but the method used for detecting that antigen may vary among manufacturers. Though the type of immunoassay used differ among manufacturers, all of them are using anti-SARS-CoV-2 nucleocapsid protein antibodies conjugated to the indicator to capture the test antigen.

The principle described here is of lateral-flow immunoasay.

Nasopharyngeal swab specimen of the patient collected in the direct swab or in viral transport medium is transferred to the extraction vial, during which time the virus particles in the sample are disrupted, exposing internal viral nucleoproteins. Extracted swab sample is then added to the sample well of the test device to initiate the test.

When the swab sample migrates in the test strip, SARS-CoV-2 viral antigens bind to anti-SARS-CoV-2 nucleocapsid protein antibodies conjugated to indicator and capture particles in the test strip forming an immune complex. The immune complex is then captured by immobilized antibodies present in the test line on the nitrocellulose membrane as it migrates through the strip.

Result Interpretation(s)

Depending on the test principle, results of COVID-19 antigen testing is available within 10-30 minutes. Test results are interpreted only after the positive control and negative controls give expected results. In the case of the immunochromatographic assay, the control line should always appear for the test procedure to be valid.

1. Positive: Positive results indicate the presence of viral antigens thus the presence of the virus, but the clinical correlation with patient history and other diagnostic information is necessary to determine infection status. Positive results do not rule out a bacterial infection or co-infection with other viruses.
2. Negative:  Negative results are presumptive and confirmation with a molecular assay, if necessary, for patient management may be performed. Negative results do not rule out SARS-CoV-2 infection and should not be used as the sole basis for treatment or patient management decisions, including infection control decisions.

Advantages and limitations of antigen-testing devices

Advantages

1.  Fast: The currently authorized devices return results in approximately 15 minutes.
2. Inexpensive: Most of the COVID-19 antigen testing devices are relatively inexpensive
3. Easy:  Antigen tests for SARS-CoV-2 can be used at the point-of-care.

Limitations

• Less sensitivity: Antigen tests for SARS-CoV-2 are generally less sensitive than viral tests that detect nucleic acid using reverse transcription polymerase chain reaction (RT-PCR). 

References and further readings

1. Hagen A. (2020, April). COVID-19 Testing FAQs. American Society for Microbiology. https://asm.org/Articles/2020/April/COVID-19-Testing-FAQs
2. In Vitro Diagnostics EUAs. U.S. Food & Drug Administration. https://www.fda.gov/medical-devices/coronavirus-disease-2019-covid-19-emergency-use-authorizations-medical-devices/vitro-diagnostics-euas#individual-antigen