COVID-19 Antigen Test- Introduction, Principle, Test Requirements

Introduction of COVID-19 Antigen Test

Table of Contents

SARS-CoV-2 antigen test for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is used as a screening test on asymptomatic persons to detect or exclude Coronavirus Disease 2019 (COVID-19) or to determine whether a person who previously was diagnosed with COVID-19 remains infectious. It is an immunoassay for a nasopharyngeal or nasal swab specimen. The test is simple, rapid, and easy to perform even in a low-level laboratory setup. The report may be dispatched within half an hour.

Principle of COVID-19 Antigen Test

The immunochromatographic test principle is the same as the sandwich ELISA method. The only difference is the immunological reaction. It is carried out on the chromatographic paper by capillary action. For this system, two kinds of specific antibodies against the antigen are used. One of the antibodies is immobilized on the chromatographic paper while another is labeled with colloidal gold and infiltrated into a sample pad. An immunochromatographic unit is completed by attaching the sample pad to the end of the membrane. The liquid sample is dropped on the sample pad. The antigen in the sample forms an immunocomplex with the antibody labeled with colloidal gold.

It’s complex moves along with the liquid sample and makes contact with the antibody immobilized on the membrane. It is followed by forming an immuno-complex with the immobilized antibody. It results in generating a colored line. The appearance of the band on the membrane indicates the presence of the antigen of interest in the sample. The liquid of the sample migrates through the membrane very fast, which makes it possible to detect the presence or absence of antigen within 15 minutes, as shown above image.

Components of an Immunochromatographic test device

Sample pad: It acts as a sponge and holds an excess of sample fluid. Once soaked, the fluid migrates to the second element.
Conjugate pad: A dried format of bio-active particles in a salt-sugar matrix that contains everything to guarantee. It is an optimized chemical reaction between the target molecule (e.g., an antigen) and its chemical partner (e.g., antibody) that has been immobilized on the particle’s surface. Control ( reagent line): It contains an antibody that picks up free latex/gold to confirm the test has operated correctly.
Test: It contains a specific capture molecule and only captures those particles onto which an analyte (antigen ) molecule has been immobilized.

Requirements for COVID-19 Antigen Test

PPE set/ Sample collecting safety cabinet for sample collection
The test kit contains- a Test device in a pouch, a buffer, and a dropper.
nasopharyngeal or nasal swab sticks
Marker
Reporting table
Waste disposal box
Hypochlorite solution

Procedure of SARS-CoV-2 Antigen Test

Take a nasopharyngeal or nasal swab.
Place directly into the assay’s extraction buffer or reagent.
Mix it properly.
Dispense 2 drops ( specimen mixed buffer) in the sample pad of the test device as shown above image and in the video clip.
Wait for 15 minutes for band observation.

Result Interpretation of COVID-19 Antigen Test

Test negative: Only on the band at the control region ( reagent region as shown above the test device)
Test positive: Both bands at the test and control (r) regions
Test invalid: No band at all or band only in the test region
Note: The Test is positive as shown above image because of has two bands at the control ( i.e., reagent line) and the test region.

Clinical Significance of COVID-19 Antigen Test

It is a screening test on asymptomatic persons to detect or exclude COVID-19 or to determine whether a person who previously was diagnosed with COVID-19 remains infectious.
It is a very simple test.
Rapid assays can be completed in 15-20 minutes.
It reduces the need for trained examiners and costly equipment.
It can be used under harsh field conditions.
Disadvantages of SARS-CoV-2 Antigen Test
The “gold standard” for clinical diagnostic detection of SARS-CoV-2 is nucleic-acid amplification tests(NAATs), such as reverse transcription PCR ( RT-PCR).
The sensitivity of antigen tests varies but is generally lower than most NAATs.
The performance of antigen tests can be affected if the test components are not stored and handled properly. They should never be frozen and should always be brought to room temperature (15-30°C) before use.
Despite the high specificity of antigen tests, false-positive results are found in this test and thus it is most commonly used in the community.
Even being a rapid test, laboratory and testing professionals who conduct diagnostic or screening testing for SARS-CoV-2 with antigen tests must also comply with Clinical Laboratory Improvement Amendments (CLIA) regulations since the organism is a high-risk group (3).

Keynotes on COVID-19 Antigen Test

Some antigen assays have explored the use of viral transport medium (VTM) during sample collection, but the use of VTM may dilute the specimen and may decrease the sensitivity of the assay (possibly causing false test results).
Decontaminate work surfaces and equipment with appropriate disinfectants by using an EPA (emergency use authorization ) -approved disinfectant for SARS-CoV-2.

COVID-19 Antigen Test- Introduction, Principle, Test Requirements, Procedure, Result Interpretation, Clinical Significance and Disadvantage