Coronaviruses are a large family of viruses which may cause illness in animals or humans. SARS-CoV-2 is an enveloped, single-stranded RNA virus of the β genus. The virus can cause mild to severe respiratory illness and has spread globally. The most common symptoms of COVID-19 include fever, cough and fatigue. The incubation period for COVID-19, which is the time between exposure to the virus (becoming infected) and the onset of symptoms, is on average 5-6 days, however can be up to 14 days. The window period for COVID-19, which is the time between exposure to the virus (becoming infected) and the appearance of detectable antibodies to the virus can be up to 24 days. 

The virus encodes 4 main types of structural proteins, which include spike (S) protein, envelope (E) protein, membrane (M) protein and nucleocapsid (N) protein. The S protein contains a receptor binding domain (RBD), and SARSCoV-2 attaches to the host cell through the interaction of its RBD with the host surface receptor, namely angiotensin converting enzyme 2 (ACE2). In the respiratory tract, ACE2 is widely distributed on the epithelial cell surface of the pulmonary alveoli, trachea and bronchi of the host. SARS-CoV-2 identifies and binds its RBD to the host’s ACE2, then enters into the host’s cell through membrane fusion, triggering an infection. 

The immune response of the body produces antibodies recognizing virus antigens, and antibodies that can block the interaction of SARS-CoV-2 with the host’s cell are called neutralizing antibodies (NAbs). Studies have shown that the NAb titer of COVID-19 patients is closely related to the IgG immunoglobulin concentration binding to the RBD protein, indicating that the RBD region on the S protein may contain the main neutralizing epitope. Detection of RBD neutralizing antibodies can be used  for the assessment of the immune response after COVID-19 infection or after vaccination.

Antibody is an immunochromatographic membrane assay that uses RBD protein to detect anti—RBD antibody in sample (human whole blood, serum or plasma) via the double antibody sandwich method. After loading, the sample will diffuse upwards from the charging end by capillary action, and then the anti- RBD antibody in the sample will combine with recombinant RBD protein colloidal gold conjugate in the marker pad to form colloidal gold antibody-antigen complex; the complex continues to diffuse upwards to the nitrocellulose membrane with the sample, and then blocked by Test (T) Line packed with mouse anti-human-IgG antibody to form colloidal gold labeled antibody-antigen antibody complex. The remaining unblocked colloidal gold conjugate continues to move upwards and combine with Control (C) line (quality control line), indicating that the reaction is completed.

• The assay time is shorter (within 10 minutes as compared to around 24 hours for enzyme-linked immunosorbent assay (ELISA))
• The cost is cheaper than ELISA
• The test can be carried out at the point-of-care
• It is a one-step assay that requires no special skills or instrumentation to achieve the result(s)
• It has a long shelf life and assay reagents do not require refrigeration or freezer storage
•  

1. Open the test cassette package and lay it flat.
2. Apply 1 drop (~ 20μL) of whole blood, serum or plasma using the disposable Pasteur pipette that is provided in the kit onto the oval charging hole (marked 💧) of the Test Cassette.
3. Apply 3 drops (~ 60μL) of sample eluent onto the charging hole of the Test Cassette. The sample will then be drawn by capillary action into the Test Strip.
4. Start a timer and read the results after 10 minutes. The result is invalid after 15 minutes.
5. Dispose of the Pipette and Test Cassette in the appropriate biohazard waste.

Note: In a valid, tested device, a red line appears at the Control Line position, confirming that the sample has flowed through the test strip and the reagents are working.

The test results are analyzed as follows:
Negative
A negative specimen will give a single red colored Control Line (C) in the top half of the window, indicating a negative result. This Control Line means that the detection part of the test was done correctly, but no anti-COVID-19 RBD antibody was detected.

Positive
A positive specimen will give two red colored lines, Control Line (C) and Test Line (T). This means that anti-COVID-19 RBD antibody was detected. Specimens with low levels of antibody may give a faint Sample Line. Any visible red colored Sample Line is positive.

Invalid
If no lines are seen, if just the red Test Line (T) is seen, or if the red Control Line is not seen, the assay is invalid. Invalid tests should be repeated.

• Huang, Chaolin, Yeming Wang, Xingwang Li, Lili Ren, Jianping Zhao, Yi Hu, Li Zhang et al. “Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China.” The lancet 395, no. 10223 (2020): 497-506.
• Astuti, Indwiani. “Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2): An overview of viral structure and host response.” Diabetes & Metabolic Syndrome: Clinical Research & Reviews 14, no. 4 (2020): 407-412.
• Jiang, Shibo, Christopher Hillyer, and Lanying Du. “Neutralizing antibodies against SARS-CoV-2 and other human coronaviruses.” Trends in immunology 41, no. 5 (2020): 355-359.
• World Health Organization (WHO). URL: https://www.who.int/emergencies/diseases/novel-coronavirus-2019. Accessed on 1st December 2020.

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