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.

The SG Diagnostics COVID-19 Immunity Strength Semi-Quantitative Detection Kit (Quantum Dot Fluorescence Immunoassay) is a competitive immunochromatographic membrane assay that uses quantum dot labelled RBD protein and recombinant human ACE2 receptor protein to detect anti—RBD antibody in sample (human whole blood, serum or plasma) by detection of the inhibition of the RBD-ACE2 interaction. 

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-quantum dot conjugates in the marker pad to form quantum dot antibody-antigen complex; the complex continues to diffuse upwards to the nitrocellulose membrane with the sample. RBD protein-quantum dot conjugates in the specimen that are not blocked by the neutralizing anti-RBD antibody from the sample are intercepted by Test (T) Line packed with recombinant human ACE2 receptor protein (hACE2), forming a quantum dot-labeled RBD-hACE2 immune complex, indicated by a red T-line. The remaining immune complex of quantum dot labeled RBD blocked by neutralizing anti-RBD antibody continues to move upwards and combine with Control (C) line (quality control line), indicating that the reaction is completed. The intensity of the detected T-line is inversely proportional to the effective activity of neutralizing anti-RBD antibodies (NAb) in the sample.

The Quantum Dot Fluorescence Analyzer scans the test cassettes by activating the quantum dots with optical signals and receiving the optical signal re-emitted. The fluorescence intensity correlates with the concentration of analyte in the sample. The machine analyses the signal it received to give a semi-quantitative result of analyte concentration.

• 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
• The samples do not need to be pre-treated before applying to the assay

Note: The test cassette must be at room temperature before use, and the test must be operated at room temperature.

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 and start a timer.
4. For specific operation of the detecting device, refer to the operating instructions of the fluorescence immunity analyzer provided along with the kit.
5. The detection must start exactly 10 minutes after the beginning of the reaction.
6. Calibration and quality control: Insert an IC card matching the batch number of this item, and read the information in the IC card for calibration. The corresponding control material is used for quality control, and measured control values shall fall within the established quality control range. If control values are out of control, the laboratory shall take appropriate corrective actions. 
7. Read the results after exactly 10 minutes.
8. 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.

Test value Explanation of results
≥ 20% This indicates that SARS-CoV-2 neutralizing anti-RBD antibodies are positive.
< 20% This indicates that SARS-CoV-2 neutralizing anti-RBD antibodies are negative.

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. The device will report the following error: “The C-line is not detected.” Invalid tests should be repeated.

Coming soon

• 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.

Coming soon