COVID-19 began sweeping across the world starting from late 2019 and the pandemic continues to this day. Since April 2022, Taiwanese society has entered a stage where tens of thousands of people may be diagnosed in a single day. The government's policy of replacing PCR results with rapid screening tests to alleviate the burden on medical capacity has led to surging demand for rapid screening. This article will briefly introduce the principles, sensitivity, and specificity of rapid screening.
Most rapid screening reagents use lateral flow immunochromatographic assay. The internal structure of the reagent comprises mainly four components: a sample pad, a conjugated pad, a nitrocellulose membrane, and a wicking pad.
Rapid screening can be classified into antigen rapid screening and antibody rapid screening, distinguished primarily by whether the rapid screening conjugated pad is plated with antigens or antibodies. The currently commercially available COVID-19 rapid screening reagents fall under the antigen rapid screening type, which mainly detect the nucleocapsid proteins of the novel coronavirus.
When the rapid screening test is performed, the specimen will first contact the sample pad; and the fibers in each layer will slowly carry the liquid molecules in the sample to the wicking pad at the end of the test via capillary action. If the specimen contains the antigen of the virus protein, then when flowing through the conjugated pad, it will be recognized and bound by the monoclonal antibody that recognizes the antigen. Since the monoclonal antibody in the conjugated pad has been bound to colloidal gold in advance, the test result of the rapid screening reagent shows a red line (T line). Moreover, each rapid screening set contains a control result (C line). The antibody with the colloidal gold marker flows through this region and binds to the antibody on the control line, accumulating color.
To summarize the above, when the added sample contains the virus, the rapid screening nitrocellulose membrane will exhibit two lines, representing a positive result. Conversely, if there is no virus in the specimen, then only a red line will appear in the control result (C line) indicating a negative result.
Regarding the virus collected by the test stick or the saliva collection tube and put into the sample buffer for reaction, the sampling method and the viral load will directly affect the results of the rapid screening. In addition, the performance of the rapid screening reagent will also affect the interpretation of the results. Each brand of rapid screening reagents has different sensitivity (true positive rate) and specificity (true negative rate). Therefore, the dilemma often arises that the rapid screening of brand A is positive but the rapid screening of brand B is negative. At this time, real-time quantitative polymerase chain reaction (Real Time PCR, hereinafter “qPCR”) is needed to assist in interpretation.
qPCR is a nucleic acid detection approach that uses a specific primer and probe. The cDNA template is amplified by PCR 2n-fold, so the sensitivity and specificity of qPCR are better than those of a rapid screening reagent. Among these, the GB SARS-CoV-2 Real-Time RT-PCR kit (4PCO052E) developed by GBC is taken as an example. This product’s determination criteria is a Ct value of 37 and a limit of detection of 1,000 copies/ml. Even with a low viral load, the virus’s presence can still be detected via an amplification of 237 times.
Even though qPCR is better in sensitivity and specificity than rapid screening, due to the long actual operation time and the need for professionals and corresponding equipment, it will cause great pressure on the demand for medical testing if the number of confirmed cases increases. Conversely, the principle behind antigen rapid screening is the interaction between antibodies and antigen proteins. The operation process only takes fifteen minutes and can be undertaken by the general public in room temperature conditions; and the interpretation of results only requires judgment with the naked eye regarding the difference between two lines (positive) and one line (negative). The process is fast and simple, suitable for use when nucleic acid detection capacity is insufficient. In response to adjustments in pandemic prevention policy and the immediate needs of clinical medical care, GBC has developed and launched the GB COVID-19 Ag POCT (4LCO009E) antigen rapid screening kit for professionals and the GB COVID-19 Ag Home test (4LCO019C) home use version antigen rapid screening kit.
1. Abduljalil, J. M. (2020). Laboratory diagnosis of SARS-CoV-2: available approaches and limitations. New microbes and new infections, 36, 100713.
2. Department of Health and Social Care (2021). Guidance: Understanding lateral flow testing for people without symptoms. GOV.UK.