1. The Global Shift to Rapid Screening

With recurring COVID-19 case surges, many regions—including Taiwan since April 2022—struggled to maintain PCR testing capacity. Rapid antigen tests gained traction because they are easy, fast, and suitable for mass screening.


2. How Rapid Antigen Tests Work (Lateral Flow Assay)

Rapid tests use lateral flow immunochromatography, composed of four key components:
• Sample pad → receives nasal or saliva sample
• Conjugate pad → carries monoclonal antibodies bound to colloidal gold
• Nitrocellulose membrane → displays results
• Wicking pad → ensures liquid flow via capillary action

If viral antigen (typically nucleocapsid protein) is present, you’ll see both T (test) and C (control) lines—indicating a positive result. Without antigen, only the C line appears, indicating a negative result.


3. RT-PCR: The Gold Standard

PCR amplifies viral RNA (converted to cDNA) using a specific primer-probe design, increasing target DNA exponentially—thus offering much higher sensitivity and specificity than antigen tests. GBC’s GB SARS-CoV-2 RT-PCR kit (4PCO052E) features:
• Ct threshold of 37 cycles
• Detection limit: 1,000 copies/mL
• Capable of detecting low viral loads through 2³⁷-fold amplification

4. Rapid antigen tests and RT-PCR serve different purposes in COVID-19 diagnostics, each with distinct strengths:

• Speed — Rapid tests deliver results in about 15 minutes, while RT-PCR usually takes several hours.
• Ease of Use — Rapid tests can be performed by anyone, even at home. RT-PCR requires trained personnel and specialized equipment.
• Sensitivity — RT-PCR is far more sensitive, capable of detecting very low viral loads. Rapid tests are better at identifying cases with high viral loads.
• Use Cases — Rapid tests are ideal for mass screening and self-checks, while RT-PCR remains the gold standard for clinical confirmation and early detection.

In short, rapid tests are designed for convenience and speed, whereas RT-PCR ensures maximum accuracy and plays a critical role in healthcare decision-making.

5. Interpreting Discordant Results

Occasionally, a rapid test may show positive, and PCR negative, or vice versa. This can be due to:
• Variations in sample collection or viral load
• Differences in kit sensitivity or quality
In such cases, PCR serves as a confirmatory test.