1. What Is Ct Value in Real-Time PCR?

Real-Time PCR (qRT-PCR), also known as quantitative PCR, leverages primers and probes (commonly TaqMan chemistry) to produce fluorescence as target DNA is amplified—enabling instant quantitative detection. The Ct value, or cycle threshold, is the number of cycles required for fluorescence to exceed a predefined threshold.

A low Ct value indicates high initial viral load, while a high Ct value suggests lower viral presence. For example, a Ct of 25 implies detection occurred after approximately 2²⁵ amplification cycles.


2. GBC’s Ct Threshold and Sensitivity

GBC’s GB SARS-CoV-2 Real-Time RT-PCR kit (4PCO052E) has a Ct cutoff of 37 cycles and a limit of detection (LoD) of 1,000 copies/mL—demonstrating high analytical sensitivity.


3. Why Ct Values Aren’t Absolute Indicators

Though Ct values are useful, they aren’t definitive. Ct can vary due to:
• Sample collection method, specimen type, and quality
• Reagent design, brand, and lot variability
• Timing of sample collection relative to disease progression
Thus, clinical context and patient symptoms remain primary for decision-making.


4. Interpreting Ct in High-Volume Settings

During surges, some regions may adjust Ct thresholds for ending isolation to balance diagnostic rigor with healthcare capacity. In these contexts, Ct should be interpreted cautiously to avoid premature release from isolation and ensure patient and public safety.


5. Clinical Takeaways
• Low Ct values usually correlate with high infectivity or early disease onset.
• High Ct values may reflect late-stage infection, testing delays, or reduced risk of transmission.
• Ct is a useful reference point but should never replace assessment based on symptoms and clinical judgment.