In real-time PCR for respiratory virus detection, what does the Ct (cycle threshold) value represent, and how does it correlate with viral load?

• A Ct is the fluorescence intensity at cycle 40; higher Ct = higher viral load
• B Ct is the number of PCR cycles at which fluorescence crosses the threshold; a LOWER Ct value indicates a HIGHER viral load ✓
• C Ct is inversely proportional to primer efficiency; it does not correlate with viral quantity
• D Ct value above 35 indicates a positive result with high viral load

Explanation

In real-time (quantitative) PCR, the Ct (cycle threshold) is the PCR cycle number at which the fluorescence signal from amplified target DNA crosses a set threshold above background. Since PCR amplification is exponential (doubling per cycle), samples with higher initial template (higher viral load) reach the threshold sooner — at a lower Ct value. Conversely, a high Ct value (e.g., Ct > 35) indicates a low viral load and may represent a borderline or late-stage infection. Each 3.3-cycle increase in Ct corresponds approximately to a 10-fold decrease in initial template concentration. Ct ≥ 40 is typically called undetected.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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