In real-time quantitative PCR (qPCR) using TaqMan probes, the probe is labeled with a fluorophore at the 5' end and a quencher at the 3' end. During extension, Taq polymerase's 5'-to-3' exonuclease activity degrades the probe, separating fluorophore from quencher. Fluorescence increases proportional to PCR product amount. A laboratory receives a sample where the qPCR amplification curve shows a high cycle threshold (Ct value of 38 out of 40 cycles). What does this indicate?

• A Very high concentration of target DNA in the original sample
• B Inhibition of Taq polymerase by sample components, producing a falsely low signal
• C Very low concentration of target DNA, with detectable amplification only near the detection limit ✓
• D Probe degradation in the absence of target, causing non-specific fluorescence

Explanation

In qPCR, the Ct (cycle threshold) value is inversely proportional to the starting amount of target DNA. A low Ct (e.g., 15-20) indicates abundant target; a high Ct (e.g., 38-40) indicates very low target concentration requiring many amplification cycles before the signal crosses the threshold. Each unit increase in Ct corresponds to approximately a 2-fold decrease in starting template concentration. A Ct of 38 in a 40-cycle assay indicates that the sample is near the limit of detection — the target is barely present. This is clinically relevant in virology (e.g., Ct >35 for SARS-CoV-2 may indicate non-infectious viral load) and prenatal diagnosis (cell-free DNA). Inhibition would cause no amplification or irregular curves, not a reproducible high Ct.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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Written and medically reviewed by the StethoPrep medical team.