CtWerte

CtWerte, or cycle threshold values, are a central metric in real-time reverse transcription PCR (RT-qPCR) assays used to detect nucleic acids. In RT-qPCR, amplified DNA is labeled with fluorescence and measured each cycle. The Ct value is the cycle number at which the fluorescence crosses a predefined threshold above background.

Interpretation: Ct values are inversely related to the amount of target nucleic acid present in the sample. A low Ct indicates a relatively high starting copies; a high Ct indicates a lower starting amount. Ct values are semi-quantitative and depend on the assay, the reagents, the instrument, and the sample type.

Positivity and quantification: Laboratories set decision thresholds; a commonly used cutoff is a Ct value below about 35 cycles for a positive result, but this varies by assay. Some protocols report weak positives at higher Ct values or require retesting. Ct values can be converted into an estimate of viral load only with a standard curve; without it, they provide relative, not absolute, amounts.

Variability and limitations: Ct values can differ across platforms, extraction methods, and sample quality. Pre-analytical factors such as collection technique and transport conditions, plus the presence of inhibitors, can affect Ct. Consequently, Ct values should not be used in isolation for clinical decisions about infection status or infectiousness.

Clinical use: In some clinical contexts, Ct values aid in monitoring disease progression or response to treatment when validated for that purpose; however, comparability between assays is limited.