PCRvärden

PCRvärden refers to the numerical outputs produced by quantitative polymerase chain reaction (qPCR) assays. The most common measure is the cycle threshold value, abbreviated Ct or Cq, which reflects how much target nucleic acid is present in a sample at the start of the assay. In Swedish reports the term Ct-värde or Cq-värde is used.

Ct value is defined as the cycle number at which fluorescence from amplified product crosses a defined threshold. Because PCR amplification is exponential, lower Ct indicates higher starting amount of target; roughly a 3.3 cycle difference corresponds to a tenfold change in copy number when reaction efficiency is near 100%.

Quantification can be absolute or relative. Absolute quantification uses a standard curve with known concentrations to calculate copy number in the sample. Relative quantification compares target gene expression between samples using reference genes and methods such as delta-delta Ct, assuming similar amplification efficiencies.

Several factors influence PCRvärden: sample quality (RNA integrity), extraction efficiency, presence of inhibitors, primer/probe performance, and platform characteristics. Reproducibility is improved using technical replicates, controls (negative and positive), and appropriate standard curves.

Interpretation of Ct values requires context: values near the limit of detection require caution; controls determine assay validity; in clinical diagnostics thresholds are assay-specific and must be validated. PCRvärden provide relative or absolute measures of target nucleic acids in a sample, but are not a direct physical quantity without calibration.