Cq (Cycle Threshold)
Definition
The PCR cycle at which fluorescence crosses a defined threshold; inversely proportional to starting template concentration and used to calculate DNA quantity from the standard curve.
- Method
- Quantitative PCR (qPCR)
- Relationship to DNA amount
- Inversely proportional, lower Cq means more starting template
- Used with
- Standard curve to calculate DNA quantity
- Older term
- Ct (cycle threshold), used interchangeably in older literature
Common questions
Why does a lower Cq value indicate more DNA in the original sample?+
Cq marks the PCR cycle at which the fluorescent signal from amplified product first crosses a set detection threshold. A sample starting with more template DNA reaches that threshold after fewer amplification cycles, so a low Cq means abundant starting DNA and a high Cq means the sample started with very little.
How is Cq converted into an actual DNA quantity for a forensic sample?+
The examiner runs a dilution series of samples with known DNA concentrations to build a standard curve plotting Cq against log concentration, then reads the unknown sample's Cq against that curve to estimate how much DNA the questioned sample contained before amplification.
Related terms
- A260/A280 Ratio
- Absorbance ratio at 260 nm and 280 nm used to assess nucleic acid purity. Values of 1.7-1.9 indicate pure DNA; lower values...
- Degradation Index (DI)
- The ratio of large-amplicon to small-amplicon Cq values in Quantifiler Trio or equivalent kits; a DI above ~3 indicates significant fragmentation of...
- InnoQuant HY
- Human-specific qPCR quantitation kit with internal PCR control, providing autosomal and Y quantitation plus degradation index; widely used in Japanese and East...
- PicoGreen
- SYBR Green I-related fluorescent dye that intercalates specifically into double-stranded DNA, used for sensitive fluorometric quantitation in plate-reader or Qubit formats.
- Plexor HY
- Promega qPCR quantitation kit using isoguanosine/isocytidine quenching chemistry, providing autosomal and Y-chromosome quantitation with degradation index.
- QuantiBlot
- Human-specific slot-blot quantitation kit (Applied Biosystems) using a probe directed at the D17Z1 alpha-satellite repeat on chromosome 17.
- Quantifiler Trio
- Applied Biosystems qPCR kit reporting autosomal total DNA (small + large amplicon targets) and a Y-chromosome target, with a computed Degradation Index...