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A Ct value obtained from qRT-PCR can be converted to viral copy number if a standard/control of known copy number was also used in the assay.
If no standard was used, the Ct value can only give an approximate magnitude of starting viral copies. This is because Ct values can vary depending on the protocol/reagents being used as well as on which part of the SARS-CoV-2 genome is being targeted.
Although an exact copy number cannot be calculated, it is generally considered that a Ct value ≥35 represents a very low viral titer (<100 genome copies).
The xGen™ SARS-CoV-2 Amplicon Panel used with the xGen Amplicon Core Kit and indexing primers supports a wide range of inputs/Ct values. A low input protocol has been provided for samples with low viral titer. We recommend using the low input protocol for Ct >28.