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How do I convert reverse transcription (RT) qPCR Ct values to SARS-CoV-2 viral copies?
A Ct value obtained from RT-qPCR 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).