Table 3.

Limitations of Using Ct Values for SARS-CoV-2 RT-PCR Testing

Definition

A Ct value is the number of PCR amplification cycles required to reach a fixed level of fluorescence at which the result of real-time PCR changes from negative (not detectable) to positive (detectable). In general, a higher Ct value indicates a lower viral RNA titer and a lower Ct value indicates a higher viral RNA titer, but these are not quantitative tests.

Cautions

• No COVID-19 test has been validated as a quantitative assay. Ct values can be used as rough estimates of the viral RNA concentration in a specimen only.

• Ct values are “not comparable” from one assay to another.

• Ct values can vary significantly depending on the NAAT, sample type, consistency in sample collection, time from infection to testing.

• There is no international standard by which results from different tests can be calibrated.

• Residual RNA may be detected from nonviable virus.

• When comparing data from different studies, Ct values “should be considered as trends” rather than absolute values.

• Ct values “should not be used” to define whether or not an individual is infectious.

Abbreviations: COVID-19, coronavirus disease 2019; Ct, cycle threshold; NAAT, nucleic acid amplification test; RNA, ribonucleic acid; RT-PCR, reverse-transcription polymerase chain reaction; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.