Polymerase chain reaction (PCR) and cycle threshold (Ct) values in COVID-19 testing

Most tests that detect the ribonucleic acid (RNA) or genetic fingerprint of the virus that causes COVID-19 (e.g., a polymerase chain reaction, or PCR test) use a process where specific bits of the genetic fingerprint are amplified using a temperature cycling reaction that repeats up to 45 times. These are called amplification cycles. The more RNA that is present in the patient sample, the fewer cycles are required for the signal to reach the detection threshold (low Ct value). The Ct value is the cut-off that calls a test positive. This is defined by the manufacturer of the test or the laboratory during the validation process.