CT value and covid testing

Subject: Science and tech

Context:

Currently in India, RT-PCR tests can tell if a person is infected with novel coronavirus or not. It does not reveal the amount of virus (viral load in scientific parlance) present in the person.

Concept:

• The PCR test amplifies the genetic material from coronavirus through multiple cycles.
• Since coronavirus has RNA, it is first converted into DNA, and each cycle of amplification doubles the amount of DNA.
• If there is just one DNA molecule to start with, the amount of DNA after 30 cycles of amplification will be 230 (2 raised to 30) times, or one billion molecules.
• If there is more genetic material to begin with, then fewer cycles of amplification would be sufficient to detect the DNA.
• While the cycle threshold (Ct) value can be suggestive of the amount of virus in an infected person, there is no reliable way of correlating the Ct value with COVID-19 disease severity or infectiousness.
• The Ct (cycle threshold) is defined as the number of cycles required for the fluorescent signal to cross the threshold (ie exceeds background level).
• Ct levels are inversely proportional to the amount of target nucleic acid in the sample (ie the lower the Ct level the greater the amount of target nucleic acid in the sample)