CT Value (Cycle Threshold) in RT-PCR: Lower Value-Higher Viral Load|Different from CT Score YouTube Lecture Handouts

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CT Value (Cycle Threshold) in RT-PCR: Lower Value - Higher Viral Load | Different from CT Score

Title: Cycle Threshold CT Value in RT-PCR

What Does It Mean?

  • An individual is considered COVID-19 negative, only if the CT value is 35 in the RT-PCR test. So, if the CT value is below 35 in the RT-PCR test, then the patient is coronavirus positive.
  • Short for cycle threshold, Ct is a value that emerges during RT-PCR tests, the gold standard for detection of the SARS-CoV-2 coronavirus. In an RT-PCR test, RNA is extracted from the swab collected from the patient.
  • It is then converted into DNA, which is then amplified. Amplification refers to the process of creating multiple copies of the genetic material — in this case, DNA. This improves the ability of the test to detect the presence of virus. Amplification takes place through a series of cycles — one copy becomes two, two becomes four, and so on — and it is after multiple cycles that a detectable amount of virus is produced.

Cycle Threshold

Less CT Value High Viral Load

More CT Value Low Viral Load

  • According to the ICMR advisory, the Ct value of an RT-PCR reaction is the number of cycles at which fluorescence of the PCR product is detectable over and above the background signal. Put simply, the Ct value refers to the number of cycles after which the virus can be detected. If a higher number of cycles is required, it implies that the virus went undetected when the number of cycles was lower. The lower the Ct value, the higher the viral load — because the virus has been spotted after fewer cycles.
  • Globally, the accepted cut-off for Ct value for Covid-19 ranges between 35 and 40, depending on instructions from the respective manufacturers of testing equipment. The ICMR has arrived at the Ct value of 35 based on laboratory experiences and inputs taken from several virology labs.
  • With a low cycle run, one can still get a positive result if the value load is high. If you lower down the CT value, that will pose a serious problem because the person is still infected, he would have a low viral load - the person would be negative. Although he may have an illness, he may not seek treatment. False-negative results could be able to transmit the infection to the family and the society. In a similar manner, a person can have a high CT value but still have a very significant level of covid 19 infection, a patient can have a low CT value, but the patient may be asymptomatic. Many factors like the collection of the sample, what day it was collected, where was it stored, what were the reagents used.

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