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Coronavirus

CT VALUE IN COVID 19 RT-PCR: Explained for the common man

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What is CT Value?

CT stands for cycle threshold value. It is defined as the number of cycles or turns taken by a sample in a molecular test like RT-PCR to release signal that reaches a certain threshold.

Let us discuss it with a simple example. I have 4 cats in a room. How many times would they have to mate (which is the no. of cycles) to reach a target of say, 10 cats in total (which is the threshold value). Higher the no. of cats, lower the no. of times they need to mate.

I have a CT value of 23. What does it mean?

As discussed in the example above, a low CT value means that the threshold value is reached with lesser no. of cycles. This indicates that the initial viral load (or the no. of cats in the room) was high. So this patient may possibly be very infectious and a source of infection to people he/she comes in contact with. So, a patient with a CT value of 18-20 may be considered highly infectious even if he/she has mild symptoms

I have a CT value of 33. What does it mean?

A high CT value means that it takes a higher no. of cycles to reach the threshold value. So it may indicate that the patient has a low viral load and may not be very infectious for the people around him.

Is it a reliable indicator of the infectivity of an individual?

This is the most important question that we need to understand. It may indicate an infectious state but is affected by various factors, which cannot be controlled.

What are the factors that can affect the CT value of an infected individual?

  • These values differ from one kit to another.
  • They also depend on how well the sample has been collected.
  • It may differ if taken from different areas (nasal or oral) from the same patient.
  • It is also affected by the temperature and duration of transportation of the sample.

What is the controversy surrounding it?

As we have understood it until now, the CT value seems to be inversely proportional to the viral load of an individual.  However, there are no reliable studies to prove this correlation definitely.  Low CT values may be associated with a high viral load. But a high viral load does not play many roles in patient management. 

Also, a high CT value, meaning low viral load, may give the patient a flawed and untrue sense of security that he/she cannot spread the infection making him/her go careless and lower his bars of containment and isolation.

The bottom line is, the severity of COVID 19 infection and disease is dependent on numerous host factors other than the viral load like the patient’s response to the infection, his immunity, and coexisting diseases. So using CT value to guide patient management is not recommended until further studies give more proof in this regard.

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