The coronavirus pandemic has had medical lingo sliding into day-to-day conversations of a layman. Pandemic, PPE, N-95, herd immunity are among some of the many technical terms that are now used and understood by many outside the medicine fraternity. Another entrant in the list is Ct value or Cycle Threshold value in a RT-PCR test, which is often hailed as the gold standard for Covid testing.
RT-PCR tests always mention a Ct count. The Ct value determines the number of cycles it takes to finally detect coronavirus in a sample. Other than helping conclude if a person is Covid-19 positive or not, the number of cycles also helps determine the viral load.
Lower the Ct value, higher is the viral load, and higher the Ct value, lower is the viral load.
A Ct value below 35 means a person is Covid-19 positive, whereas a Ct value above 35 means the suspected patient has tested negative. To put it in another way Ct value does not correlate with severity but only with infectivity. It tells of the viral load in the throat and not in the lungs.
That said, viral load or Ct value don't necessarily indicate the degree of sickness or help determine possible symptoms in anyway. For example, a person can have a low Ct (high viral load) and still have little or no symptoms.
Then why is Ct count important?
The benchmark 35 for Ct value has been established by the Indian Council of Medical Research (ICMR) for India on the basis of inputs from various virology labs and lab experiences. Globally, the Ct value may range from 35-40 on the basis of testing kits used in various countries.
Ct value's benchmark i.e. 35 was set such so that no infectious patients are missed from the count.
Suppose this benchmark is lowered to 24, suspected patients with Ct value in the 25-35 range would not be considered as positive, even though they may have mild symptoms or be asymptomatic.
This could prevent these people from getting the right medication or treatment and they may even become carriers of the virus.
Furthermore, this could drastically impact the entire picture of the daily total count of Covid-19 positive patients, misleading the government and the healthcare system in its fight against the pandemic.
Leading virologist Dr Shahid Jameel, Director of Trivedi School of Biosciences, Ashoka University has also been reported considering Ct value as a measure of transmission potential.
Incorrect Ct Values
Even though regarded as the best in class, an RT-PCR test is not foolproof. As per the ICMR, the ability of these tests to detect coronavirus is 95 percent, meaning that there is a five percent chance of faulty negative reports.
The reasons why a report turns out to be faulty negative could be varied; low viral load, virus mutations, incorrectly collected samples or human error being some reasons.
This explains why lately many a people have been complaining of Covid-like symptoms despite having a negative RT-PCR report in hand. Although there is no official data or study to back these anecdotes yet, experts are advising such suspected patients to follow all Covid-appropriate behavior including isolating, continuously monitoring symptoms, and seeking medical attention when necessary.
Difference between Ct value and CT score
While the Ct value determines the viral load, CT score is a value derived at in a chest scan. It is a measure of the impact the virus has had on the lungs. And unlike Ct value, higher the CT score, higher the infection.
Also, while a person may choose to go for a test anytime they think they have been exposed to the virus or because a Covid-19 negative report is needed for another purpose, getting a CT scan done from time-to-time is not advisable.
"One CT scan is equivalent to 300-400 chest x-rays. According to data, repeated CT scans in younger age groups increase the risk of cancer in later life. Exposing yourself to radiation, again and again, may cause damage. So, there is no point in doing a CT scan in mild COVID-19 if the oxygen saturation is normal," says Dr Randeep Guleria, Director, All India Institute of Medical Sciences (AIIMS).
The AIIMS chief also cites that a CT scan in asymptomatic or mild Covid cases may show patches in the lungs that eventually go away without treatment, eliminating the need for such a test entirely. For those in doubt, a chest x-ray is recommended instead.