“What is CT value in Covid-19 Test & What crucial information we’ll get from this”

CT value: As we all are seeing the devastating 2nd wave of the Covid-19 pandemic, we should remain to keep the social distance, wear masks, sanitize your hand regularly & try to get vaccinated as soon as possible.

According to the Indian Council of Medical Research (ICMR), the current positivity criteria are based on the cycle threshold (CT). Soon after an RT-PCR test, the SARS-CoV-2 is detected in a sample.

Scientifically, a CT value signals the number of cycles for a sample to go through to amplify and bring up the viral DNA to a traceable level under given settings. It gives the suspected patient an idea as to how many cycles were needed to detect the DNA of the SARS-COV-2 virus. 

The ICMR states that the globally accepted cut-off for CT value for Covid-19 ranges from 35-40 depending upon the instructions laid down by individual manufacturers.


After the sample is collected, RNA is extracted and treated with a reverse transcriptase enzyme. A complementary DNA is extracted from an initial RNA

Now the DNA can be easily amplified by using a polymerise chain reaction to make billions of copies of a fragment.

The DNA is amplified by each cycle

Each cycle amplifies the DNA by double x2

IT calculates in real-time by fluorescent dye signals. Fluorescent signals data collected in the exponential phase are noted- this is the starting amount of the target DNA

Let’s try to understand this with an example-

Suppose we’ve two samples: one with 80ng of cDNA – fewer cycles for making it detectable – will give me a lower value

Another with 30ng cDNA -needs more cycles – higher value.




Why The CT Value is so important-

 It gives us a clear picture of the infection in our body. False-negative results could be able to transmit the infection to the family and the society. 

“A patient can have a high value but still have a very significant level of covid19 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,” said Dr. Dhiren Gupta, Senior Intensivist at Sir Ganga Ram Hospital.

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