What is CT value in RT-PCR test, how it determines test positivity rate

RT-PCR test
Swab being collected from a passenger at Ranchi airport to conduct the COVID test. Image courtesy: PTI

Amid the massive rise in the daily COVID 19 cases, Maharashtra has urged the central government to regulate its testing guidelines. The state has asked to reduce the CT value or the Cycle Threshold value in Reverse Transcription-Polymerase Chain Reaction (RT-PCR) tests. Maharashtra’s main argument is that it could significantly reduce the number of people testing positive for the virus.

What is CT value?

The CT value refers to the number of cycles needed to amplify the viral RNA of the SARS-CoV-2 virus in a sample to detectable levels, in a standard RT-PCR test. Samples that show a CT value of 35 or less are considered positive while the samples that have a CT value above 35 are usually considered negative.

How is CT value calculated?

The RNA is extracted from the sample and is treated with a reverse transcriptase enzyme. A complimentary DNA is then extracted from the initial RNA. The DNA can be amplified using the polymerize chain reaction to make billions of copies of it. The DNA that is extracted is amplified in cycles.

How is it done?

The DNA gets amplified to double its size during each cycle. The CT value is then calculated by measuring the DNA using fluorescent dye signals. If the DNA in a sample could be detected in a few cycles, then it means the viral load is significantly high. So, low CT value indicates higher viral load. Meanwhile, if the CT value is high, then it means that the DNA could not be detected despite amplifying it for quite a number of cycles. This shows lesser viral load.

Indian Council of Medical Research says that if the CT value is lowered, as demanded by Maharashtra, then the test reports could show erroneous negative results. The patient who gets a negative result like this would roam about freely and might infect others. The manufacturers of the RT – PCR kits too have determined the CT value between 35 and 40. It means that if the CT value is above 40, then the test result of the sample would be negative. 

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