Ct value stands for cycle threshold. During the RT-PCR test, the Ct value determines the detectable level of the virus in a given sample based on which it is decided if the person is COVID positive or negative.
In RT PCR test, the RNA is extracted from the sample and is converted to DNA, which is then amplified or multiplied. This multiplication happens over a number of cycles in order to find out the viral load.
If the virus is found within fewer cycles, it mean that the body has a high viral load.
If the virus is found after more cycles, this means the body has lesser viruses which could only be detected after the sample was passed through many cycles.
In this way, the Ct value is inversely proportional to the amount of viral load in the human body.
Higher the Ct value lesser the viral load and lower the Ct value, higher the viral load.
A higher viral load is associated with being more infectious.
Interpretation of Ct value in COVID test A patient is considered Covid-positive if the Ct value is below 35.
In other words, if the virus is detectable after 35 cycles or earlier, then the patient is considered positive.
Any correlation between a Ct value and severity of COVID?
Although Ct value is inversely correlated with viral load, It does not have any bearing on the severity of the disease.
A patient can have a low Ct value, which means her viral load is high enough to be detected rapidly, but she may still be asymptomatic.
The Ct value tells us about the viral load in the throat and not in the lungs.
Does a high Ct value always mean a low viral load?
Factors that may vary the results of Ct value includes:-
A poorly collected sample may reflect inappropriate Ct values.
Technical competence of the person performing the test.
Calibration of the equipment.
Analytical skills of the interpreters.
Ct values may differ between nasal and oropharyngeal specimens collected from the same individual.
The temperature of transportation of the sample.
The time taken from collection of the sample to receipt in the lab, can also adversely impact Ct values.