The full form of CT value is ‘Cycle Threshold value’. It tells us about virus concentration in a sample.
TESTING OF SAMPLE
RT-PCR test detects amount of viral DNA or RNA molecule in a testing sample.
For details about PCR (Polymerase Chain Reaction) CLICK HERE
The instrument cannot detect very small amount of viral DNA or RNA. When the concentration reaches up to a threshold or optimum concentration, then the instrument can detect it and give positive result.
As most or almost all test sample contain small amount of viral DNA or RNA, the first step is to increase the concentration of DNA or RNA of the collected sample.
In PCR (polymerase Chain Reaction) technique, the concentration can be increased.
In one step or 1 cycle, the amount become doubled. For example, if the initial sample contain 10 viral DNA or RNA, after one cycle that will be 20. After second cycle 20×2 =40, and value after 3rd cycle will stand as 80 and so on.
CT value indicate the number of cycle needed for virus to be detected and give positive result.
For examples, say 2 sample designated as A and B were taken.
A show positivity after 20 cycle, and
B show positivity after 30 cycle
The interpretation is, initial concentration of virus in test sample A is higher than B, as the sample needed less cycle to reach optimum concentration.
In other words, we can say, viral load of sample A is more than B.
In this context, it must be noted that a minimum number or concentration of virus inside body is necessary to manifest a disease. For example, if 1000 virus are needed for disease manifestation, then 10 virus inside body could not produce the disease. When this 10 virus will multiply inside body, become 1000 in number, then only the person will suffer from the disease.
If person contain less number of virus, collected sample will also contain less virus- that sample need more cycle to amplify to reach a threshold value to become positive.
After repeated cycle of multiplication by PCR technique, this sample will show positive reaction. In this way, a negative sample may give positive result after 40 cycles. Therefore if the number is less than 40, the sample is considered ‘positive’ in case of COVID-19.
Thus, less CT-value indicate more virus in the sample and patient will be more contagious, will spread disease and infect other.
It is claimed that CT-value in case of COVID-19, if more than 24 will not spread infection if patient were mask. But CT-value of 30 without a mask is more likely to spread infection more rapidly.
CT value in Covid-19
- No reliable study to definitely prove and correlate CT-value and severity or infectivity
- Viral load does not have much role in patient management
- CT-value differ from one testing kit to other
- CT-value also depends on how the sample has been collected. Poorly collected sample will show more CT-value.
- CT-value also depends on technical persons, technical competence and equipment calibration.
- CT-value of nasal and oro-pharyngeal sample may differ in same person.
- Sample transport, temperature may affect CT-value.
- Asymptomatic patient may show higher value- it may change over time. If a person show CT-value 30 on day 1, he may show CT-value 24 on day 3 and so on. So, early symptomatic patient’s CT-value may give a false sense of security about severity of the disease- which may change latter.
- Severity of the disease not only depends on CT-value, but also depends on host factors (Immune status of the person and co-morbidity). Same viral load may manifest differently in different people.
- RT-PCR done presently is qualitative in nature. It gives a rough estimation of viral load.
- All patients with a Ct value less than 35 may be considered as positive.
In a study (sciencemag.org) La Scola et all found that virus from 70% of the sample could be cultured of CT-value <25, compared with 3% of CT-value >35. Culturable sample means viable virus and related to infectivity.
In another study, it was found that among 76% hospitalized patient, 35% of CT-value, 25 died compared to 17.6% with CT-value 25-35, and 6% above 35.
Finally, it can be stated that CT-value is a rough estimation of viral load and severity of the disease. Lower CT-value related with high morbidity, but remembers CT-value may change with time and higher value may give a false sense of security.
What is Corona Virus? : See Video in Bengali Language