‘CT’ Value or Cycle Threshold
As swab tests become a more common experience for Malaysians, we may question the meaning behind our results and particularly the significance of the CT value for RT-PCR tests. CT value refers to the number of cycles needed for the PCR machine to determine a positive result. The test focuses on 3 genes which are: E, N and RdRP and confirms if these genes have been infected by the COVID-19 virus.
For an RT-PCR test, the process requires a temperature cycling reaction called the amplification cycle. For a single test, the amplification cycle is repeated at least 35-45 times before obtaining the results. The total number of amplification cycles required to detect the virus’ genetic material or viral RNA is defined as the ‘CT’ value or Cycle Threshold.
What Does the CT Value Mean?
For RT-PCR tests, the more viral RNA present in your body, the fewer cycles (low CT value) are needed for the machine to detect the virus. Whereas, if you have very little to no virus in your body, more cycles (high CT value) are required to detect the virus. The results can be simplified as below:
However, having a high CT value does not guarantee that you cannot transmit the virus to others.
If I have a low CT value, does that mean I have not been infected?
No. If you have a low CT value, you are infected, have a high viral load and are highly contagious. You are required to isolate yourself immediately and practice good hygiene. Take all the necessary measures to disinfect your home and avoid sharing the same space and household items as your family. It can also be assumed that you were recently exposed to the virus and completed the swab test at an early stage of your infection.
An example of a positive RT-PCR result (detected)
If I have a high CT value, does that mean I have been infected?
Yes. If you have a high CT value, you are infected but considered to have a lower viral load than those that obtained a low CT value for their results. However, as you are still infected with the virus, you are required to isolate yourself as you can still infect others. A high CT value may also indicate that you have been tested at a later stage of your infectious period.
An example of a negative RT-PCR result (not detected)
Stage of Infection
CT values can also change based on the infectious period. A person that was very recently exposed to COVID-19 may have an extremely low viral load to the point that it is undetectable. Within 24 hours, this can change to a low CT value (high viral load).
Factors That Can Affect CT Value
Swab Test Stick
Other than the infectious period, the type of swab test conducted can also affect your CT value. A nasopharyngeal swab test that collects samples from the back of your throat can provide a more sensitive or accurate sample for testing. On top of that, the type of swab used can also affect the quality of the sample collected.
Laboratories and Test Differences
CT values from different laboratories or RT-PCR tests cannot be compared to one another as there may be differences in the interpretation of results by the machine and varying end-to-end processes.
What about the new variant?
The current tests available in Malaysia are effective in detecting the Delta variant. Since the emergence of the new variant, it has been documented that those infected with the Delta variant had a low CT value (highly infectious) compared to those with non-variant infection.
What do I do if I have a high CT value?
Although your infectivity is lower than someone with a low CT value, you can still infect others. If you have not undergone self-isolation, please quarantine yourself at home for a minimum of 10 days and monitor your symptoms on the MySejahtera app. Those that have been in contact with you for the last 10 days should also get themselves tested.
What do I do if I have a low CT value?
A low CT value indicates that you have a high viral load and can infect others more easily. You must undergo home quarantine immediately and monitor your symptoms closely. If you develop any serious symptoms such as breathing difficulties, go to the nearest hospital as soon as possible. As a standard practice of exposure to the virus, those that have been in contact with you must get themselves tested immediately.
What if My Results are Inconclusive?
An ‘Inconclusive’ result on an RT-PCR test can be assumed as a positive result with a low viral load. You are not advised to immediately get a second swab test but should instead self-quarantine and follow the same guidelines as a confirmed positive result. You may decide to get a second swab test 48-72 hours after your inconclusive results to reconfirm your health status.
How Can I Be Infected When I Am Fully Vaccinated?
The COVID-19 vaccine is designed to reduce the likelihood of you contracting the virus, preventing hospitalisation and death and enabling you to return to normal living. It does not provide you with full (100%) protection. You can still contract the virus, remain asymptomatic and spread it to others.
Adjusting To The New Normal
As we continue to embrace the new normal and prepare for the endemic phase of the COVID-19, we must keep our guards up and adhere to the government-mandated SOPs in order to reduce the pressure on our healthcare systems. Although coming in contact with the virus appears to be unavoidable, we still have the ability to prevent it from spreading to others through vaccination and swab tests.
If you have been exposed to the virus or been in close contact with a positive case, it is essential to get yourself tested and protect those closest to you. With a deeper understanding of the swab test results and CT value, we can take better precautions to protect those that may not be able to get themselves vaccinated, including those who are immunocompromised and younger children.
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