askST: How do CT values and serology tests tell if a person still has Covid-19 infection?

A serology test involves a blood sample, to detect the presence of antibodies.
A serology test involves a blood sample, to detect the presence of antibodies.PHOTO: ST FILE

SINGAPORE - Looking at serology tests and CT values are ways to determine if a person is still infectious with Covid-19 or has recovered from a past infection.

The Straits Times sheds light on what these mean.

Q: What is a serology test?

A: A serology test involves a blood sample, to detect the presence of antibodies. It can show if the person might have been previously infected. Patients have antibodies around two weeks after they recover from the infection and will not have them at the point of infection. Serology tests are done by the Health Ministry to understand if a patient has the virus antibodies, which could imply an earlier infection.

Q: What is CT value and how does it relate to viral load?

A: Otherwise known as cycle threshold, CT value is a number that comes up during a real-time reverse transcription polymerase chain reaction (real-time RT-PCR) test, used widely here and touted as the gold standard for Covid-19 testing.

The test looks for genetic sequences specific to Sars-CoV-2, the virus causing Covid-19.

During a PCR reaction, genetic material from the virus is amplified, and multiple copies are created. Amplification takes place through a series of cycles - a single copy becomes two, two becomes four, and so on. Only after multiple cycles is a detectable quantity of virus produced.

CT value hence refers to the number of cycles it takes to detect the virus.

If a higher number of cycles is needed - hence a higher CT value - it means that the amount of virus, otherwise known as viral load, is low. This is because the virus is spotted only after many cycles.

Conversely, if a low number of cycles is required, this means that the viral load is high, as few cycles are needed to detect the virus.

CT value hence has an inverse correlation with viral load.

But CT value is not an absolute scale and the same sample can give different CT values on different testing machines. So CT values cannot be compared between machines.

CT values may differ between nasal and oropharyngeal samples collected from the same person and also if the nose swab was done properly. The temperature of transportation and time taken from collection of samples to testing in the lab can also impact CT values.

Q: Why is CT value important?

A: CT value can be used as an estimate of transmission potential.

Patients with a low CT value (high viral load) will be actively shedding the virus and likely to still be infectious, while patients with high CT value (low viral load) might be recovered individuals who are no longer infectious, as they are shedding very minute quantities of virus.

A high CT value, typically above 30, suggests that it is not a recent infection.