AS Covid-19 self-test kits become as integral to our homes as toiletries, the red lines are no longer as confounding as they once were.
But what about polymerase chain reaction (PCR) tests? What do they show and how do we interpret the results?
A CT value is the Cycle Threshold value, says IMU consultant infectious diseases specialist and professor of medicine, Dr James Koh Kwee Choy.
During a PCR test, a nose or throat swab or both are taken and run through a PCR machine. The PCR test detects the RNA that the virus carries. However, it cannot detect this just from the original swab sample taken from the patient as the genetic material of the virus is too little for detection.
The sample needs to be processed through a PCR machine which amplifies the genetic material to a sufficient amount for detection.
The CT value tells us the number of cycles it took to multiply enough of the viral genetic material for us to detect it, explains Dr Koh.
INTERPRETING CT VALUES
When we get our PCR results, a low CT value indicates that it only took a few cycles to multiply enough of the viral genetic material for it to be detected. This means that you have a high viral load and that you are Covid-19 positive.
On the other hand, a high CT value means that the machine ran through many cycles before the viral genetic material could be detected. In essence, the lower the CT value, the higher the viral load and risk of transmission. Conversely, the higher the CT value, the lower the viral load and risk of transmission.
The CT value however, often does not correlate with the severity of a person's symptoms.
"Some people may have a low CT value (high infectivity) but are asymptomatic. Some people may have a high CT value (low infectivity) but are symptomatic," says Dr Koh.
Different countries also have different CT values for interpretation of PCR results. In Malaysia, the threshold is 40, meaning a CT value of less than 40 means you are positive while more than 40 means you have tested negative. India uses a CT value threshold of 24 and China uses 30.
In addition to CT values, a laboratory may also indicate an infectivity value. The infectivity value uses a CT threshold of 30-35 depending on the individual laboratory standards. Above 30 means less risk of transmission, below 30 means high risk of transmission.
DIFFERING RESULTS
Why are there times when repeated RTK tests show negative, but a PCR shows up positive?
Dr Koh explains that if we test too soon using the RTK, the test won't be able to detect the virus as viral replication takes time.
The PCR is a lot more sensitive in picking up genetic material that the RTK is not able to.
However, the reverse can also be true, where an RTK test comes up positive, but a PCR is negative.
"Many factors can contribute to a false-positive RTK test or even a false-negative PCR test, including time and method of sampling and contamination of samples," explains Dr Koh.
TOO CLOSE FOR COMFORT
What should you do if you are a close contact? First, it is important to know what close contact means.
Dr Koh says you are a close contact if you were together with someone who tested positive for more than 15 minutes and were less than 1 metre apart, without masks, for example when dining together.
Although close contacts who do not have symptoms are not required to self-quarantine now, it is prudent to be mindful of any symptoms that may develop within the next few days.
Based on current guidelines from the Ministry of Health, you are required to perform the RTK test on day 1 and 3.
If you are symptomatic, you are required to self-quarantine and the duration depends on your vaccination status.
For those who have just recovered from an infection, there is no "honeymoon" period where you are certain you won't get the virus again. There has been no study to show what the immunity duration is after contracting Covid-19. It's very dependent on the individual and how infectious the subsequent infection is and what variant the person is exposed to.
Dr Koh adds that it's safe to say recovery from Covid-19 does not guarantee immunity to subsequent infections.
"As we prepare to enter an endemic stage, we will see many more cases, but life will start to return to some semblance of what it was before the pandemic."
Shop for Covid-19 self test kit and face masks at discounted prices using Shopee Voucher Code.