Here’s How to read RT-PCR test and understand cycle threshold value?

What you should Know:
• What is RT-PCR test?
• What is the cycle threshold value?
• How accurate is the RT-PCR test for COVID-19?

What is RT-PCR test?
In the midst of the second wave of COVID-19, many people want to test whether they contain the virus. The most common test to check for infection is the RT-PCR test. RT-PCR means reverse transcriptase polymerase chain reaction. This test is done by taking a sample of the swab from the person’s nose or throat. In the laboratory, this sample is used to extract viral RNA (ribonucleic acid).

The RNA then undergoes an RT-PCR technique that strangles viral DNA. The DNA is run through PCR for several cycles until it reaches a level that is detectable. A dye is added to the DNA while it is being synthesized and when the solution contains a sufficient amount of dye, it will change color. The dye is specific and will only work for the DNA of SARS-CoV-2, which causes the virus COVID-19.

What is the cycle threshold value?
As previously mentioned, the DNA strand is run through multiple cycles of PCR to replicate itself. The cycle limit value or CT value is the number of cycles it takes the DNA to reach a detectable level. For example, if the DNA has to go through 20 cycles of PCR before the dye is detected, the CT value is 20.

So, how do you know whether the cycle limit value is positive or negative? According to other international organizations such as ICMR (Indian Council of Medical Research) and CDC (Center for Disease Control) in the United States, the cut-off for a positive COVID-19 test is 35. This means that if viral, DNA is detected within a maximum of 35 cycles, the test is positive. If the test takes longer to show results, it is negative.

This is because a higher viral load will provide more viral RNA per sample. In turn, this RNA will generate more DNA via RT-PCR. The more DNA is present, the sooner it will be detected in the sample. Therefore, while reading the RT-PCR report you need to check whether the CT value is above 35 or not. The low value is positive for COVID-19 and the high value is negative for the disease.

How accurate is the RT-PCR test for COVID-19?

The RT-PCR test is considered the standard test for COVID-19. However, because there are many factors that can affect test accuracy, it is recommended that the test be performed with other tests as well as a chest X-ray or antigen test.

Some of the factors affecting the test include:
• The way the sample was taken.
• Storage of samples.
• Phase of transition when the sample was taken.
• Human error in the laboratory.

Some experts claim that a positive RT-PCR test with fever, cough, difficulty breathing and similar COVID-19 symptoms may be considered a good indicator of the disease.

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