To confirm a Covid-19 diagnosis, a swab or PCR test is performed by collecting mucus from the nose or throat. The report usually includes a qualitative result that is positive or negative, and a quantitative figure called the cycle threshold, or CT value. So, what exactly is a CT value, and how does it relate to Covid-19 infection?

Understanding the CT Value in Covid-19 PCR Test

Put simply, the CT value in PCR test reflects how many amplification cycles are needed to detect viral genetic material in a swab sample. After the sample is taken, the material is extracted with a dedicated kit. The extracted target is then amplified in a real-time PCR machine.

This instrument measures fluorescent signals. As amplification proceeds, fluorescence increases and is detected by the machine. When the fluorescent signal crosses a defined minimum level, the system calls the result positive. That crossing point is the cycle threshold, known as the CT value.

How to Interpret the CT Value from a Covid-19 PCR Test

What does your CT value tell you? The CT value is inversely related to the amount of viral RNA in the sample. A higher CT value generally indicates a lower concentration of viral genetic material. In other words, the higher the CT value, the lower the estimated chance of transmitting the virus.

On the other hand, a low CT value suggests that virus is present in larger amounts and the risk of passing the infection to others is higher. In many Covid-19 cases, CT values around 35 to 40 are often viewed as indicating a lower risk of causing new infections. This range is sometimes used as a practical reference in patient management, including decisions about whether a person should continue self-isolation.

Even so, clinicians must be careful about using CT values as the sole basis for decisions. Several technical factors can shift a person’s CT value, such as how the specimen was collected, the extraction approach, and the brand or type of test kit.

A CT value cannot tell you the exact number of viruses in the body of a person with Covid-19. It also cannot predict how severe the illness will be or the risk of death. A low CT value is not a direct indicator of whether a patient will have mild or severe symptoms.

If you are self-isolating or have recovered from Covid-19 and have had a PCR swab test, discuss your CT value with your doctor or a nearby health facility. Avoid drawing conclusions or starting treatment based only on the PCR report. Management can differ from one person to another depending on clinical findings.

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