Article by Dr. Nguyen Thi Thuy Hang – Laboratory Department – Vinmec Times City International General Hospital
Currently, according to recommendations from the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) in the United States, the Ct value in PCR testing is used to diagnose COVID-19 infection. This is a molecular biology technique that allows real-time reading of signals in a sample (if present). The Ct (cycle threshold) value appears in RT-PCR tests for coronavirus and determines whether a person is positive for COVID-19.
1. What is the Ct value in PCR testing?
Ct stands for cycle threshold – a value that appears in RT-PCR tests. At the same time, this value is the “gold standard” for detecting the SARS-CoV-2 virus.
In RT-PCR testing, RNA is extracted from samples taken from patients, converted into DNA, and then amplified. The amplification process occurs through a series of cycles, with one copy becoming two, two copies becoming four… and after several cycles, a detectable amount of the SARS-CoV-2 virus is generated.
The Ct value of the RT-PCR reaction is the number of cycles required to detect a signal in the sample that exceeds the background signal and is recorded by the device. In simple terms, the Ct value refers to the number of cycles after which the SARS-CoV-2 virus can be detected.
The Ct value can be viewed as an indirect indicator of the amount of viral genetic material detected from a specific sample at a given time. A lower Ct value means a higher viral load of SARS-CoV-2 because the virus was detected after fewer cycles. Conversely, a higher Ct value means a lower viral load of SARS-CoV-2, and beyond a certain point, it may indicate the absence of infectiousness.
2. Factors affecting the Ct value in PCR testing
The Ct value is influenced by several factors, including:
- The timing of sample collection (related to the number of viruses replicating and the onset of symptoms).
- The type of sample (nasopharyngeal swab, nasal swab, throat swab…), whether the sample was collected and preserved correctly, and the time from sample collection to testing.
- The presence of mucus or inhibitors in the sample.
- The sensitivity and specificity of the testing method.
The Ct value in PCR testing is affected by the timing of sample collection.
3. Significance of the Ct value in PCR testing
The Ct value is one of the components used to interpret the results of real-time RT-PCR tests that are positive for the SARS-CoV-2 virus and to make decisions regarding isolation and quarantine.
In situations requiring complex interpretation (e.g., decisions about isolating or re-isolating a fully vaccinated asymptomatic person with a positive PCR test in a residential care facility) and when infection control measures are needed, the Ct value is one of the variables that can be considered in making decisions to prevent transmission.
Clinically, the Ct value alone is not sufficient to demonstrate a correlation with the severity of illness or to decide on treatment options and isolation for individual patients. It must be combined with other factors such as clinical symptoms, exposure history, and vaccination status.
In Vietnam, on July 14, 2021, the Ministry of Health issued guidelines regarding home isolation for COVID-19 positive cases based on Ct values. Specifically:
For asymptomatic COVID-19 patients being cared for and treated in healthcare facilities, they may be discharged after 10 days if two consecutive samples (taken at least 24 hours apart) test negative or positive by RT-PCR but with a low viral load (Ct value ≥ 30), allowing for monitoring and treatment at home for 14 days.
For cases detected in the community (with RT-PCR test results positive for SARS-CoV-2) that are asymptomatic: if the viral load is low (Ct value ≥ 30), they are to be isolated and monitored in a healthcare facility. After 24 hours, if the viral load remains low (Ct value ≥ 30) or the RT-PCR test result is negative for SARS-CoV-2, the patient may be discharged and monitored as above. Home monitoring is based on the principle that with such a low viral load (Ct ≥ 30) or a negative result, the risk of transmission to family members and the community is minimized.
