After the pandemic hit India in March 2020, the population was suddenly bombarded with new vocabulary and other important pieces of information. Words like RT-PCR, CT value, viral load and oxygen concentrator, among others, became a part of our (almost) daily conversations. Many studies and reports helped spread awareness about the deadly virus, the tests that were to be done to determine the infection, and ways to prevent infection, among other aspects. Many platforms went a step ahead and helped the users understand the process of various COVID-19 tests, too. However, there is still a large population that either does not understand the process or is confused about the reports.
The ongoing wave of COVID-19 has ravaged the country. In April, alone, the country recorded 69 lakh new COVID-19 cases and a grim 48,768 deaths. The high positivity rate (the measure of the spread of any disease in the society) of the new variant has led to a higher number of active cases, over 24 lakhs, in the country. Yet, there are many (infected or not) who are struggling with the tests and their subsequent reports. For many, while the word ‘Confirmed’ or ‘Positive’ stare back, the words ‘CT value’, and ‘Amplification Plot,’ among others just confuse the concerned party.
Such alarming numbers of cases should be reason enough to revisit the basics of decoding RT-PCR reports, irrespective of the results. The sooner and better you understand the process and its report, there may be higher the chances for a quicker course of action by your doctor.
Types of tests
There are two main types of tests that India has been focusing on since the pandemic hit the country in 2020 – Rapid Antibodies Test and RT-PCR. The two tests are fundamentally different in the way that while one determines the person’s response to SARS-CoV-2, the other detects its presence in a person’s body. Even though no organisation or study considers any one test to be 100 per cent accurate, many support RT-PCR way of testing the infection as it is elaborate and gives almost accurate results.
The two main types of tests used in India during the COVID-19 pandemic are Rapid Antibodies Test and RT-PCR.
As mentioned, the Rapid Antibodies Test does not pick up on the current infection, it instead aims to detect coronavirus-specific (as the name suggests) antibodies to determine if the person was exposed to the virus. This test is cheaper than RT-PCR and is also called the immunity test. In India, this test was more commonly used in serosurveys to map the COVID-19 exposure in geographical regions. On Tuesday, however, the Indian Council of Medical Research (ICMR) release new guidelines suggesting an increase in rapid antigen tests for a suspected COVID-19 patient to decrease the load on the laboratories.
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The reverse transcription-polymerase chain reaction test (RT-PCR) is recognised by the World Health Organisation (WHO) as the ‘gold standard' in detecting coronavirus. Because of this validation from WHO, the Indian Government has made this test mandatory before travel. Due to its higher percentage of accuracy, it also the only test acceptable in workplaces and hospitals.
The process of this test is a little longer than that of antibodies test since the SARS-CoV-2 virus is an RNA (a single helix genetic material) and in order to detect it early, it is converted into a DNA (a double helix genetic material). This process is called – reverse transcription. In India, the sample for this process is collected from a person’s nasopharyngeal (back of the nose) and oropharyngeal (middle part of the throat) area where the virus usually gathers.
The CT value, short for cycle threshold, of an RT-PCR test, is the number of cycles after which the virus is detected.
All about CT value and Amplification Plot
Once the reverse transcription is done, the newly-formed DNA is then amplified. It is a process that creates multiple copies of DNA which gives a bigger sample for the evaluation and eventual determination of the infection. It is after many cycles that a detectable virus is produced through this process.
The CT value, short for cycle threshold, of an RT-PCR test, is the number of cycles after which the virus is detected. The number mentioned in your report under CT Value is the number of cycles after which either the virus was detected or not detected in your sample. One needs to also understand that the higher number of cycles means low viral load (a measure of the total number of viral particles inside the individual), while the lower number of cycles indicate high viral load.
If a higher number of cycles are required to detect the virus, it implies that the virus either went undetected or the viral load is less to be noticed. While lower CT value indicates a higher viral load because the virus was spotted after the initial few cycles. Viral load is the measure of the total number of viral particles inside the individual. For example, if the number written on your report is 26, it means the viral load in the patient is lower than that of someone who’s report indicates a CT value of 18.
Now, according to the ICMR, a patient is considered COVID-positive if the CT value is below 35. Several virology labs and reports indicate that the CT value can help determine the transmission potential of a patient. CT value, however, does not determine the severity of the disease. The target gene mentioned in some of the RT-PCR reports is the detection of a particular gene present in DNAs and RNAs, but the ones mentioned in the report are specific to the virus and human DNA.