More and more states are now relying on Rapid Antigen Detection Test (RADT) which is not so reliable because of ‘false negative’ results are too high. “RADT is not so reliable as the percentage of ‘false negative’ results is too high,” said Dr. Kapil Kochhar, Director – Department of Minimal Access Surgery, Fortis.
Despite this, rapid antigen tests comprise nearly 25-30 per cent of the total daily tests conducted for the detection of the deadly coronavirus in the country at present. As on Monday, the total number of tests conducted so far in the country was 2,08,64,750 and the Tests per Million (TPM) was 15,119.
Of the total 2.08 crore tests conducted so far, around 26.5 lakh are antigen tests, ICMR officials said.
Dr. Inder Maurya – Consultant, Emergency Medicine & CEO & Founder, Foreign OPD – also expressed his apprehension about the accuracy of the Rapid Strep Test (RST) or rapid antigen detection test (RADT).
“An RT-PCR test is considered very reliable because it can detect even a single virus particle in swabs taken from inside the mouth or nose, where the virus particles are most prevalent. Accuracy is the single largest problem with RST, which are much less sensitive than RT-PCR as a diagnostic tool,” Dr. Maurya said.
However, the Indian Council of Medical Research (ICMR) Balram Bhargava recently said that a rapid antigen detection test has a very high specificity (ability to detect true negatives) of 99.3 per cent to 100 per cent, however, their sensitivity is moderate ranging between 55 to 85 per cent.
What is RST or RADT?
According to Dr. Kapil Kochhar, the Rapid Strep Test (RST) is a rapid antigen detection test (RADT) that is widely used in clinics to assist in the diagnosis of bacterial pharyngitis caused by group A streptococci (GAS), sometimes termed strep throat. The test works by detecting the presence of GAS in the throat of a person by responding to GAS-specific antigens on a throat swab. An RST may help in deciding whether to prescribe an antibiotic to a person with pharyngitis, a common infection of the throat.
“RST seek out specific proteins only found in the virus, which the body’s immune response recognizes as ‘foreign’. Most COVID-19 antigen tests target the ‘spike protein’ that studs the surface of the coronavirus. A swab from the nose is collected for this test, where there’s a high likelihood of virus particles being present. The swab is then dipped in a solution that inactivates the virus, and then transferred onto a test strip. The test strip houses antibodies that bind to coronavirus proteins and hold them in place as the fluid spreads. If the sample is positive for coronavirus, colored lines will show up on the paper strip in 15-20 minutes,” explained Dr. Maurya.
What is RT-PCR?
“The Reverse Transcription Polymerase Chain Reaction (RT-PCR) is a laboratory technique combining reverse transcription of RNA into DNA and amplification of specific DNA targets using Polymerase Chain Reaction (PCR). It is primarily used to measure the amount of a specific RNA which is achieved by monitoring the amplification reaction using fluorescence, a technique called real-time PCR or quantitative PCR (qPCR),” Dr. Kochhar said.
Dr Maurya said that the version of PCR testing used to detect viruses like the COVID-19-causing SARS-CoV-2 is called RT-PCR (reverse-transcription PCR).
“While some viruses have only DNA, others like SARS-CoV-2, only contain RNA. To detect an RNA virus like SARS-CoV-2, experts use an enzyme (reverse transcriptase) to convert the virus’s RNA into DNA, in a simple and widely-used one-step process called ‘reverse transcription’. This allows a single molecule of DNA to be amplified exponentially (millions of times), which is the main goal of the PCR process, so even virus particles in single digits can be detected in the final result,” he added.
