Weapons of Mass Detection: RADT
Apart from washing your hands, wearing a mask, and keeping your distance from everyone, there’s another mantra that’s become familiar to our ears over the last year: test and trace. It’s important to identify who’s infected and get them out of circulation as fast as possible by isolating them. What better way than rapid antigen testing with results in minutes. But, till now, the testing is performed using a system called real-time reverse transcription polymerase chain reaction (rRT-PCR). It’s a great test that amplifies the genetic material in the coronavirus and detects a positive case with a sensitivity of over 99%. But there are drawbacks. And now there’s a new kid on the block.
The problem with rRT-PCR is it’s a pretty laborious process that requires a proper lab, trained technicians, and time. Sometime it takes a couple of days to get a result back. That’s a big issue if someone is waiting for a test, because it’s harder to get them to strictly isolate when they’re not quite sure if they have COVID-19 or not. There are a few other issues with rRT-PCR too. It’s expensive—well over €130 a test and the Irish government are planning to spend about €600 million on testing in the coming months. And because this is a global pandemic, there haven been supply issues and shortages of one of the most important components required for the test—the reagent.
Enter Rapid Antigen Detection Tests, or RADT.
Rapid Antigen Detection Tests (RADT) also identify the presence of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), the virus that causes COVID-19. But rapid antigen tests tests for proteins in the virus, not the genetic material. Some rapid antigen tests look for the so-called spike protein, and others, the nucleocapsid protein. Similar to the rRT-PCR, a swab is taken from the nose or throat. Then, unlike rRT-PCR, the swab is mixed with a buffer, and a couple of drops of the reagent is then dropped into a cassette that works similarly to a pregnancy test. An antibody within the strip of the cassette quickly identifies if the SARS-CoV-2 protein is present, giving a positive or negative result in less than 15 minutes. Gamechanger—sort of.
The most cited issue with rapid antigen tests for COVID-19 is that it is less sensitive than rRT-PCR. The best ones in validation studies approach a sensitivity of 93-94%. Read about sensitivity in our dedicated blog post here. Compared to rRT-PCR’s sensitivity of over 99%, that means with rapid antigen detection test you are bound to get more false negative results. So cases of COVID-19 can slip through the net.
Here’s an important caveat though.
An RADT is less likely to yield a false negative result if the viral load in the individual is higher. And a high viral load indicates infectiousness. So the clinical utility of RADT could be extremely significant, picking up highly infectious people, even though they might not have symptoms. Furthermore, rRT-PCR is so sensitive that it can pick up cases of COVID-19 long after they have recovered and become non-infectious. Clearly, it’s less important to isolate someone when they don’t pose a risk of transmission anymore.
Rapid antigen swab tests are also much cheaper than rRT-PCR, and don’t require a laboratory. This means it can be rolled out in near-patient or point-of-care settings: in nursing homes, hospitals, clinics, schools, factories, even airports. In fact, in Slovenia the government has just announced a plan to use RADT to test the entire population of over 5 million people, establishing 5,000 near-patient pop-up centres to do so. Sure, there are likely to be false negatives, but it’s the detection of many asymptomatic carriers of SARS-CoV-2 where the benefits lie.
Read PK Medica’s White Paper to read about the leading scientists who raise concerns about the silent threat of asymptomatic spread and the major role mass screening can play in living with COVID-19.