Science Behind Virus: How to test for new coronavirus
Updated 09:24, 11-Feb-2020
By Yang Zhao, Yang Xiao
The outbreak of the novel coronavirus in China this January saw people suffering from fever, cough, shortness of breath and even severe pneumonia. But these are only symptoms which could be caused by other diseases. How do we test for a virus that we haven't seen before?
You might have come across this term a lot recently: nucleic acid test. As one can tell from the name, it is about testing the presence of the virus's genetic material, the RNA or DNA.
Medical personnel first take a sample from the suspected patient, usually a throat swab, to test for coronavirus.
We know that the genetic material of the new coronavirus is RNA. But most of our tools for this test are specific to DNA. So scientists need to take virus's RNA as a template, create a complementary strand and make a DNA copy of it. This process is called "reverse transcription." Transcription is the copying of DNA into RNA, and here the process is reversed.
Now we have a mess of DNA fragments from everything in this sample: patients' own cells, harmless bacteria and so on. How do we find the coronavirus?
Scientists in China sequenced the virus's genome and made it available to the world on January 10. This means, we know the entire RNA sequence of the new coronavirus. It allows us to compare it with other virus strains in this family and find what's unique – in other words, its distinctive signature.
If this signature can be found in the sample, it means the virus exists. But how is this done? Scientists copy small pieces from this distinctive fragment of DNA, what they called "primers." They then heat up a sample to separate the strands of all DNA inside. If there are any traces of the new coronavirus, the primer will grab onto it and start to make copies. One DNA becomes two, two becomes four, until there are billions of copies. To make the process easier to observe, scientists add fluorescent labels to the primers, meaning the more viral bits there are, the brighter it glows, telling scientists that virus exists.
One problem is the test can have minor errors. Primers can grab onto a piece of DNA in the wrong way or be led astray by contamination from a previous sample or the throat swab may not contain enough virus. All these could lead to a wrong result. Chinese scientists say in practice, just 30-50 percent of infected patients test positive.
On top of this, testing for the highly infectious coronavirus needs to be undertaken in a lab with high bio-safety levels, usually within a country's disease control authority. There are four levels and only labs with protection levels 3 and 4 are allowed to test such virus. That's why testing has been relatively slow, especially during the outbreak.
China has announced that CT screening of lungs will now be part of the standard diagnosis process, along with the nucleic acid test. This serves as a double check in case the test fails.
It's a new process. But the more we know about the virus, the faster we can track it down.