Testing for Covid-19

Living in a world that is currently dominated by Covid-19; locally, nationally, internationally, and at almost every turn, you may be wondering how it is that we can so easily ‘test’ people for the virus, and determine the daily caseload, right across the globe?

Well I’m glad that you asked! In today’s piece I thought it might be useful to address this process which, like so many things in our modern-day, busy, technical lives, is seemingly ‘glossed over’ as if it’s just expected that we would have a test for it. The fact is that several things must come together in order to successfully identify a viral infection in a mere swab of mucous taken from the back of your airways – and to do so within 24-48 hours of sampling. Moreover, the track-and-trace approach to containing the spread of disease relies heavily upon both the accuracy, and the rapidity, with which a test can be performed.

So, what are these factors?

Well essentially, we need a reliable, reproducible method of both obtaining a specimen, then screening it, to detect components of the virus, i.e.

  • Effective specimen collection

  • Techniques for DNA isolation

  • Knowledge of key genetic markers

  • Screening for sections of genetic code

Sounds easy, but there’s a lot to it and thankfully we’ve made enormous progress over the last 100 years.

Effective specimen collection:

If you’ve been tested for Covid-19 then you’ll appreciate that it’s quite an uncomfortable process collecting the specimen – usually via a long cotton-tip swab guided to the back of your nose and throat. You may have wondered why it must be collected from there, rather than the more acceptable methods you might see on TV, where a person’s DNA is obtained by swabbing the sides of their cheek?

It’s a fair question but, given that we’re testing for the virus, then we need to isolate the viral DNA, not yours! Corona-viruses (like most cold and flu bugs), fall into the category of ‘rhinoviruses’ which means that they tend to inhabit the back of the airways, i.e. back of the nose and throat. So, if we’re looking to obtain a specimen that has viral DNA in it for testing purposes, then it makes sense to obtain that specimen from the back of the airways.

Techniques for DNA isolation:

Once we have a specimen to test, the next step is to isolate some DNA – the building blocks of all life on earth. There are several established, basic methods for the chemical extraction of DNA from any tissue sample, however the amount obtained is usually quite small (less than millionths of a gram), so not enough to work with.

However, around 30 years ago, a new technique known as the Polymerase Chain Reaction (PCR), was developed by researchers in molecular biology. It allows investigators to create millions and millions of additional copies, from a single stretch of DNA, thereby amplifying the amount of material that can be made available for further testing.

Now that’s good news!

Knowledge of key genetic markers:

With enough material to examine, the next step in this chain of events is to look for specific sections of the virus (or more specifically, the viral DNA), by ‘screening’ the amplified sample. But how do we know what to screen for?

One of the key steps in unraveling the spread of any disease, is to understand the pathogen (i.e. the responsible bug), and how it ticks; a key part of which is to examine its genetic code. When we look at a virus there are often stretches of DNA coding for genes that are common among variants of that viral family – just as you have some genes that are very similar, if not identical at times, to your own close relatives. Viral families are no different so, given that we already have data available for other corona-viruses, then it makes sense that we should be able to identify stretches of DNA (called genetic markers), that will always occur in the Covid-19 virus, no matter what the other variations might be. Pretty cool detective work really!

Screening for sections of genetic code:

Now that we have obtained a suitable specimen; isolated (and amplified) some DNA to test, and established some stretches of genetic code that the virus will contain, then the last step is to use some very reliable techniques to ‘screen’ that sample, and look for those genetic markers! Easy as 1, 2, 3.

Ironically, the same PCR technique that was used to amplify the original test material, is now used to check for the presence of certain stretches of code within that DNA. If a match is obtained then we can say with an incredibly high level of certainty, that the virus we were looking for, is indeed present in the original sample, at which point the patient can be confirmed as testing positive.

Sometimes in the modern world we take for granted the technology and knowledge that already underpins our day-to-day lives, never realizing just how much effort has gone into the development, or indeed the current application, of those life-changing techniques. So next time you hear the news piece about how many cases were reported overnight, just take a moment to reflect on all the people, steps and critical applications that came into play, just to give you that result! 😊

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