Testing for viruses

24th March 2020

The head of WHO said at a news conference on 16th March: “We have a simple message to all countries: test, test, test. All countries should be able to test all suspected cases, they cannot fight this pandemic blindfolded”. (The Economist).

Well said but how can the virus be actually tested, you might wonder.

OK, let’s start from the beginning. First of all, a good test must be accurate. There are three elements of accuracy: 1/ how the sample for testing is collected and I will explain it later; 2/ sensitivity; 3/ specificity. Let me explain the two latter terms. Let’s take a silly example of lentil soup. We want to detect lentils in the soup. If we are able to detect every and each lentil and not mistake it for, say, beans our test has 100% sensitivity. But there are different types of lentils, i.e. red, green and so on. If our test is able to distinguish between each type of lentil, it is highly specific.

Let’s go back to our virus. There are two types of tests used for virus identification: genetic and serological. Let’s take a closer look at each of them.

Genetic tests

In order to be able to create genetic test scientists have to identify virus’ genetic code. In case of SARS-Cov-2 virus the code was broken in January 2020. Few days later the first genetic test was ready. The testing involves taking a sample from throat, using a swab. And this is the problem number one: the sample must be taken correctly. If it is taken too superficially there will be no virus on it. Going deeper into the throat causes gagging and coughing, posing a threat for person taking the sample. Problem number two is time required for processing the sample. The process is centralised, i.e. the sample must be sent to laboratory and testing itself takes as long as 48 hours. The genetic test is very sensitive and specific. The downsides are: it detects existing infection only, it cannot (at least for now) be used at bedside and it takes time to get results.

Serological tests

The alternative to genetic tests is a serological one, i.e. based on blood sample. Usually just a little droplet is enough. It is easy, cheap, fast and can be used at bedside, clinic, even home. However the way it works makes it far less specific compared to genetic test. The serological one does not search for a virus in a sample. It searches for antibodies. Antibodies are small molecules circulating in our bloodstream as a response to infection. It is our internal defence system triggered by any pathogen, such as bacteria and virus entering our body. Antibodies stay in our blood for sometime even when the infection is gone. And this is the reason why this test is not particularly specific. Coronavirus is a whole family of viruses, there are seven types of human ones, of which four cause common cold. The test is unable to distinguish between different types of coronavirus. So somebody who had a common cold not long time ago will test positively as if was infected by SARS-Cov-2.

The upshot?

Testing is crucial and tests should be widely available. But, please, don’t fall into marketing trap and buy on-line a test kit which will be sent to your home for self-testing. If it is a genetic test chances are that you won’t be able to take a swab correctly and your result might be false negative. If it is a serological one, most likely it will test you blood sample for some immunoglobulins which are not specific for SARS-Cov-2 and your result might be false positive. The test are improving though and what I have written above is correct at the time of writing only. I will keep you posted on any new developments. Please, check our blog frequently. (ZK)