What’s the deal with COVID19 testing?

What’s the deal with COVID19 testing? Why can’t I get a test if I think I have it?


We’ve been receiving variants of this question. Someone has mild symptoms of respiratory disease and when calling their country’s health authorities, they are asked to stay home and advised that no testing will be done. Why? What is happening? Let’s break it down.

What type of information do the tests provide

First it’s important to understand that testing comes in mainly two “flavours” This is true for any virus, not just the virus that causes COVID19 – called SARS-CoV-2.

Tests detecting the virus per se.

These tests can detect part of the genome of a virus. These are called quantitative or real-time PCR tests (q-PCR) and are done in fluids from the body where we are most likely to find the virus.

In the case of #COVID19 the virus is most easily found in the respiratory system of the body: swabs of nasal cavities or throat, sputum (the mucus that comes up when a person coughs), or something called bronchoalveolar lavage – fluid collected from the lungs through a medical procedure.

Tests detecting immune defences against the virus.

These tests are called “serology”. They detect a type of defenses that the body’s immune system has made against the virus, called antibodies. Serology tests are normally performed in blood (actually in the more liquid part of the blood called serum, hence the name). These tests do NOT show that the person has the virus right now, but they can prove that the person HAD the virus at some point in the past or that they were recently infected, depending on the type of serology test done.

As of this writing, existing tests for COVID19 fall into the first category only. They detect the genome of the virus. Thus it only makes sense to carry them out when the person has an active infection (i.e. has symptoms). They could also be carried out in people who are at high risk of infection during active surveillance and contact tracing efforts (e.g. people who have been in close contact with someone known to have the COVID19 virus).

In people with mild symptoms or asymptomatic, these tests become harder because the person may not be able to cough up sputum, and material from the nasal or throat swabs may not be enough to produce reliable testing results.

If a person has no symptoms or very mild ones, a serology test may provide better information, as it can show if the person has been infected at any point in time and now has immunity against the virus. However, as of this writing, serology tests for COVID19 are not yet available.

Priorities during different stages of the outbreak

Testing requires having specialized personnel collecting the samples, transporting them to the laboratories, and processing them for testing, as well as sharing back results. Sample collection and handling needs to be carried out with appropriate protective equipment so that the healthcare workers do not risk infection.

All these logistic efforts need to be weighed out against all other competing priorities during an outbreak.

When the first cases start in a country, normally contact tracing and testing of individuals suspected to have the virus will be prioritized. This is because health authorities are trying to contain the virus from spreading more widely in the community and need to monitor how it’s moving around.

However, when the number of cases increases and there is wide community spread, it becomes very laborious and time consuming to follow up all contacts of COVID19 cases. At this point, testing of asymptomatic contacts or of people with mild symptoms is weighed out against all other priorities, such as managing the surge in cases, providing healthcare to those who are sick, ensuring safety of healthcare workers, and ensuring adequate supplies of hospital materials and equipment. Testing at this point may be prioritized for those with severe symptoms and those requiring hospitalization.

In this setting, when testing more broadly in the community may not be as highly prioritized, to ensure that those who might be sick do not transmit the virus to others, blanket recommendations or mandates to stay home may be issued.

It is also important to remember that, because to date there is no treatment for COVID19, a test is not required to determine a treatment course.

Importance of testing.

While testing needs to be weighted against all other priorities at the height of an outbreak, testing of asymptomatic or mild cases is important for a number of reasons.

  • People who test positive for COVID19 may be more likely to stay home than those who are not sure if they have the infection.
  • People who have symptoms may feel very anxious if they can’t get tested.
  • Testing a wider number of people can give a more reliable picture of the extent of the epidemic.
  • Widespread testing also allows to get a more accurate case fatality rate. For example, if you only test hospitalized cases and you have 100 cases and 5 deaths, that results in a case fatality rate of 5% (5/100). But if you test in the community and find an additional 400 mild and asymptomatic cases, your case fatality rate would decrease to 1% (5/500).

Different approaches to testing

Countries have taken different approaches to testing, with some testing more broadly as many contacts and mild symptomatic cases as possible, while others have focused testing at the hospital and specific “sentinel” sites, as well as testing contacts in localities where there is not as yet widespread community transmission. As we have seen, these approaches may also vary depending on the stage of the epidemic in a country.

Other reasons why testing may vary among countries is availability of testing kits, laboratory equipment and materials and skilled human resources.

When looking at testing, it is important to consider not only the absolute number of tests a country is carrying out, but also the number of tests compared to the total population in that country (which you can express as number of tests per hundred thousand (100K) people, for instance).

As an example, as of this writing, US, Spain and South Korea are on top of the chart in the number of tests performed . But when seen as tests per 100K, it is Iceland, Bahrain and Norway (all countries with smaller total populations) that top the charts.

When serology tests become available, more widespread community testing might occur. This will help us to understand how many people have actually been infected with the virus without showing symptoms. In the meantime, with the current tests that only show who is infected when the testing takes place, testing approaches will continue to vary depending on the phase of the outbreak a country is in, the country’s own capacity, and a number of other factors.

More info

Here you can find WHO’s interim guidance for laboratory testing of COVID19 (technical document).

This older video from Feb 26, from the Wall Street Journal does a good job of pointing out all the challenges with testing.

This Wikipedia page is the best site I’ve been able to find that tracks number of tests performed in each country, using different sources of data.

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