3.4 What Tests can we use during the COVID-19 Pandemic?
By Charlotte Kerr
PhD Student, Faculty of Epidemiology and Population Health, LSHTM
There has been a lot of discussion about testing for COVID-19. We hear about it on the news - the numbers of tests conducted, and new tests that have been developed.
But what tests are being used, and
are different tests important?
Different types of test for COVID-19. Image credit: Wikimedia commons.
There are three different types of test; PCR tests, antibody tests, and antigen tests. They each look at different things, and will indicate whether someone is currently infected, or has been infected in the past. In this article, we’ll discuss how they’re used now, and how they might be used in
What tests do we use to diagnose COVID-19?
Currently (as of 18 May 2020) if someone becomes sick with symptoms of COVID-19, they are tested for the virus using a PCR test. This tests for an active infection, i.e. an infection that’s happening now.
A swab is taken from the back of the throat or the back of the nose. This swab is then sent to a laboratory where technicians run a test called rt PCR, which is short for real-time Polymerase chain reaction. This test can tell us if genetic
material (RNA in the case of SARS-CoV-2) is present. If the swab contains viral RNA, the PCR process will make lots of copies of it, and tag each copy with a fluorescent marker, or stain. By reading these markers, we can diagnose whether a
person has the infection.
tests have been very important in this outbreak but they take time to conduct, and need technicians who are specially trained. They also need specific equipment, which is often expensive and only available in certain laboratories. And as we
mentioned before, this is a test for an active infection- it won’t tell us if someone has had the infection and recovered from it.
What about antibody tests?
Infectious agents such as coronavirus have unique molecular structures on their surface that our bodies recognise as something that shouldn’t be there. These stuctures are known as antigens. One of the ways our immune system reacts is to produce
These antibodies are specific to a particular antigen of a particular infectious agent, which makes them useful in testing. The antibodies attach to the antigen, and then either neutralise the microbe directly, or activate other parts of
the immune system to attack the microbe.
Different types of antibody are produced at different stages of the immune response- It will take a few days after initial infection before antibody responses can be detected.
Timing of an Antibody Response
The timeline of the antibody response is shown in the image below. As you can see, immediately after infection, we wouldn’t record an antibody response- we have to wait 2-3 weeks after infection before we’d get reliable results.
For SARS-CoV-2 IgM and IgG antibodies both increase at about the same time, but IgM antibodies decrease again and disappear by about 2 months. IgM antibodies indicate recent infection. IgG antibodies persist for longer (although we do not yet know
for how long), so can tell us about infection that was recent or longer ago.
Not everyone produces the same immune response, time to response varies, and level of antibodies
produced can vary between people.
Figure 1. The variation in test efficacy with time since symptom onset (time since symptoms are first seen). Adapted
from figure in Sethuraman et al. 2020 
As shown in the graph, an antibody test can only tell us about past infections. It can’t be used to diagnose active cases
Why do we want to know about people who’ve already recovered?
An antibody test is useful in looking at past infections. Some people will have been asymptomatic during their infection, so would not have come forward for a PCR test. But by mass screening with antibody tests, you can retrospectively count case numbers
in your population.
Antibody tests might also be important to help us understand whether infection provides you with immunity. We would be able to tell how many people in the population might already be immune to the coronavirus. We expect that there will be some immunity
to a second infection, but at the time of writing, we don’t know how long this will last.
Current Antibody Tests
At the time of writing, one antibody test has been developed, and certified for use in the UK. Its use will initially be limited to particular high risk groups (e.g. health care workers),
but it may eventually become available to the wider public.
As we have seen, an antigen is the part of a microbe that the body recognises as foreign. It stimulates the immune response, often resulting in production of antibodies.
An antigen test, like the PCR test, could be used to detect a current infection. At the time of writing, antigen tests that detect proteins from the viral capsid (the protein coat of the virus) are under development, but aren’t yet
approved for use.
The PCR test for viral RNA is used by the NHS to diagnose patients. But as we said before, it needs specialist equipment and training to perform, is expensive, and takes several hours to return results. In future it may be possible to develop more user-friendly tests.
Tests for the antigen or the antibody could theoretically be cheaper and easier to use than the PCR test: they don’t require the complicated equipment, or specialist training. It may even be possible to develop antigen and antibody
tests into a Rapid detection test (RDT) kits to give a result within a few minutes (like a pregnancy test). Although these are already being used in some places they are not yet reliable.
Before any test can be used, it has to go through checks to ensure it gives reliable results. We’ll be talking about this in the next step of this course.
So we now have tests that can detect current and past infections. Where should we be using these tests? What do you think?
A lot depends on our resources, and the stage of the outbreak we are at. Different groups and the reasons for testing them are given below.
Different countries have taken different approaches. In some countries testing capacity has been very limited, so only seriously ill patients receive tests. Other countries, like Hong Kong, Singapore, Germany and Iceland have used extensive testing,
including testing people who haven’t shown symptoms of the disease. At the end of section 3 we’ll be discussing the different COVID-19 control strategies taken by countries around the world. In general, the more tests we do, the better we understand
how much transmission is happening where, and the better we can control the spread of the virus.
Antibody - Proteins produced by our immune cells that bind to an antigen
Antibody Test – Tests that detect disease-specific
antibodies in a person’s blood. Presence of antibodies suggests that a person has been infected with the disease in the past.
Antigen – A substance inducing an immune response in the body
. Often stimulates the production
Antigen test – A test which assesses presence of disease-specific antigens in the body. If antigens are found, this indicates
a current infection.
DNA - deoxyribonucleic acid, a self-replicating material which is present in nearly all living organisms as the main constituent of chromosomes. It is the carrier of genetic information.
Polymerase Chain Reaction (PCR) - polymerase chain reaction a method of making multiple copies of a DNA sequence, involving repeated reactions with a polymerase
Rapid Detection Test - a medical diagnostic test that is quick and easy to perform
RNA - ribonucleic acid, a nucleic acid present in all living cells. Its principal role is to act as a messenger carrying instructions from DNA for controlling the synthesis of proteins, although in some viruses
(including coronaviruses) RNA rather than DNA carries the genetic information.
1. Sethuraman N, Jeremiah SS, Ryo A. Interpreting diagnostic tests for SARS-CoV-2. Jama. 2020 May 6.