Only two years ago, the thought of regularly shoving a stick up your nose to check you’re safe to meet family or go to work would have seemed a little, well, crazy. Now it’s the stuff of Twitter feeds, school-gate conversations and an awful lot of angst.
Testing, contact tracing and isolation of infectious people has been a large part of the strategy for tackling the COVID-19 pandemic worldwide, but knowing which test to take when, and why, has many healthcare professionals confused, let alone the public. Let’s take a look at the different types of test, how they work and when they might be useful.
Rapid Antigen or Lateral Flow Tests
These are perhaps the most commonly used type of test. In countries such as the UK, rapid antigen testing kits are made freely available to the general public. They are relatively easy to carry out and the result is available within minutes. They test for the spike proteins (antigens) specific to SARS-CoV 2, the virus responsible for COVID-19, which are present in the nose and throat of people with the infection.
The more antigen in someone’s nose, the more likely it is the test will be positive. The people with more antigen in their noses are those who are more infectious and have more in the way of symptoms. Peak viral load takes several days to achieve, so someone may be infected and infectious for several days before a lateral flow test returns a positive result.
Sensitivity (the ability of the test to correctly identify someone who has the infection) varies depending on the brand of test, the viral load of the patient and how the test is actually performed. To pick up the spike proteins, the swab has to be stuck in the right place, for the right time, and mixed with the right chemical for the right time before being dropped onto the testing strip in the right amount. A Cochrane review showed that lateral flow tests pick up a positive case about 72-78% of the time. A UCL study showed lateral flow tests had a sensitivity of 80% for all cases and 90% for those with symptoms.
In symptomatic patients, a positive test is very unlikely to be false positive – i.e. the person almost certainly has COVID-19 if they have a positive lateral flow test with symptoms. Some governments still recommend a confirmatory PCR, but there is some debate about whether this is necessary.
In a person without symptoms, a positive test is also unlikely to be false positive – i.e. the person probably has COVID-19 and should isolate. This is an important factor in limiting the spread of the disease, as one in three cases in the UK are asymptomatic. A confirmatory PCR test may be recommended in many countries.
The combination of no symptoms and a negative test is likely to be accurate enough to exclude infection.
The issue comes when there are symptoms of infection but a negative result. The sensitivity of rapid antigen tests is not high enough to exclude COVID-19 infection. A common misconception amongst the public is that a negative lateral flow means no need to isolate. This is only true in the context of someone with no symptoms. If there are symptoms present, a negative lateral flow does not rule out COVID-19 and a PCR test is required.
So… lateral flow tests (rapid antigen tests) are useful if you have no symptoms and want to check you are safe to work or socialise.
Polymerase Chain Reaction, PCR, tests identify genetic material specific to the organism they are manufactured to test for. SARS-CoV 2 PCR tests look specifically for the virus causing COVID-19. Genetic material collected on the swab (inserted in the nose, throat or both) undergoes a process in which it is multiplied many times, so even a small amount of virus can be detected. PCR tests are highly sensitive, i.e. very good at correctly picking up someone who has the infection. They can return a positive test much earlier in the course of an infection, compared to a rapid antigen or lateral flow test, even before someone becomes symptomatic. A positive test, symptoms or not, means you have COVID-19. A negative one means you very likely don’t.
They are more awkward to carry out though. The test has to be arranged, often involving a visit to a test centre or delivery of a testing kit, and then taken correctly, before being processed in a laboratory. Depending on the circumstances and conditions involved, results may take several hours to several days.
The other downside is that, due to the same system that makes PCR tests so accurate, they can detect tiny amounts of genetic material which may be present weeks after an active infection. The pay-off? An accurate result that can guide isolation, identify variants and assist in public health measures.
So…. PCR tests are useful if you have symptoms, or if you need a high degree of accuracy.
Loop-mediated isothermal amplification testing involves amplification of viral RNA, usually in samples of saliva. It’s reasonably quick (results within about an hour of arrival in the lab) and has high sensitivity and specificity but does require training and laboratory equipment (albeit much less than for PCR), refrigerated transportation of samples to the laboratory setting and spitting into a pot doesn’t appear (at least in the UK population) to be as attractive as sticking a swab up the nose.
Seroconversion, the development of antibodies to the SARS-CoV 2 virus, happens about one to two weeks after the onset of infection. Antibodies can be detected in the blood from this point until about a year post-infection. This test is not a useful test for rapid identification of infectious people but can be used to guide research and monitor responses to vaccines or infection in the population.
The tests are important but even more important is awareness of them and what those results might mean. In a world where we need everyone to play their role, information and understanding are vital. Let’s hope, one day, we can all start to forget what it’s like to shove that stick up our nose though.