A guide to COVID-19 tests for the public

HERE WE COVER THE MAIN TESTS AVAILABLE FOR COVID-19

VIRUS STRUCTURE

To understand testing methods, it is helpful to understand the structure of the coronavirus SARS-CoV-2, which causes the disease COVID-19.

The virus is composed of a core made up of nucleic acid (nucleic acids are what makes up the virus's genetic code) in the form of RNA, surrounded by a coat called the envelope which contains various proteins. Spikes formed of a protein called the S (spike) protrude from the envelope. It is the S protein that attaches to cells of the human respiratory tract. 

TESTING

The tests commonly available for SARS-CoV-2 can detect either:

  • the RNA − detected by the PCR test
  • the surrounding proteins − detected by the rapid lateral flow devices
  • the human body’s response to the virus – detected by antibody tests.

PCR TESTINGnucleic acids are what makes up the virus's genetic code

PCR tests detect the virus’ RNA. These tests are normally carried out in a laboratory using a swab of the nose and/or throat.

PCR tests can detect very tiny amounts of RNA, meaning they are extremely sensitive. They are the best test for current infection.

Patients with COVID-19 usually start to become positive by PCR testing a day or two before symptoms start and will continue to test positive by PCR afterwards for some time. Repeat PCR once a diagnosis has been made is not necessary.

The period the patient must isolate for is defined by time from the start of symptoms or, if there are no symptoms, from the first positive test. Current UK policy is that patients must self-isolate for ten days after this time.

WHEN SHOULD I HAVE A PCR TEST?

  • If you currently have symptoms that may indicate COVID-19, this is the test you should have to diagnose the infection.
  • If a lateral flow test is positive. The purpose of the PCR test is to confirm the diagnosis, since it is a more accurate test than the lateral flow test.

Is a PCR test accurate?

PCR is the most accurate test available for current infection. In a person with symptoms, a positive PCR test is likely to accurately indicate infection. If a person has symptoms suggesting infection but a negative PCR test, doctors may decide to repeat the test if they still suspect infection (e.g. in hospitalised patients).

LATERAL FLOW TESTS

These are the rapid tests that are used in the community. They are convenient because they can give a result within 30 minutes and do not need a laboratory.

They detect proteins from the virus, not RNA. They use a swab of the nose and/or throat and are carried out on a small flat plastic device like a pregnancy test.

These tests are very different from PCR. They are not suitable for diagnosing individual patients who suspect they may be infected because they have symptoms.

People with symptoms need a PCR test. Lateral flow tests are intended for picking up additional infected cases who would otherwise be missed because they don’t have any symptoms.

WHEN SHOULD I HAVE A LATERAL FLOW TEST?

  • You should only have this test if you don’t have any symptoms and have been invited to take one as part of an exercise to identify infected people without symptoms.

ARE LATERAL FLOW TESTS ACCURATE?

These tests are not as sensitive as PCR. They are simply a convenient way of picking up a proportion of undiagnosed people who have no symptoms.

The way to look at these tests is that every additional positive case picked up is a bonus, preventing further unknown transmission of the virus.

If a person tests positive with these tests, they need to confirm this by having a more accurate PCR. In the meantime, they must self-isolate.

If these tests are negative, the person may or may not be infected and that person must continue to take the usual precautions such as hand washing, wearing a mask and social distancing.

A negative lateral flow test should not be used to rule out infection or indicate that it is safe to do something such as visit relatives. 

You can read more about the accuracy of lateral flow tests here.

ANTIBODY TESTS

These tests detect the body’s response to a previous infection, by looking for antibodies that the body has produced.

It takes some time after infection for the body to produce antibodies. So, antibody tests are not suitable for diagnosing people at the time they have symptoms.

They are useful for finding out if someone has been infected in the past. This is useful, for example, for studying how many people in a population have been infected.

It is not known for how long after infection antibody tests remain positive. Levels of antibody are likely to decline with time, over months or years.

WHEN SHOULD I HAVE AN ANTIBODY TEST?

  • You might be asked to have an antibody test as part of a study to see how many people have been infected in the past.
  • In future, if a doctor wanted to know if you had been infected in the past, they might perform an antibody test.

LAMP and LamPORE testing 

Like RT-PCR, LAMP and LampORE tests detect the viral RNA. They have the advantage of being able to use saliva as a sample, as well as swabs. Recently LamPORE has been deployed for local community testing in some locations. This test can be carried out in mobile laboratories around the country. LamPORE tests have high accuracy. They can be used for people with or without symptoms, but they are currently being deployed in the UK to detect people without symptoms in the community.  

As these tests are more accurate than lateral flow tests, positive tests do not require a confirmation by PCR.