Lateral flow device tests for COVID-19

Two biological tests are being used in the UK to control the spread of COVID-19: (1) the reverse transcription polymerase chain reaction (RT-PCR, or PCR) test; and (2) the lateral flow device (LFD) test. 

PCR tests tend to be used for people showing symptoms associated with COVID-19, while LFD tests are used for more widespread testing, including identifying asymptomatic people (who do not show symptoms but may be carrying the virus).

This article explains current understanding around LFD tests for COVID-19. FactCheckNI has previously published an article that looked in detail at PCR tests.

What are LFD tests?

Lateral flow devices can detect the presence of a target substance in a liquid. Home pregnancy tests use LFDs.

LFD tests for COVID-19 target the SARS-CoV-2 antigen. An antigen is a substance that stimulates an immune response; this differs from an antibody (a substance your body produces that interacts with a targeted antigen to inactive it). This LFD test is sometimes called a “rapid antigen test”. Being vaccinated will not by itself cause a positive LFD test result, because the LFD test targets the presence of antigens, not your antibodies.

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As part of the LFD test, swabs are taken from your throat and nose. LFD tests for COVID-19 can be self-administered at home, though this can affect the reliability of the result.

How accurate are LFD tests for COVID-19?

The accuracy for LFD tests for COVID-19 is measured in terms of “sensitivity” and “specificity”.

    • “Sensitivity” refers to the proportion of people who have the disease and test positive for it — the true positive rate. Research suggests that the true positive rate varies from 28% to 79%. Some of this wide variation is explained by the quality of the testing conditions, for example whether the test was administered by laboratory scientists or not, and how well the test was processed and read. It is suggested that home testing may reduce the true positive rate than testing performed in supervised test centres.
    • “Specificity” refers to the proportion of people without the disease and test negative for it — the true negative rate. Research suggests that the true negative rate varies from 99.5% to 99.9%.

False positives?

Imagine that you test positive for the SARS-CoV-2 antigen, using an LFD. However, you are vaccinated, display no symptoms, and are feeling well; you haven’t travelled internationally and the number of cases of COVID-19 in your locale is low. These factors could suggest the possibility of a false positive. The current official guidance in Northern Ireland is that, no matter the circumstances, if you receive a positive test result using an LFD, then you should have a confirmatory PCR test within 24 hours and isolate yourself and the rest of your household, while waiting for the results of the PCR test.

Meanwhile, social media posts have claimed you can mischievously generate a false positive test result by manipulating the acidity of the liquid buffer solution.

False negatives?

On the other hand, if you test negative for the SARS-CoV-2 antigen, using an LFD, but you are showing symptoms of a cough, fever, headache, and fatigue (that’s not an exhaustive list of possible COVID-19 symptoms), like anyone displaying symptoms associated with COVID-19, you should immediately book a PCR test.

Summary

A lateral flow device is one of two common methods used to test for the presence of SARS-CoV-2 antigens. It is used to identify people who may have the virus but are not evidencing any symptoms of COVID-19.

Research suggests that the true positive rate (the “sensitivity”) varies from 28% to 79%. This can depend on how well the test is administered.

Any positive LFD test result currently requires a PCR test to confirm it. Likewise, if you test negative but show symptoms associated with COVID-19, you should go ahead and book a PCR test.

You can learn more about LFD tests for COVID-19 at the Department of Health.