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Dr Mark Ali discusses Lateral Flow Test Accuracy

There has been much discussion recently about Lateral Flow Tests, the role they play in symptomatic and asymptomatic testing and what it could mean if you receive mixed results. In this blog, Private Harley Street Clinic's Medical Director Dr Mark Ali answers some of the most commonly asked questions regarding the tests and their results.

When should I take a lateral flow test?

Lateral flow tests should be used by someone with no symptoms of COVID-19. This could be as a precautionary measure or if there has been known significant exposure. They are thought to be an excellent screening tool in this scenario. Previous advice e.g. to healthcare professionals, was to take these tests twice per week but, with the new Omicron variant, we know that the time from exposure to infection is shortened (~two days). Therefore testing more often is key here; literally “flow before you go”.

How accurate are lateral flow tests?

Lateral flow tests are very good at picking up positive results when a person is infectious. This can include a day or two before becoming symptomatic. This reinforces the need for regular lateral flow tests and why it is recommended that they are taken daily in vaccinated individuals with known exposure.

A peer-reviewed study published Clinical Epidemiology found that “lateral flow tests (LFTs) are likely more than 80% effective at detecting any level of COVID-19 infection and likely more than 90% effective at detecting those who are most infectious when using the test”.Similarly, a Cochrane review of 64 studies found that lateral flow tests correctly identify 72 per cent of infected people who have symptoms, and 78 per cent within the first week of becoming ill. This is because LFTs detect material from the surface proteins of viruses and so are likely to give a positive result when someone is infectious. A PCR test, however, detects viral genetic material that could remain for weeks after infection and doesn’t necessarily imply infectiousness.

Are lateral flow test results ever wrong, and why?

Lateral flow tests can be “wrong”- the false negative (sensitivity) and false positive (specificity) rates are not 100%. As described above, the specificity is excellent (low false positive rate)- if it’s positive, you likely are infectious with COVID-19. The sensitivity (false negative rate) depends on how infectious someone is and when in the course of the illness the test is taken. These figures are assuming the tests are performed correctly. A major factor to consider is that these tests require good technique. Eating food and drink prior to the tests can interfere with the results. Having a snotty nose that is not blown before the test might reduce the chance of picking up COVID-19. Or the user may not swab high enough/long enough. This is why having a test taken by a healthcare professional is important. In A&E departments, every patient has a lateral flow test and these are administered by healthcare professionals, not the patients themselves. If in doubt, it is best to ask a healthcare professional to perform the test as the results can be user-dependent.

Are lateral flow tests still useful for Omicron?

A study from the UK Health Security Agency found that lateral flow tests are as useful at detecting Omicron as Delta variants of COVID-19. 

I've had negative and positive results on my lateral flow tests, what should I do?

The most important thing in this scenario is to be safe and assume a positive result is a positive result. Lateral flow tests will pick up people at their most infectious period and this should be confirmed by a PCR test. A person may test negative one day and positive the next as they reach the threshold for detection on the lateral flow. Alternatively, different techniques may be used such that one sample was more thoroughly taken than they other. In general, lateral flow tests are sensitive when someone is infectious (low false negative rate for infectivity) and are specific (low false positive rate). The most important next step is to 1) isolate 2) obtain a PCR test as soon as possible 3) alert contacts so they can lateral flow test themselves more frequently

I’ve had a positive lateral flow test but negative PCR test, what does this mean?

The interpretation of a test depends on the pre-test probability- “what is the likelihood of the result being positive before taking the test”.

With Omicron rates so high at the moment and the sensitivity of lateral flow tests being good, the pre-test probability of a PCR test being positive is high. Tom Chivers explains this very well here.

As a *very best-case scenario*, then, if you get a positive LFT followed by a negative PCR, there’s a roughly one in three chance that you actually have COVID-19.

The true figure could well be much higher than that, but that’s a plausible lower boundary.