How accurate are the new rapid lateral flow tests?

We’ve been asked by readers about the accuracy of new Covid-19 “lateral-flow” tests being trialled in the UK. 

To date, tests to determine whether someone currently has the virus which causes Covid-19 have been PCR tests. These work by adding a specific reagent to the sample, which identifies any genetic material of the virus that’s present, and replicates it to the point at which it can be detected in a lab. However, these tests take considerable time to perform.

You can read more about how PCR tests work here.

Now the government is piloting a new diagnostic test, called a lateral flow test, for detecting the virus that causes Covid-19, known as SARS-CoV-2. 

These tests do not require lab processing and so can return a result much quicker than a PCR test. 

With a lateral flow test, a liquid sample is placed on a small absorbent pad. The liquid is drawn along the pad via capillary action (in a similar way to how liquid wax is drawn up a candle wick).

At a point along the pad, there are strips coated with components which react with the material of interest. 

In the case of Covid-19 tests, samples are taken from the patient’s throat and nose using a swab. The swab is added to a liquid, which is then applied to the end of the pad.

A strip on the pad is coated in antibodies which bind to proteins on the SARS-CoV-2 virus. If these proteins are present, that will show as a coloured line on the test. 

If this sounds familiar, that may be because pregnancy tests are also a type of lateral flow test, which detect the “pregnancy hormone” human chorionic gonadotropin. 

How accurate are they?

A number of different lateral flow tests have been put forward to be tested by Public Health England and the University of Oxford, and their accuracy has varied in testing. 

The tests being trialled, most notably in Liverpool, are the furthest along this testing process, and details of their accuracy were published recently.

These tests, made by Innova, return very few false positives. 99.68% of people who did not have the virus received a negative test result. In other words, very few people who do not have the virus which causes Covid-19 will be told wrongly that they do have it, after doing a lateral flow test.

However, the tests do return considerably more false negatives. Just 76.8% of people who did have the virus received a positive result (meaning the rest received false negatives). 

This percentage can vary though depending on how much virus is in the person’s system—the test detected over 95% of individuals with “high viral loads”.

PCR tests are still considered the gold standard for testing for the Covid-19 virus but like lateral flow tests, tend to return a higher rate of false negatives than false positives.