What does a faint line on lateral flow tests mean?

A second line on a lateral flow test means you have tested positive for Covid, but is it still the same conclusion if the line is hardly visible?

Rapid antigen tests, or lateral flow tests, have become a vital part of our daily lives as the Covid pandemic persists.

They are used for travel testing, attending public events, and cutting down one’s self-isolation period, among other uses.

An Image Detailing How Antigen Test Result Should be Submitted

Lateral flow tests are convenient and effective. Using these tests, an asymptomatic person can determine whether they have a Covid infection or not within 15 – 30 minutes. They are extremely handy in that the tests can be carried out virtually everywhere – hospitals, offices, homes, or anywhere else.

Basically, lateral flow tests do not require samples to be sent to a laboratory. You can take the test in your living room and get your result in minutes. Since the lab requisite is non-existent, these rapid Covid tests cost a fraction of what you would otherwise pay if you were to use a PCR test instead.

Understand lateral flow test results

The tests involve collecting an individual’s swab samples and simply mixing them with a testing solution. Then the mixture is analyzed using a lateral flow device (similar to that of a pregnancy test’s).

Understanding the results from lateral flow tests are quite straightforward. The lateral flow device has two lines: one is marked ‘C’ and the other ‘T’. These are the control and test lines, respectively. The control line indicates that the test is working correctly, and the test line specifies the results of the test.

Approximately 15 minutes after the sample mixture is added to the device, if only the control line appears, then it is a negative result. Test samples that trigger both the control and test lines to appear indicate a positive result.

A test that only makes the ‘T’ visible should be considered inconclusive, and the test must be carried out again with a new lateral flow kit.

What if the ‘T’ line is faint on a lateral flow test?

If you have a visible ‘T’ line, no matter how faint, your test result indicates you are positive for Covid.

In a social media post, Dr Nathan Hudson-Peacock, who works for the NHS, wrote: ‘Essentially, if *any* line appears before the end of the interpretation window (check leaflet, often this is 30 minutes), then this is a *positive* test and you must isolate and book a PCR. The picture above is my positive LFT from this morning, and I have confirmed COVID.’

He also went on to point out that results that appear after the 15–30-minute time window of the tests do not count as valid.

In the same post, he wrote, ‘if a line appears *after* the interpretation window then this does NOT count as a positive test. NHS guidance is that you do not need to isolate and you do not need to book a PCR, unless of course you have symptoms.’

According to the government’s current guidance, if you test positive on a lateral flow test, you must enter self-isolation straightaway without requiring a confirmation through a PCR test.

How reliable are lateral flow tests?

The statement that ‘no tests are 100% accurate’ is applicable for lateral flow tests as well. However, in terms of reliability, they should not be ruled out.

According to a review published on Cochrane Library in March 2021, antigen tests correctly identified the absence of SARS-CoV-2 in 99.5% of people with symptoms similar to the ones of Covid, and in 98.9% of people who were asymptomatic.


Current foreign travel guidelines permit fully vaccinated travellers to enjoy their travels using the more affordable lateral flow tests wherever applicable. Check out our full range of travel tests that include Premium Lateral Flow tests: https://vivoclinic.com/shop/tests/