What does a faint line mean on a lateral flow test?

The UK has experienced another rise in Covid-19 infections in recent weeks, the latest surge driven by a sub-lineage of the Omicron variant known as BA.2.

However, the uptick now appears to be subsiding, with the UK Health Security Agency (UKHSA) recording 281,269 positive tests within seven days on 11 April, as well as 1,613 deaths and 16,438 people admitted to hospital.

The sub-lineage is currently estimated to account for approximately 93.7 per cent of cases in England, with the highest prevalence in the South East (96.4 per cent) of the country and the lowest in the East Midlands (91.1 per cent), according to the UKHSA.

Boris Johnson’s government ended all social restrictions in England on 24 February and followed that by scrapping the provision of free lateral flow tests (LFTs) on 1 April as part of its pivot towards a “Living with Covid” strategy, assuming the worst of the pandemic to be over.

But the latest wave of cases has already created renewed demand for LFTs in pharmacies, which reported a 400 per cent increase in sales in the final week of last month.

Still the most straightforward means of determining whether or not you have contracted Covid from home, LFTs typically retail for £2.50 each or £12 for a pack of five and use a test strip that changes colour to indicate whether the saliva sample you provide contains traces of the coronavirus.

Once you have squeezed droplets from your sample onto the plastic reader, a single hard line will appear next to the letter C (for “control”) at the testing window to confirm that the test has worked successfully.

A second red line will duly appear next to the letter T (for “test”) if the virus is present in your sample, although this line can sometimes appear very faint.

According to the government’s guidance, the reason for this is that LFTs can be less sensitive when a person has lower levels of the virus in their system, although the appearance of any line at all means that your test is positive, that you have contracted Covid and that you should consider yourself infectious.

“Even faint lines indicate the test is positive,” the official advice states, adding: “Result lines may appear smudged or faint, but they are still valid results and must be reported.”

However, no test is 100 per cent accurate all the time and it is possible for the LFTs to return a false positive, typically if the sample you provide has inadvertently been contaminated or if you wait beyond the 15-30 minute recommended in the instruction ket to determine your result.

If you do test positive for Covid, you are no longer legally required to self-isolate, although the NHS does recommend working from home where possible to avoid passing on the disease to colleagues, friends and family, particularly as others may be more vulnerable than you.

Prior to the scrapping of social restrictions, the public were advised to leave isolation after five days if they tested negative on two consecutive days or after 10 days of quarantine if they continued to test positive.

This article was amended on 22 April 2022 to remove a reference to litmus paper. While the test cartridge contains a strip that changes colour to indicate a positive test, it does not use litmus paper.