For months many of us have used a single red line in a plastic cartridge as a kind of social traffic light – swaying whether go out or stay at home to protect others from Covid. But from 1 April, lateral flow tests (LFTs) for those not showing symptoms will cease to be free of charge in England, and from 18 April in Scotland, meaning people will want to use them as economically as possible (if they bother testing at all). Wales and Northern Ireland are implementing a phased approach to charging for tests.

So, when is the optimal time to test if LFTs are limited and you want the greatest chance of knowing if you’re infected?

Visiting vulnerable friends or relatives

Doing an LFT immediately before visiting a vulnerable person helps to reduce the risk of transmission, but doesn’t entirely negate it. This is because LFTs only produce a second red line (a positive result) above a certain threshold of virus. “You can be infectious without noticing symptoms, or before symptoms develop, and lateral flow tests may miss this,” said Prof Ajit Lalvani, chair of infectious diseases at Imperial College London.

In a recent study published in the BMJ, Lalvani and colleagues found that LFTs would miss 20% of positive cases, when used to screen symptomatic people at an NHS Test and Trace centre, or 29% of cases in city-wide mass testing of asymptomatic people.

Using LFTs as a green light for visiting those at greatest risk of severe Covid could therefore provide false reassurance – although it is better than not testing at all. To further reduce risks, Lalvani advises ensuring that both parties are fully vaccinated; limiting unnecessary social exposure in the five days before the visit; washing hands immediately before the visit; wearing a face mask and maintaining social distancing; and meeting outside or in a well-ventilated room.

You’ve got ambiguous cold-like symptoms but no fever, loss of smell/taste, or cough

According to users of the Zoe Covid app, the most common symptoms associated with the Omicron variant closely match those of common colds or influenza: a runny nose, headaches, fatigue, sneezing and a sore throat.

Although a positive LFT can confirm such symptoms are due to Covid, virus levels may not be high enough during the first few days to give a positive result. This could be a particular issue in people who have been well-vaccinated, because symptoms are often caused by the immune response to the virus, and these responses may kick in faster if you’ve been vaccinated or previously had Covid. This means you may be borderline infectious during the first few days of symptoms – but still testing negative on an LFT.

“The problem is that once the virus starts to replicate it goes really, really fast – so, you may have symptoms in the morning and not be infectious, but by midday you may be infectious,” said Irene Petersen, a professor of epidemiology and health informatics at University College London.

She recommends staying at home if you have any symptoms, and if LFTs are limited, “don’t waste your test at the beginning – keep it until day two or three”.

Staying at home is particularly advisable if you’ve developed symptoms after being in contact with a known Covid case, or local infection numbers are high. If you must leave the house, wear an FFP2 mask, keep your distance from other people, conduct meetings outside where possible, and regularly wash your hands.

You’ve spent time with somebody who definitely has Covid, but you don’t have symptoms

Although there is no longer any legal requirement to self-isolate if you’ve been in close contact with an infected person, the government still advises you to work from home where possible; avoid contact with vulnerable individuals; limit close contact with other people, especially in crowded, enclosed or poorly ventilated spaces; and to wear a mask in these situations.

“After contact with a confirmed Covid case, there is a risk of contracting (and then transmitting) the infection for up to 10 days after the contact,” Lalvani said.

Testing may be advisable in some circumstances, but doing so in the first few days after exposure is likely to be a waste of time (and tests), because it takes time for the virus to produce enough copies of itself to become detectable. According to a recent challenge trial, which involved deliberately infecting 36 young healthy people with the original strain of coronavirus, the average time from first exposure to early symptoms was 42 hours, with virus levels peaking at around five days after exposure, and remaining high for a further four days, on average.

If you never develop symptoms, it’s impossible to know if you’ve escaped infection without testing, so should you use your last remaining LFTs during those five to 10 days after exposure?

“If you do not develop symptoms, then a possible reason for using LFTs after contact would be if you plan to visit a vulnerable person within 10 days of the contact,” said Lalvani. Health or care workers should follow the testing guidelines of their hospital, clinic or care organisation, he added.

You’ve had Covid and are wondering if it is safe to return to work or school

According to current NHS guidance, if you have Covid you should stay at home for up to 10 days from when your symptoms start (day zero), but if you get a negative LFT on days six and seven, and don’t have a fever, you can go back to business as normal. If you have two consecutive negative LFTs, you can be highly confident that you are no longer infectious – so testing at this point is advisable, if you feel well enough to leave the house.

If you continue to test positive after day seven, the picture is less clear. “Often the LFTs continue to test positive even though the person is no longer infectious, but there is no better easily available test to determine the end of infectiousness,” said Lalvani. “Most adults are no longer infectious 10 days after the onset of symptoms, regardless of their LFT results. However, some people, particularly those with health conditions that severely impair their immune systems, may remain infectious for longer.”