Explainer
The new rules on testing for Covid - and why the UK has moved to lateral flows over PCRs
Covid testing rules across the UK have been adjusted to reduce disruption to businesses caused by staff self-isolating, as the country continues to report high numbers of daily cases due to the spread of Omicron.
What are the existing rules?
Up recently, people without symptoms who test positive on a lateral flow had been asked to take a PCR test and only begin their isolation period when they receive a positive result of that, meaning they could quarantine for more than the minimum of seven days as they should isolate from their positive lateral flow result and seven days after their positive PCR one.
This only applied to people who were fully vaccinated and test negative on lateral flows on days six and seven of their isolation are then released - if not, it's the full 10 days of quarantine.
Boris Johnson is also said to be considering reducing the isolation period to five days in order to help ease staff absences across the economy and public services.
The prime minister's official spokesperson said “if it is possible to go further then we will do so”.
What are the new rules?
The news rules mean symptomless people who get an initial positive result on their lateral flow test now don't need a PCR test to begin their period of self-isolation.
In theory it will allow people to get back to work earlier, having begun their period of quarantine as soon as they received one positive result.
The government has said that people who have one of the three main Covid symptoms - high temperature, a new, continuous cough and a loss or change to the sense of smell or taste - must still get a PCR test.
Test and trace will still operate in the same way so long as people register their result.
People eligible for the £500 Test and Trace Support Payment will still need to do a PCR to access the financial support, as will those involved in research studies and those at risk of becoming seriously ill with the virus. People in that vulnerable group will be sent a PCR kit by mid-January in case they develop symptoms at any point.
The measure is likely to be temporary while Covid rates remain high, the government has said.
The change came into force on Tuesday, January 11 in England.
The same rules have already come into force in Wales, Scotland and Northern Ireland.
Why has the government brought in the rule changes?
The rule changes mean people can take action immediately, beginning self-isolation and having their contacts traced.
Capacity in laboratories processing PCR results should also increase, as they'll only be fielding tests from those people with symptoms.
Amid disruption to businesses (and growing pressure on the NHS) the adjustment should plug staff shortages as workers will be able to return to work earlier - having completed their isolation quicker.
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New rules for workers in certain industries have also been introduced to ease disruption.
The rule changes also follow weeks of disruption to the supply of Covid tests across England.
The government admitted the country was feeling the affects of a global shortage, but Labour has branded the situation a "shambles".
Is it safe to just rely on lateral flow results?
Lateral flow tests tell people in 30 minutes if they have coronavirus, by detecting proteins from the virus in nose and throat samples. The government says they deliver fewer than one false positive in every 1,000 tests carried out.
A study by the Queen Mary University of London, the University of Oxford, the Institute for Advanced Studies based in Vienna, and the Medical University of Graz published in July found lateral flow tests detected more than 95% of the cases found by PCR tests and correctly identified 89% of cases as negative.
In December, the Health Security Agency (HSA) said lateral flow tests are as likely to detect Omicron as other variants of coronavirus.
PCR tests are extremely accurate but take up to three days for results to come through. They detect the genetic material from a specific organism, specifically coronavirus.
Coronavirus: What you need to know
When should you take lateral flow tests?
According to the Royal College of Pathologists, lateral flow tests should be taken by people who do not have symptoms.
A factsheet on the RCP’s website states: "These tests are very different from PCR. They are not suitable for diagnosing individual patients who suspect they may be infected because they have symptoms.
"People with symptoms need a PCR test. Lateral flow tests are intended for picking up additional infected cases who would otherwise be missed because they don’t have any symptoms."
People are advised to do lateral flow tests before mixing with crowds in indoor places or visiting someone who is at high risk of getting Covid-19.
It's also advised if you’re vaccinated, but have been in contact with someone who then tests positive for coronavirus, you should do a lateral flow test.