When’s best to take a PCR test? COVID incubation times and isolation guidelines explained.

Here’s everything you need to know about when it’s best to take a COVID test – and some handy insight into what terms like ‘incubation period’ and ‘symptomatic period’ actually mean.

5, 10 or 14 days?

We all know that we’re meant to isolate if we – or someone we live with – shows symptoms of COVID-19, or if we’ve travelled to a ‘hotspot’ country. But the guidelines are changing all the time. Then there’s the Test-to-Release scheme, which lets you leave isolation after travelling if you test negative using a PCR test after 5 days. It can be hard to understand what it all means, what you should be doing, and when you should take a COVID test. So let’s break it all down…

When is the best time to take a COVID test?

Studies have reported that the highest coronavirus viral load (the total amount of virus a person has inside them during infection) is seen in the throat and nasal passage when your symptoms begin to show.
So if you have one or more of the following, it’s time to take a COVID test:

  • New continuous cough
  • High temperature
  • Loss of – or change in – your normal sense of taste or smell (anosmia).


This time of early symptoms is not only the best time to test if you think you might have the virus – it is also when you will be eligible for a free NHS test – you can order one here.

Why is it best to test when you’re beginning to show symptoms?

The reason the onset of early symptoms is such a good time to test is because it’s when the virus is showing in very high quantities in the nose and throat. So if you swab at this point, there’s a higher chance of collecting viral particles on the swab successfully which improves the likelihood of detecting its presence in the laboratory using a molecular testing kit.

10 days self-isolation if you show symptoms or test positive

Here in the UK, we’ve been living with a 10-day isolation period – from the day symptoms first appear – or when a person tests positive. This also applies after returning from a ‘hotspot’ country.


The policy aligns with many other countries, and data published in the journal nature on how the virus behaves. This is because it appears to typically takes around 10 days to go from infection to no longer testing positive for the disease. Some people actually test positive for longer – some shorter – but this seems to be the average.


The 10-day rule applies even if you don’t have symptoms, and everyone in your household has to isolate also – there are guidelines here.

10 days isolation post-exposure or travel

The 14-day isolation period post-contact or travel was recently reduced to 10 days, when scientists and government officials discussed the behaviour of the virus – and the general public!


To increase compliance whilst still providing good levels of safeguarding, it was decided that 10 days gives plenty of time for the bug to incubate if people have come into contact with someone infected, or have travelled to a COVID hotspot. According to the BBC:


“People are most infectious around the time they first develop symptoms and, 10 days into an infection, only about 2% will still be capable of passing on the virus to others.”

What is all this based on?

It comes down to how we understand we understand COVID to progress. So far, the science indicates that the virus passes through two stages:


Incubation period – 4-5 days
Symptomatic period – 5-15 days

What is the incubation period?

This is the time when you might have COVID, but you don’t realise it yet. You feel normal, but if you have been exposed to the virus and your immune system hasn’t fought off the attack, it is multiplying in your body. In these early days, it’s more difficult to diagnose – though not impossible, when you choose the most sensitive tests such as molecular PCR test.


The incubation period seems to last around 5 days, which is why the test-to-release scheme makes travellers isolate for 5 days (to incubate the virus), then take a PCR test. If that test comes back negative, the traveller can be then released from quarantine early.

What is the symptomatic period?

The symptomatic period is when you start to show symptoms – until those symptoms have gone and you are no longer infectious. This period can last from 5-15 days, though as your infection progresses, you will show less and less virus when you get tested (the ‘viral load’ is lower).


However, this doesn’t necessarily mean that you are less infectious – it’s just that the virus is harder to spot.

So when are people most infectious?

It’s less about ‘infectiousness’ and more about how we behave when we might be contagious. So far, it has been estimated that nearly 50% of COVID-19 infections have been passed on from people who were in the incubation phase – when ‘presymptomatic’. This means they did not have symptoms at the point they passed the virus on to others, and were out and about behaving normally.


This is what makes the virus very efficient at spreading; people are less likely to know they are infected at the point where they are feeling healthy, out in public and possibly spreading it to others unwittingly.

When is the best time to test before visiting loved ones?

In an ideal world, you would isolate for a full 14 days, then test to be 100% sure that you’re COVID clear. But being realistic, isolating for 5 days – to allow any virus you might have picked up to incubate – is a good option before testing. This aligns you with the government’s test and release programme.

Is it worth testing if you don’t isolate?

Though you won’t be as sure that you’re COVID free, it is worth testing even without an isolation period if you choose a high sensitivity test, such as molecular PCR. These tests are analysed in a unique way that means even the tiniest traces of the virus’s genetic material can be detected – so you’re less likely to get a ‘false negative’. Read more about different types of tests here.

Should I get tested if I have no COVID-19 symptoms?

Even if you do not develop any COVID-19 symptoms at all, you may still be infected and pass the virus on to others without knowing it. This is called asymptomatic COVID. That’s why – if you want to visit someone vulnerable – it can be a good idea to isolate and take a private COVID test.

What can I do if I don’t want to get tested?

Whatever your circumstances, it’s really important to follow local guidelines, and the national advice:

  • Wash hands – regularly with soap or use sanitiser
  • Cover face – with a mask that you change regularly
  • Make space – 2 metres where possible
  • It’s also a good idea to meet outside if you’re allowed to do so, and to be mindful that all family members (however small!) should regularly sanitise or wash hands. Staggering festive meetings to give time for any symptoms to show is also sensible if feasible.

Meet the expert…
MyHealthChecked Editor Ray Mills-Morrow was speaking to Dr Greg Fitzgibbon, PhD, DipRCPath, an eminent clinical scientist and entrepreneur in the world of biotechnology. Greg is MyHealthChecked Clinical Director, with New York State Department of Health Certificate of Qualification. Greg holds a Doctorate in Molecular Biology, is a Diplomate of the Royal College of Pathologists, and brings 8 years’ experience in Clinical Science for the NHS, plus a wealth of experience leading in biotech.


REFERENCES:
https://www.nature.com/articles/s41591-020-0869-5
https://www.gov.uk/government/publications/guidance-for-contacts-of-people-with-possible-or-confirmed-coronavirus-covid-19-infection-who-do-not-live-with-the-person/guidance-for-contacts-of-people-with-possible-or-confirmed-coronavirus-covid-19-infection-who-do-not-live-with-the-person#:~:text=You%20should%20not%20arrange%20for,smell%20 (anosmia).

Last reviewed on 21st December, 2020

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