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PCR test

Dr Karen Martin
Reviewed by Dr Karen MartinReviewed on 19.10.2023 | 3 minutes read

A PCR test is short for polymerase chain reaction, a test that works by detecting the genetic material of a particular bacteria or virus. If it detects the genetic material of the target infection, then the result comes back as positive.

PCR testing is used for various conditions, including influenza, herpes, and COVID-19. It's more accurate for detecting COVID-19 than lateral flow tests, but requires time as a laboratory needs to process the result, rather than a quick test at home.

It identifies people with a current COVID-19 infection, but it cannot detect if you have had it. Anecdotally, doctors have found that PCR tests usually turn positive a couple of days before a lateral flow test.

What does the test involve?

A clean, sterile swab (a thin plastic stick with a soft end, like a cotton bud) is used to collect secretions from up your nose. The swab is moved around for 5 seconds in each nostril. This can be uncomfortable, make your eyes water, or make you gag.

The swab then goes into a sealed test tube and gets sent to a laboratory to do the PCR testing. It takes between 24 to 72 hours to come back.

Will I keep testing positive?

If you have had COVID-19 within the last 90 days, even once you've cleared it, it's possible that fragments of dead or remnant COVID-19 virus could still be present in your body and may be picked up on a PCR test, resulting in a falsely positive result.

A lateral flow test may turn negative more quickly, and is used to guide people on when they can come out of self-isolation, along with government guidelines.

When should I arrange a test?

If you have symptoms of COVID-19, such as a high temperature, a new continuous cough, or a change in your sense of smell or taste, then you should consider getting a PCR test, either at a local test site or by ordering a kit online.

If your PCR test comes back positive, it means that you are currently infected with COVID-19 and should avoid others and isolate for at least five days. This is regardless of whether you have been vaccinated, but this is no longer a legal requirement. The government requests everyone take responsibility for themselves to prevent spreading the infection, and should therefore avoid others while they could be contagious.

Others who should consider a PCR test are:

  • those with certain health conditions that might require COVID-19 treatments
  • if you live with someone who has tested positive for COVID or has COVID symptoms
  • or if you have been advised to do so by a doctor

You no longer need to do a confirmatory PCR test after testing positive on a lateral flow test.

PCR tests are usually not required for traveling abroad or returning to the US, but these requirements are subject to change.

When should I see my doctor?

You do not need to see a doctor for a PCR test. If you are in the US and have COVID-19 symptoms or have been advised by your doctor to get a test, then you can book a test at local testing centers.

*Information correct as of 25 January 2023

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This article has been written by UK-based doctors and pharmacists, so some advice may not apply to US users and some suggested treatments may not be available. For more information, please see our T&Cs.
Dr Karen Martin
Reviewed by Dr Karen Martin
Reviewed on 19.10.2023
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