COVID-19 test results and their accuracy

Information about how COVID-19 test results are reported, why they are sometimes uncertain, and what they can and cannot tell us.

Last updated: 22 March 2022

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The information on this page is included in the factsheet COVID-19 testing, testing uncertainty and what we know about infection and immunity, available from What we know about infection and immunity.


How the results are reported

COVID-19 test results are reported as positive or negative. If the test result is:

  • positive - the virus was detected in the sample
  • negative - the virus was not detected in the sample.

Sometimes the test result might be reported as a ‘weak positive’, which means a very small amount of virus was detected in the sample. This could mean the person was tested towards the beginning or end of the illness when the level of virus in their body was low.

RAT results and their accuracy

Rapid antigen testing (RAT) helps to tell if a person is infectious with COVID-19 and can pass the virus to others. They are quick and relatively simple to do but are not as accurate at telling whether you have COVID-19 as the PCR test you may have had before. A nose sample for a RAT works better than a throat sample.

RATs are better at detecting COVID-19 when people have symptoms. They are recommended for people who feel unwell with Covid-19 symptoms, or for those who live in the same household as someone who has COVID-19. RAT results are less reliable for people who don’t fit these categories.

At the start of a COVID-19 infection, it may take some days before a rapid antigen test (RAT) can detect the virus, even if a person has symptoms and can infect others. A positive RAT at the start of an illness is a good indication that you have COVID-19. But if you’ve tested negative and still have symptoms that could be COVID-19, please stay home and repeat the test on day 3.

In contrast, some people continue to test positive even after they feel well and are no longer infectious. If you’re isolating after a positive RAT, there’s no need to test again at end of the seven-day isolation period - but remember to stay home if you still have symptoms.

If someone at higher risk of severe Covid-19 illness is a household contact and has a negative RAT for COVID-19, a more sensitive PCR test maybe needed. They could discuss this with their usual health provider.

Don’t forget to upload your RAT result to your My COVID Record account. Information on how to do that can be found on the Unite against COVID-19 website. Uploading a positive result helps you access support to successfully complete your isolation period. Uploading a negative result helps us understand how the outbreak is progressing.

If you do a RAT self-test, make sure you do these 3 things:

  1. follow the instructions
  2. only use the swab inside your nose
  3. record the result.

Anyone with COVID-19 (cold or flu like) symptoms should stay at home until they are feeling better. We also encourage people to continue to wear masks, physically distance where possible and maintain good hand hygiene.


PCR test results accuracy

A PCR test for COVID-19 is highly accurate when taken in ideal conditions. A laboratory study reported in 2020 found that different COVID-19 testing kits correctly detected SARS-CoV-2 (the virus that causes COVID-19) in samples more than 95% (and frequently 100%) of the time. When tests were done on samples without the virus, the tests correctly gave a negative result 96% of the time. 

Read more about the study on the accuracy of different COVID-19 testing kits

But it is important to remember that PCR tests don’t work as well in the real world. No test is 100% accurate. There is a very small chance with a PCR test that someone may have a negative result even though they are infected (i.e. a false negative result).

Reasons for a false negative test result could be because: 

  • the sample was taken at the wrong time (too early or too late) 
  • the swab was not taken correctly and did not pick up any pieces of the virus
  • the sample is contaminated during collection

That’s why it remains very important to isolate yourself if you have COVID-19 symptoms or if a health professional asks you to, even if your test result is negative.

We expect very few (if any) false positive test results with PCR (a false positive being a positive test result for someone who does not have the disease).


What the test results can and cannot tell us

Even when we take the uncertainties of testing into account, the results can tell us a few things.

A positive test tells us that a person either has COVID-19 (whether they have symptoms, or not) or has had COVID-19 recently. We may not be able to distinguish whether the person is currently infectious or not so we take a precautionary approach. 

A positive test cannot tell us:

  • if the person is currently infectious
  • how ill the person is likely to become.

A negative test can tell us:

  • the person was unlikely to be infectious at the time of the test.

A negative test cannot tell us:

  • if the person was exposed to the virus or not
  • if they are in the early stages of incubating the disease
  • if they had COVID-19 in the past 
  • if they were infectious in the past
  • that they will not get COVID-19 in the future.

Note: if a person has a negative test result and they are at a higher risk of having COVID-19, they may want to be tested again.

 

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