Information about how COVID-19 test results are reported, why they are sometimes uncertain, and what they can and cannot tell us.
Last updated: 14 August 2020
On this page:
- How the results are reported
- The accuracy of test results
- What the test results can and cannot tell us
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.
COVID-19 test results are reported as positive or negative. If the test result is:
- positive - the virus (its genes) 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 is in the sample. This could mean the person was tested towards the end of the illness when the level of virus in their body was low.
The viral test for COVID-19 is accurate when taken in ideal conditions. A recent laboratory study found that different COVID-19 testing kits correctly detected 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.
But it is important to remember that tests don’t work as well in the real world. No viral test is 100% accurate. In real-world use, the viral test for COVID-19 is not 100% ‘sensitive’ (meaning able to correctly identify people with the disease all of the time). This means that if 100 people who have the disease are tested – some will have a negative result (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 did not pick up any pieces of the virus, or
- the sample of mucus (or liquid from the lungs) wasn’t big enough.
This means that it is very important to isolate yourself if a health professional asks you to, even if your test result is negative.
The viral test for COVID-19 is much better at correctly identifying people who don’t have COVID-19 (this is known as a higher ‘specificity’). We expect very few (if any) false positive test results (a false positive being a positive test result for someone who does not have the disease).
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 will 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 caught 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 be tested again.