False positive and false negative COVID tests

False positive and false negative COVID tests: what do they mean?


COVID testing is complicated. In this post we will try to simplify it for you. One thing that has made the current COVID-19 epidemic so difficult to control is that many of the tests for this virus give inaccurate results. The test result may say a person is “negative” (not have the virus), but is wrong, and the person actually does have the virus. 

Fortunately, for most diseases, blood tests are much more accurate—many at the 99% level. But tests for COVID give an incorrect result in up to 50% of tests performed!  Over time, the accuracy of COVID tests will improve, but for now, a negative COVID result must be viewed with caution. A negative result does not mean for sure the person does not have the virus.

You may have read of situations where before a certain event, everyone is tested, and only the people who test “negative” are admitted. Later, however, it is discovered that multiple people caught COVID at the event. How?

What is a “false negative” ?

The test result is “negative”, meaning the test shows the person does not have the virus, but this is incorrect, and the person does indeed have the virus. For one of various reasons, the test just gave a wrong result. 

What is a “false positive” ?

A person receives a “positive” result showing he has the virus, but in fact, he does not have the virus. The lab test was incorrect, probably due to contamination of the specimen at the lab. 

What type of COVID tests are done?

There are some (blood) tests to show if you have had a COVID infection in the past. But in this post, we are concerned with the tests performed to see if you are infected right now.

There are two types of tests. The test considered the “gold standard” is the PCR test, which involves taking a swab deep inside your nose. This is considered the most accurate type, but some PCR tests use saliva or simple swabs inside your nostril. The disadvantage is that this test is more expensive, and may take days to get a result. 

The second type of test is called an “antigen” test, also done with a nasal or throat swab. This test is less expensive, and gives results quickly, often in under 30 minutes. The major problem with this antigen test is that it is more likely to give an incorrect result.

How common is a “false negative” result?

For the PCR test, it depends on when during the course of infection the person was tested. If a person is tested within 4 days after exposure to the virus, the false negative rate is about 67%; very high. Even when the patient begins displaying symptoms (an average of 5 days after exposure), the false negative rate of the PCR test is 38%. Still high. About three days after the patient first notices symptoms, the false negative rate has fallen, to 20%, but this is still a significant number of people who are told they do not have the virus, but actually do have it. 

For the antigen test—quicker and cheaper than the PCR test—the false negative rate is as high as 50%, depending on the brand of the test and the laboratory. This means there is a very significant chance that even if you test negative with this test, you still might have the virus.

How common is a “false positive” result?

It is much less common than a false negative result, probably in the low single digits. 

If I suspect COVID, when should I get tested?

Testing within the first 3 days of suspected or known COVID exposure is very likely to give a false negative result.  So it is probably best to wait at least 3 or 4 days after suspected exposure to be tested to have a reasonable degree of accuracy. 

So if these tests are so inaccurate, why bother doing a test?

Tests are still valuable, but you have to be smart about interpreting the results.  It depends on the type of test you do, and when in the course of illness the test was done  If the result is positive, you can be rather certain that you are infected. You should seek medical advice and isolate yourself so you don’t infect other people.

If your test result is negative, that is good news, but still, you should assume that your result could be incorrect, and you really do have the virus (false negative). You probably can go out with some degree of confidence, but you should still wear a mask and socially distance from others. You might want to be retested in several days. And, avoid indoor situations outside of your home as much as you can. Good luck, and test smart!

To find a doctor, of any specialty, anywhere in Brazil, check out our website: www.Procuramed.com.

See also in ProcuraMed:

Why the US is in trouble now with COVID

Learn how to prevent COVID in restaurants and on public transport

Beyond the Dr. Dave Blog:

Which test is best for COVID-19? (from Harvard, good for general public)

False Negative Tests for SARS-CoV-2 Infection — Challenges and Implications (more for medical professionals)

Esta postagem também está disponível em: Portuguese (Brazil)

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