Why Do False Negatives Happen In COVID-19 Tests?

Do false negative tests have the power to stall reopening?

At-home Covid test

UPDATED June 10, 2020 — 12:00 p.m. EST

Every day, information about the COVID-19 pandemic fills the news. One of the most common issues you’ve probably heard being discussed is testing. Where can you get a test? Why aren’t more tests available? Will more testing mean the end of social distancing? But recently, a new concern popped up: the issue of false negative COVID-19 test results. This means people who are being tested for COVID-19 get results that say they don’t (or didn’t) have the virus, but they actually do (or did).

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This can be a bad thing, because a false negative test result could be falsely reassuring to a patient. Imagine if you got tested and found out you were negative. You may be more willing to ignore your own symptoms, which could make things worse for you. Or you may be more willing to socialize with other people, which could make things worse for them. With a highly contagious virus like COVID-19, it’s important to understand how testing is performed and whether or not you should be concerned about receiving a false negative test. Additionally, how might the evidence of a false negative or false positive impact the government’s ability to reopen? Should this be a serious concern once more tests become available?

How COVID-19 Tests Are Performed

The test used to diagnose COVID-19 is called a reverse-transcription polymerase chain reaction (RT-PCR) test. This is a test that looks for the genetic material of the virus that causes COVID-19. It uses a sample that usually comes from a nasal swab (but can also come from the mouth or from mucus that is coughed up).

There is another type of test, called a serologic test, which looks for antibodies in the blood and can determine if somebody has ever been infected with the COVID-19 virus. Serologic tests don’t diagnose active infections and may not be positive until 1–3 weeks after somebody has been infected. There are several different companies that currently offer serologic testing, and the accuracy of each test varies.

So, Why Do False Negatives Occur?

The reason is more simple than you might think. Essentially, designing a medical test is hard. There are many different kinds of tests, but they often involve using small samples (in this case, a swab or blood sample) to look for something even smaller (in this case, a virus or antibody). Making a test that is 100% accurate 100% of the time is quite a feat when you consider all of the things that could possibly go wrong. These include getting a bad sample, medical equipment not working perfectly, and human error. But there are also other things that can cause false negative test results. For example, if you have COVID-19, but the virus is only present in very small amounts, the virus may not be able to send a strong enough signal to turn the test positive. Or, if you were infected too recently, you may not have developed enough antibodies yet for a serologic test to detect.

Although the exact rate of false negatives for COVID-19 is currently unknown, researchers have noted that both false negative and false positive test results may occur with the RT-PCR test. False negatives and false positives also occur with serologic tests. The COVID-19 Testing Project did not comment on exactly how many false negatives results some tests have, but it did find false positive results up to 15.7% of the time.

What If I Get a COVID-19 Test & It Comes Out Negative?

If you get a negative result but feel like there’s a chance something went wrong, talk to your healthcare provider. Ask, "How likely is it that I have COVID-19 or that I had it in the past?" This can give you a sense of your “pretest probability," which is a measure of how likely you are to have the disease before finding out your test results.

For example: Imagine you are (or were) sick with all of the classic symptoms of COVID-19 (fever, cough, and shortness of breath). Imagine you were also recently in contact with somebody else who knows they had COVID-19. For you, since your history and symptoms are classic for COVID-19, your pretest probability of having the disease is high. So, even if your test comes back negative, since your pretest probability was so high, it might be appropriate to assume this is a false negative result.

In limited circumstances, it may be appropriate to get retested. If you got a false negative on the RT-PCR test, finding out if you are actually positive may change your life in a few ways. You may need to notify other people you have come into contact with and you may need to self-isolate away from others. (If you have COVID-19 symptoms, you should be self-isolating regardless.) However, if you got a false negative on the serologic test, you probably don’t need to be retested. This is because we don’t really know how to interpret serologic tests anyway, since it’s still unclear if it’s possible to get COVID-19 more than once or not.

Will Any of This Impact the Reopening of the Country?

The short answer is, probably not. False negatives have always occurred with medical testing and they always will. Any plans for reopening have likely taken false negatives into consideration. Again, the likelihood of actually getting a false negative test result is very low. Therefore, if large portions of the population are getting tested, we would know if there are still cases of COVID-19 out there (because the vast majority of people with COVID-19 will still test positive). Some governments have considered the option of giving people who have antibodies “immunity passports” in order to participate in society again. However, for a number of reasons — false negative test results being one of them — this is largely a bad idea.

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