Why Do False Negatives Happen In COVID-19 Tests?

April 15, 2020

By Michael Bohl, MD, MPH
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April 15, 2020 — 2 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 have the virus, but they actually do.

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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 

Currently, the main form of testing for 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 is not widely available yet, but should be in the near future. The serologic test looks for antibodies in the blood and can determine if somebody has ever been infected with the COVID-19 virus — even if the RT-PCR test is negative.

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) to look for something even smaller (in this case, a virus). 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.

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, though false positives are more unlikely.

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

False negatives can happen with human error (poor swabbing technique, testing at the wrong time in the course of the illness, etc.) If you still feel like there’s a chance something went wrong, consider talking to your healthcare provider, "how likely is it that I could have COVID-19?" also known as your “pretest probability." The pretest probability is a measure of how likely you are to have the disease before finding out your test results.

For example: Imagine you are 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 have 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 for COVID-19. However, a retest isn’t always necessary. Though there is no specific treatment for COVID-19, finding out whether you are actually positive will change your life in a few ways. You may need to notify people you have come in contact with and take care to self-isolate away from other family members. If you have COVID-19 symptoms, you should be self-isolating regardless. Get plenty of rest and use over-the-counter medications to help control your symptoms. Additionally, do whatever you can to help prevent spreading your illness to others — this includes staying home, wearing a cloth face covering if you do go outside, coughing and sneezing into your elbow, and frequently washing your hands.

Will Any of This Impact the Reopening of the Government?

The short answer is, probably not. False negatives have always occurred with medical testing and they always will. Any projections for when the government will reopen 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 — which is what some governors are calling for prior to reopening — 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.


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Article written by Michael Bohl, MD, MPH