False Positive COVID-19 Test Results

false positive COVID-19 test results chart

How accurate are the laboratory tests used to diagnose COVID-19? I was wondering if someone could have a false positive due to having prior flu shots or for some other reason.

A false positive means that the test shows a positive result, but in reality, it should be a negative result. This means that a patient may be told that they have COVID-19, but they actually do not. A false negative means that the test shows a negative result, but it should have been a positive result. This means that the patient may be told they don’t have COVID-19, but they actually are infected. 

Why could this happen?

For a variety of factors, this can happen. For one thing, the SARS-CoV-2 coronavirus could have contaminated the sample, leading to a false positive. There's also the human aspect: depending on how much is automated, a human needs to process the sample to varying levels of participation, and so the process is vulnerable to human error.  But let’s assume that hasn’t happened. Regarding the tests, it comes down to sensitivity and specificity. And this depends on how the test works.

When we speak about the accuracy of a laboratory test, such as the one used to diagnose COVID-19 infection, we are basically talking about two different kinds of precision: "sensitivity," which is the ability of the test to tell us when a person is infected, and "specificity," its ability to tell us when a person is not infected. A rather sensitive test is less likely to yield false-negative results, and a highly precise test is less likely to yield false positives.

A molecular test is a test used to diagnose COVID-19. This means that it operates by detecting SARS-CoV-2, the virus that causes COVID-19, from genetic material. This genetic material can be detected in the nose and upper throat when someone is sick. The test utilizes a sample that is obtained by inserting a long swab through the nostril into the back of the nasal passage. It is difficult to confuse the genetic material from SARS-CoV-2 with the genetic material from other viruses, so the COVID-19 test is highly precise. This implies that it almost never produces a false positive. You may be very sure that you are infected with this virus if you are screened for COVID-19, and the test comes back positive. And, no, no positive test will be caused by a prior flu shot.

The test is not, sadly, equally sensitive. If the selection of the specimen is not done perfectly, or if the patient is in an early stage of infection or has already recovered partially from the disease, the sample of the nasal swab may not contain enough viral material to return positive. There are many stories about patients who, shortly after their symptoms started, tested negative, only to test positive on a test done later. A 34-year-old man who tested negative four times before eventually testing positive five days after being admitted to a hospital is portrayed in one case study.

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