April 29: MIT Medical answers your COVID-19 questions. Got a question about COVID-19? Send it to us at CovidQ@mit.edu, and we’ll do our best to provide an answer.
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.
When we talk about the accuracy of a laboratory test, like the one used to diagnose infection with COVID-19, we’re actually talking about two different types of accuracy —“sensitivity,” which is the test’s ability to tell us when an individual is infected, and “specificity,” its ability to tell us when an individual is not infected. A test that is very sensitive is less likely to give false-negative results, and a test that is highly specific is less likely to give false positives.
The test used to diagnose COVID-19 is a molecular test. This means that it works by detecting genetic material from SARS-CoV-2, the virus that causes COVID-19. When someone is infected, this genetic material can be found in the nose and upper throat. The test uses a sample that is collected by inserting a long swab into the back of the nasal passage through the nostril. The genetic material from SARS-CoV-2 cannot be confused with the genetic material from other viruses, so the COVID-19 test is highly specific. This means it almost never gives a false positive. If you are tested for COVID-19, and the test comes back positive, you can be very sure that you are infected with this virus. And, no, a previous flu shot will not trigger a positive test.
Unfortunately, the test is not equally sensitive. If the specimen collection is not done perfectly, or if a patient is in an early stage of infection or already partially recovered from the illness, their nasal-swab sample might not contain enough viral material to come back positive. There are many stories about patients who tested negative soon after their symptoms began, only to test positive on a test done later. One case study describes a 34-year-old man who tested negative four times before finally testing positive five days after being admitted to a hospital.
Experts worry that a negative test can give people a sense of false security, leading them to be less careful about social distancing or other safety measures. Some have suggested that anyone who has coronavirus symptoms should assume they are infected, even if they test negative, a concept that is somewhat in line with the CDC’s newly updated diagnostic criteria. “That’s not a bad idea,” says Dr. Shawn Ferullo, MIT Medical’s chief of student health. “Though we don’t know exactly how high the false-negative rate really is, if you have any COVID-19 symptoms at all, you should be taking extreme precautions to make sure you’re not exposing others.”
And whether or not you have symptoms, given what we know about infected but asymptomatic individuals, none of us should assume we’re uninfected and incapable of spreading the infection to others. “That’s the assumption behind asking everyone to wear masks,” Ferullo notes. “We wear a mask and practice social distancing to protect others. Others do the same to protect us.”