When you get tested for COVID, how sure can you be that the test results are accurate? The answer, unfortunately, may be more complicated than you think.
The accuracy of a COVID test depends on a number of factors, including the type of test, where and how the specimen was collected, and how long you’ve been sick before being tested.
There are two types of test that detect active infections: PCR tests and Rapid (Antigen) tests. PCR tests have to go to a lab, whereas rapid tests are usually done on-site and offer quicker results. But the two tests are very different - PCR tests are significantly more accurate.
Experts talk about two parts of testing accuracy: sensitivity and selectivity. Both types of tests for active infection have high selectivity, but they don’t have the same sensitivity.
Sensitivity is basically the likelihood of a false negative result. Good PCR tests have about a 98% sensitivity, meaning about 98% of true positives are detected. That means that about 2% of those are false-negatives. That’s pretty good - but still means that if 100 people with COVID go get tested, 2 of them will be told they don’t have COVID when they actually do.
Antigen or rapid tests are a different story - they are much faster, but have much less sensitivity. Some rapid tests have anywhere from 50%-90 sensitivity, which means if those same 100 people with COVID got that rapid test, anywhere from 10 to 50 of them will get false-negative results. Yikes!
Selectivity is basically the likelihood of a false positive result. Both antigen and PCR tests have very high selectivity - so any positive result is almost certainly a true positive. There are some extremely rare cases of false positives, of course (the governor of Ohio is one that was in the news recently), but it’s much, much less than 1% for nearly all types of tests, regardless of type.
When you’re tested matters. If you get tested on the day after you were exposed, you may still test negative, even though you’re already incubating the virus. Three, five, or seven days later you might finally test positive. A good example of this was the White House Press Secretary. She tested negative for four days after her exposure - until she tested positive on the fifth day.
The average incubation period for the virus before symptoms show up is five to eight days after exposure. It may take a few days for your viral levels to become high enough to show up on a positive test.
Self-collected swabs often have higher error rates - either because someone didn’t get the swab far enough up their nostrils, or they didn’t put it in the collection tube correctly, etc.
Nasal swabs work best, but sometimes patients can’t be swabbed there for one reason or another, so medical professionals will take throat swabs or saliva samples, which vary in accuracy.
Antibody tests, also known as serology tests, aren’t testing for active infection. Instead, they tell you if you’ve developed some sort of immune response to COVID. If you test positive for antibodies to the coronavirus, you probably had it in the recent past.
This is one to keep an eye out for because “antibody” and “antigen” tests sound similar, even though they’re very different.
There are some instances of false positives (especially in areas with low infection rates) and false negatives (especially when tested too soon after infection). One important thing to note is that we don’t know for sure how long antibodies last, or that having no antibodies means you were never infected.
All in all, the results of an antibody test generally have no impact on whether it’s safe for someone to work. Positive or negative, they can’t tell us whether you have an active infection and shouldn’t be used to indicate that you have immunity, because we just don’t know enough to know if that’s true.
If you get a positive PCR or rapid test result, you almost certainly do have an active COVID infection.
If you get a negative COVID test result, it doesn’t mean that you are definitely COVID-free... If it’s a PCR test, it means you probably didn’t have COVID at the moment that you were swabbed. If it’s a rapid test, it doesn’t mean much of anything at all, unfortunately.
And if you get a positive antibody result, you probably had COVID at some point, but it won't tell you when or whether you still have protection from reinfection, so it doesn't mean much in terms of your actions.
So, take your test results with a grain of salt. If you have symptoms, assume it's COVID and self-isolate accordingly. Everyone thinks it's "just a cold"... until their positive test results come in.