Many medical experts and public health officials are calling for widespread testing before we open up businesses, schools and other aspects of normal life. But how accurate are these tests?
First, let’s look at the one to diagnose someone with the infection right now – a swab test looking for genetic material of the virus in a person’s cells.
Studies have shown the sensitivity of these tests ranges from 66% to 80%. That means nearly one in three infected people will receive false negative results. They have COVID-19 even though the test says they don’t.
Errors in sample collection are probably the biggest reason for these false negative tests. The tester has to get the swab really far back in the nose to try to catch the virus. One of my patients told me she could feel it in the back of her brain for days after the test. Often, the tester doesn’t get the swab deep enough in the nose.
These false negative are why doctors don’t just rely on the test. If a person presents with symptoms of COVID-19 and potentially has been exposed, doctors will often diagnose a person with the disease is spite of a negative test.
In terms of accuracy of the tests for COVID-19, the second one we’ll look at it is the antibody test, looking for antibodies that fight the coronavirus in a person’s blood.
Antibody tests aren’t ideal to diagnose someone with a current infection. It takes one to two weeks to produce antibodies to a virus. There might not be enough antibodies in the blood to be detected by this test if it’s taken too soon after infection.
But antibody tests can tell someone they’ve had the disease and have protection going forward. Most of the antibody tests have very few false positives – tests that tell someone they are safe when they really aren’t.
There might be a couple of problems with antibody tests. Asymptomatic people and people with mild symptoms might produce fewer antibodies than sicker people, causing tests that are negative even though they’ve had COVID-19.
Plus, it’s possible that these antibody tests may cross-react with other coronaviruses. Even the common cold is caused by a coronavirus. Do people with antibodies to that and other coronaviruses falsely test positive for COVID-19? We don’t know yet.
These tests aren’t perfect, but right now they’re better than nothing. But the accuracy of them is worth considering as we push for widespread testing for COVID-19.
