We want a test to tell us whether someone has COVID-19, but that test does not exist. Instead, laboratory tests look for the presence of SARS-CoV-2, the virus known to cause COVID-19. A positive test does not necessarily mean that someone has COVID-19 and a negative test does not always mean that someone does not have COVID-19. How can this be? And if this is true, what is all the fuss about testing?
Let me be clear. Testing is the most important tool available to determine who has COVID-19 and who does not. But it is not quite as simple as “positive” equals COVID-19 and “negative” does not. At least not quite. The test must be interpreted, and to do so, a few simple nuances must be considered.
The most reliable test uses PCR methodology, which includes both amplification and detection steps, making it highly sensitive and specific for the detection of virus.
A negative PCR test means that an individual is not currently infected and has not been recently. But a negative PCR test cannot be used to predict the future. In other words, a negative test last week does not prove someone is virus-free today. Instead, a negative test simply tells us the last time a person was known to be negative for virus. The significance of a negative test result diminishes rapidly as the collection time fades into the past.
A positive PCR test means that the person tested is or has been infected by the virus. If the individual is also symptomatic, we can say the person has COVID-19. That patient is also infectious (i.e., can spread the virus to others), and should be considered infectious for at least ten days after the first positive test, or for 24-hours after symptoms resolve, whichever is longer.
Our best understanding is that a person does not need to have COVID-19 to spread the virus to others. Most people test positive five to seven days after exposure, but they can infect others for ten days from time of exposure, longer in exceptional cases. That is why a person is considered infectious and instructed to quarantine for at least ten days after the first positive test, even without symptoms. A PCR test may continue to be positive for weeks to months after infection, making it impossible to know whether asymptomatic people with a first-time positive test are newly infected or were infected in the past. Because we cannot know when the person became infected, the asymptomatic individual should be considered infectious for at least ten days after the first positive test.
Another consequence of the persistence of positive PCR tests after the infection has cleared is that there is no need to require a negative test to prove an individual is no longer infectious. After the appropriate amount of time has passed since the first positive test and/or resolution of symptoms, an asymptomatic individual should be considered virus-free for at least three months.
If you may have been exposed, when should you test? A test for SARS-CoV-2 becomes positive 2 to 14 days after infection, with most patients turning positive five to seven days later. Most authorities suggest testing no earlier than five days after possible exposure, unless you have symptoms earlier. But if you have to wait five days, should you even test at all? Whether you test or not may be a decision that addresses your peace of mind more than anything else. The most important thing you can do if you think you may have been exposed is to self-quarantine for two weeks. That is how you keep others safe and stop the virus spread.
