Don’t Dismiss Testing
MEDICAL AND MENTAL HEALTH UPDATE
Today I’ll go over a real patient story (shared with permission) and a review of testing.
My patient is a 38-year-old woman who had close contact to a friend with asymptomatic COVID-19. The two friends were dining outside at a restaurant, unmasked as they ate, seated across from one another at a small cafe table. The next morning, my patient’s friend attributed her nasal congestion and sore throat to allergies, but when she developed body aches that afternoon, she got a rapid antigen test and was diagnosed with COVID-19.
My patient was naturally concerned about the contact. She was well aware that people are most contagious starting up to 48 hours before symptoms develop. And even though they were outside, they were closer than six feet and not wearing masks when they ate. So, like a good kid, she began her quarantine and, three days later, took herself to an urgent care center for a rapid COVID antigen test. The test was negative, and she called me the next day for advice.
We discussed her situation and reviewed some TESTING BASICS:
Recall that there are two tests for the presence of virus in the body:
The gold standard PCR test which detects the presence of viral RNA, the genetic material of coronavirus. (Pictured here is me collecting a patient sample!)
Upsides: more sensitive.
Downsides: more expensive, longer turnaround time, and almost too sensitive, e.g. it can detect “dead” virus for weeks after an infection which can make the window of infectivity—the stretch of time where people can transmit the virus to others—difficult to identify.
The antigen test which detects the presence of viral proteins.
Upsides: rapid turnaround, less expensive, and most importantly the rapid antigen test is quite sensitive for the presence of infectious amounts of virus to help define the window of contagiousness to others.
Downsides: less sensitive overall, particularly in asymptomatic patients.
We discussed the kinetics of the virus. The incubation period, the time between an exposure and the appearance of symptoms, is typically 4-5 days but can be up to 14 days. We discussed that a negative test—PCR or antigen—during the incubation period is expected to be negative. The virus simply doesn’t have enough time to replicate and produce a high enough viral load for either test to turn positive.
The graphic below is poached from a great New England Journal of Medicine article about COVID testing. I highly recommend reading it. As you can see on the upward blue curve of the virus where it is multiplying exponentially, even a highly sensitive test like the PCR cannot detect the presence of the virus in that early phase. The PCR test can, however, detect the virus before the window of contagiousness—and also can remain positive for up to 90 days after an infection, well after a patient is contagious or even sick.