How long (after infection) does it take for COVID to show up on a test?
November 12, 2020 12:44 PM   Subscribe

How long (after infection) does it take for COVID to show up on a test?

Getting ready to visit my dad, wanting to be safe.

Let's say (hypothetically) I get exposed to COVID on a Monday, and the virus sets up shop in my body and I become infected. How long will it take for the virus to show up on a test?

I'm not super familiar with the different types of test that are out there. I know there's the PCR test that my doctor can give me, and there's the rapid (but less-accurate?) test I can get at the urgent care clinic. And I know the PCR test takes 3-4 days to come back from the lab, but the one at the urgent care clinic comes back in 15 mins. But that's about all I know.
posted by panama joe to Health & Fitness (4 answers total) 3 users marked this as a favorite
 
From the CDC:

The rate of false negative nucleic acid tests, a type of viral test, after exposure have been reported as:
day 1: 100%;
day 4: 67%;
day 5: 38%;
day 8: 20%;
day 9: 21%;
and then rising to 66% on day 21.

See: https://www.acpjournals.org/doi/full/10.7326/M20-1495

This is one of the insidious aspects of the coronavirus. Suppose you were (unknowingly) exposed and get tested 4 days later. A few days later you receive your negative result, but (unbeknownst to you) it's actually a false negative. You visit a relative, believing yourself to be free of the virus, when in fact you're now at day 7 or so and in a highly transmissive phase.

This is not to say that a negative result is meaningless or totally unhelpful, but it can lead to a false sense of security.
posted by jedicus at 1:18 PM on November 12, 2020 [4 favorites]


There's still some uncertainty around this.

Part of the problem is that the tests have a high false negative rate for asymptomatic people, even if you wait "long enough". So if you have no symptoms and test negative, that's not a guarantee that you don't have the virus.

The WHO says [pdf] that some people may start testing positive 1-3 days before symptoms develop, and symptoms generally develop after about 5-6 days though possibly as many as 14 days later (hence the 14-day quarantine period). People generally shed the most virus for the first 3 days after symptoms develop.

So doing the math, that would imply there's no point in getting tested earlier than 2 days after exposure, and it's probably better to wait until 5-7 days after exposure, which is in line with advice I've seen in other articles. But the bottom line is that the test results are not super reliable unless you have symptoms.

It's still helpful for reducing risk, but it's not a guarantee.
posted by mekily at 1:25 PM on November 12, 2020


The incubation period--that is, how long it takes after you are exposed for you to develop the virus(where it is detectable by a test and where you are contagious)-- for the virus is thought to be 14 days, so to be sure you are free of the virus you would have to quarantine completely for two weeks. You cannot "test out of quarantine," since you can develop the virus all the way up to Day 14, even if you previously tested negative on, say, Day 7.

Here is a good explanation.

And an infographic to represent this idea.

The rapid tests are not as sensitive. What types of tests are available will vary by location, but where I am they have the deep nasal swab tests, less deep nasal swabs and saliva tests that only became available more recently and are the same sensitivity. Turn around for results will also vary, but I recently had to do a saliva test and got my results the next day.

My non-expert understanding from listening to many hours of the "This Week in Virology" podcast and reading a lot about this is that you are actually most contagious right before you begin to show symptoms, which is very insidious indeed.
posted by ialwayscryatendings at 1:35 PM on November 12, 2020 [1 favorite]


And here is a perfect example of the dangers of relying on testing: multiple people have tested positive on a cruise ship in the Caribbean. Everyone was tested twice before beginning the cruise and therefore everyone felt comfortable not wearing masks.
posted by Bresciabouvier at 6:12 AM on November 13, 2020 [2 favorites]


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