CDC say wha?
December 28, 2021 6:03 AM   Subscribe

Should I wait until I have a negative result on a rapid antigen test before leaving COVID isolation--even if it's been 10+ days? Everything I am reading says to go by a symptom/time-based strategy for ending isolation. However, it's my understanding that a rapid test is supposed to measure infectiousness and if I'm still testing positive then I must still be contagious... right? Are there any legitimate sources discussing this?

For context:
- Triple vaccinated
- Symptoms started on December 16 + negative PCR
- Positive rapid test on December 19
- Currently no symptoms aside from a little extra phlegm. Never had a fever. Felt like a bad cold
- Day 12 of isolation still testing positive on a rapid
posted by joeyjoejoejr to Health & Fitness (12 answers total) 3 users marked this as a favorite
Rapid test is subject to a bit more both false negatives and false positives, some brands are more prone than others. I'd give it another few days and test again.

If you are no longer showing symptoms, you may want to get the more expensive PCR test / molecular test than the rapid antigen test. PCR test is more sensitive. Though generally, false positive is much rarer than false negative.

posted by kschang at 6:14 AM on December 28, 2021

I am one of a group of 12 family members/partners who took a rapid antigen yesterday, the kind where they just swab your nostrils. 7 out of 12 tested positive. A second round of antigen tests (the kind where they go deep) for some and PCR tests for others came back with 6 out of those 7 people actually being negative.

Anecdotes are not data, but if rapid antigen tests are telling you something this unusual, go with the PCR test to resolve it.

(Sad trombone: I was the only one who tested positive in both rapid antigen tests! I’ll be getting a PCR test today because my trust in the rapid antigen tests has been broken.)
posted by ejs at 6:34 AM on December 28, 2021 [4 favorites]

Definitely get a PCR. Also anecdata but you can get a bad batch of antigen tests (QuickVue in my case). Also, whether it was COVID or a cold, sounds like you're on the mend.
posted by kingdead at 6:54 AM on December 28, 2021

I had a false positive on a rapid test (negative on the PCR, and was told that the PCR trumped the lateral flow though I did end up isolating just in case.)

The only explanation given was that the rapid test picked up on my previous Covid infection (which had been confirmed by PCR) a few months before but the PCR could tell whether there was an actual live infection.

So, yes, would get a PCR.
posted by heavenknows at 7:13 AM on December 28, 2021

Response by poster: Trying not to threadsit... I am unable to get a PCR because I already have/had COVID. Most places in NYC will not retest if you've already had COVID, because it is likely I will continue to shed virus in small amounts for the next 90 days.

I'm almost positive that I have COVID because I contracted COVID along with 8 other people at the same time.
posted by joeyjoejoejr at 7:28 AM on December 28, 2021 [1 favorite]

Best answer: This article mentions that some people who are immunocompromised or have another underlying condition may be infectious longer than usual and have to isolate longer, so it doesn't seem completely impossible that you still might be infectious. Have you talked to your doctor about whether that might be the case? The false positive rate of rapid tests is very, very low (more likely that someone would get a positive from a PCR test but not be infectious because it's picking up bits of old DNA from an infection that has resolved). If you have symptoms, it's even more likely that it's correct.
posted by pinochiette at 7:34 AM on December 28, 2021 [6 favorites]

Yes, you should wait.
posted by rrrrrrrrrt at 8:44 AM on December 28, 2021

Just to add on to pinochiette’s comment: rapid tests can have false positives, but they are quite rare. PCR tests also give false negatives, missing up to 30% of true positive cases. That means that if you get a positive rapid test followed by a negative PCR, there is still a decent chance that you have Covid. The exact chances depend on the prevalence of Covid in your area. The more Covid around, the higher the chances are that you actually do have it.

Long story short, to anybody who is reading this thread: please do not take a negative PCR as permission to let down your guard.
posted by wyzewoman at 10:04 AM on December 28, 2021 [1 favorite]

Best answer: i found this radio segment from WNYC this morning (12-28) of Brian Lehrer interviewing a virologist and taking listener calls to be very helpful to my ongoing questions about isolation and testing.
posted by wowenthusiast at 10:43 AM on December 28, 2021

Best answer: Assuming that you did not have a severe Covid not are you immune compromised, the
CDC says "For patients with mild-to-moderate COVID-19, replication-competent virus has not been recovered after 10 days following symptom onset for most patients.(8, 9, 20- 24) Outliers exist; in one case report, an adult with mild illness provided specimens that yielded replication-competent virus for up to 18 days after symptom onset.(25)"

I don't know why people are telling to get a PCR - it is almost certain to be positive due to traces of COVID that are typically not replication competent but hang on for weeks after the illness. Again from the same CDC page: Studies of patients who were hospitalized and recovered indicate that SARS-CoV-2 RNA can be detected in upper respiratory tract specimens for up to 3 months (12 weeks) after symptom onset.
posted by metahawk at 5:00 PM on December 28, 2021 [4 favorites]

Per NYT today on PCR's: "(Another type of test, known as a P.C.R., is not useful for releasing people from isolation because it can return positive results after someone is no longer contagious.)"
posted by jenfullmoon at 5:38 PM on December 28, 2021

The reason the CDC uses a time-based strategy rather than a test-based strategy is that people can continue to test positive for up to 3 months after diagnosis and not be infectious to others. From the CDC. From UTSouthwestern medical center.
posted by selfmedicating at 5:58 PM on December 28, 2021

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