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I'm planning to travel to the USA and I feel concerned about the false positive rate of antigen tests for Covid-19. Unfortunately, there aren't many places that offer PCR tests with quick enough results to work with the current 24 hour test window mandated by the USA. So, say I get an antigen Covid-19 test and it comes out positive despite that I have no symptoms, and then I get another (either antigen or PCR) that comes out negative, can I use the negative one at the airport to prove that I'm not infected, or am I "tainted" and unable to travel there until some time has passed?

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    If you get a positive test result you should self isolate (and so should your housemates if the country you are in requires it) so you should not go out to get a new test done.
    – Willeke
    Dec 11, 2021 at 14:48
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    @Willeke I disagree. In many countries, if you have a positive rapid test, it is required to get a PCR test as soon as possible, to confirm or deny the first test (as rapid tests have a higher rate of false results). I think it is also required to self-isolate until this 2nd test results are out (and of course continue isolation if the 2nd is positive as well). Dec 12, 2021 at 0:27
  • Comments are not for extended discussion; this conversation has been moved to chat.
    – JonathanReez
    Dec 13, 2021 at 19:16

5 Answers 5

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You didn't ask whether you should do this, you asked whether you could. This is a question answered by simply looking at the actual CDC order:

this Notice and Amended Order prohibits the boarding of any passenger [..] unless the passenger:

Presents paper or digital documentation of one of the following requirements:

(i) A negative pre-departure viral test result for SARS-CoV-2 conducted on a specimen collected no more than 1 calendar day before the flight’s departure from a foreign country (Qualifying Test)

The order goes on to discuss past positive tests as a way to show recovery, but there is no mention of a positive test superseding a negative test taken in the last 24 hours.

According to the order, you need only to produce a negative test taken in the previous 24 hours. If you can do that, you can board your flight.

Use your judgement, as always.

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  • Perhaps you could include something on the comment by Willeke. Will it be illegal for the OP to go get tested with a PCR test once the OP has tested positive and is possibly required by law to self isolate?
    – Kvothe
    Dec 11, 2021 at 23:39
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    @Kvothe That's a matter of local law; I have no idea what restrictions the OP's country is placing on movement (which would probably be some flavor of unconstitutional in the US).
    – Undo
    Dec 12, 2021 at 0:12
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    This answer looks at the US rules, but misses the law in the country the OP is coming from. I've addressed this side of it in my answer. I have some wider issues with this answer, but that probably wouldn't lead to a helpful discussion.
    – djr
    Dec 12, 2021 at 14:49
  • @Kvothe As said, this depends on the location. But generally, laws are like this: if a test is positive, the person goes into quarantine for period X, or until Y number of negative test results are obtained over the period of Z days. This logic would be flawlessly followed here.
    – Neinstein
    Dec 13, 2021 at 12:23
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There's a saying

A person with one watch always knows what time it is.

A person with two watches never knows.

The basis of this saying is that if you have two watches you don't know which one is correct.

In your case you posit that if the first test shows a positive and the second test a negative, then you should go with the negative test. The trouble with this logic is that you don't know which test gave a false result. Was the first test a false positive? or was the second test a false negative?

Ideally if you tested positive with one test, I'd want to see multiple, consistent negative tests before assuming that the first test was a false positive.

Pragmatically I would treat a positive test as meaning you are infected and that you should self isolate, no matter what a later test says. While it may screw up your travel plans, it does the most to protect your fellow travelers against potentially being infected by you.

As an example of your logic, (and not making a political judgement), prior to the (1st?) US presidential debate for the election, Trump apparently tested positive and then on a subsequent test, tested negative. The negative test was the assumed state, yet a short time later he was in hospital getting treated for Covid.

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  • Comments are not for extended discussion; this conversation has been moved to chat.
    – JonathanReez
    Dec 13, 2021 at 19:16
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This may depend on the country you are coming from, but in general no. I've written the answer with the example of the UK, but similar rules apply elsewhere.

If you test positive on a pre-departure test, you must follow the required next steps for someone who tests positive in the country you are in. In the UK that would mean self-isolating and getting a PCR test for confirmation. If you test negative on the PCR test you can stop self isolating. I am not aware of any country where the appropriate next steps are to take further lateral flow tests until you get lucky.

In many countries, Covid-19 is a notifiable disease, meaning that the organisation which tests you is obliged by law to report positive tests to the local health authorities.

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  • OPs strategy would still be valid as they could get a positive antigen test, then follow up with a negative PCR and still fly.
    – JonathanReez
    Dec 12, 2021 at 16:54
  • @JonathanReez The question just says "another", which I interpreted as "another lateral flow test" rather than "another test, but a PCR one this time". I don't really know whether following up with a PCR test would be OK, but in practical terms getting a result in time would be challenging (which is why the OP wants to take a LFT in the first place).
    – djr
    Dec 12, 2021 at 17:15
  • Locally in the US if I get a free PCR test before noon, I can get a result by midnight. There’s also services which provide a PCR result within 90 minutes for extra payment.
    – JonathanReez
    Dec 12, 2021 at 17:19
  • @djr Yeah, I realise now that in the question it sounds like that I want to take two antigen tests, while my intent was to have the second one be PCR. As JonathanReez suggests, there is a place near where I live that provides results within two hours for PCR tests, but it's expensive and they have mixed reviews. Either way, I'm not sure about changing the question now given that I have received many answers around how it's currently written.
    – Newbyte
    Dec 13, 2021 at 7:39
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    @gerrit. There are many reasons at this time to want/need to travel which are not just for tourism, particularly visiting family or for necessary work/business reasons.
    – Dragonel
    Dec 13, 2021 at 17:29
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I'm planning to travel to the USA and I feel concerned about the false positive rate of antigen tests for Covid-19.

You should feel concerned about the false negative rate, not the false positive rate. It is shocking that any country allows intercontinental travel without a negative PCR test.

False positive — worst that can happen, your travel doesn't occur. Nobody gets hurt.

False negative — worst that can happen, you bring a new variant to another continent and that variant kills a million people. More likely, you infect some fellow passengers or people in the destination country with an already-present variant, and you hospitalise just a handful of people, some of whom might recover without long covid.

Can you travel after a positive test? Maybe. You can do many things that are either illegal or immoral or both.

Should you travel after a positive test? No, most certainly not. Even if your positive test is followed by two negative tests, JohnatanReez's answer illustrates there's still a 1.3% chance you are actually positive, which makes travelling entirely irresponsible. If 400 passengers reason that way, that would mean on average 5-6 infectious passengers boarding a plane. How many are infected after a 10 hour flight is anybody's guess. FFP2 masks are great, if worn continuously and perfectly by all; that condition may not be met on an intercontinental flight, and 10 hours is longer than what those masks are designed for — so it would be wrong to think your infection state is not important as long as you're wearing a mask.

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    "worst that can happen, your travel doesn't occur. Nobody gets hurt." This callously ignores many reasons to travel that we would all consider important. Perhaps the traveller needs an urgent medical procedure only available in another region, perhaps they are needed to unblock a ventilator factory (substitute the current supply crisis of the day), perhaps their child was seriously injured in the destination country. Yes, people can get hurt if travel doesn't occur. We can't make prescriptive assumptions about someone's reason to travel - if not the OP's reason, the next reader's reason.
    – Undo
    Dec 13, 2021 at 18:02
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    @Undo, that's something for OP to know (he didn't tell us his reason to fly) and decide. Nothing in the way his question is formulated makes me feel that there are human lifes at stake if he does not fly. The quote you quoted is not "callous" and ignores nothing; it makes a clear statement about the COVID-related injuries we're talking about here.
    – AnoE
    Dec 14, 2021 at 10:14
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Short answer, no, as per Undo's answer above & CDC regulations. It would also likely be considered misleading to do so, which is expressly prohibited.

Any passenger who fails to comply with the requirements of section 2, “Requirements for Aircraft Passengers,” may be subject to criminal penalties under, inter alia, 42 U.S.C. 271 and 42 CFR 71.2, in conjunction with 18 U.S.C. 3559 and 3571. Willfully giving false or misleading information to the government may result in criminal penalties under, inter alia, 18 U.S.C. 1001.

Some additional notes regarding the practical validity of a positive vs a negative result: Before you start showing symptoms, false negatives are 100% when testing for the virus. Viral load (a strong determinant of transmission risk) reaches its maximal level two days prior to symptom onset. Additionally, 50% of cases are asymptomatic. Even more broadly, a study on false negative rates found:

  • Negative rates were 100% two days prior to symptom onset
  • Negative rates were a minimum of 25% two days after symptom onset (or date of maximal viral load)

It is likely your negative result has a minimum 25% chance of being a false result based on this research.


Sources:

Temporal dynamics of viral load and false negative rate influence the levels of testing necessary to combat COVID-19 spread

Recalibrating SARS-CoV-2 Antigen Rapid Lateral Flow Test Relative Sensitivity from Validation Studies to Absolute Sensitivity for Indicating Individuals Shedding Transmissible Virus

CDC Global Testing Order

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    This would not be "Willfully giving false or misleading information to the government" - this would be "giving the government what they asked for", specifically, a negative test. The government is more than able to write "you must provide all tests taken in the last 24 hours" if they wanted to. Unless you can show something saying withholding positive test results is prohibited, all of this is outside the question.
    – Undo
    Dec 12, 2021 at 3:21
  • "It is likely your negative result has a minimum 25% chance of being a false result based on this research." This conclusion is completely wrong. The 25% (actually 20%?) chance is that your result will be negative if you were actually infected. You cannot make any determination on the chance of actually being infected in the first place based on this information. Also, the chart and the text in the cited source disagree. The text says what you say, but the chart clearly shows 25% at 1 day after becoming symptomic and a minimum of 20% at 2 days after.
    – reirab
    Dec 13, 2021 at 21:59
  • Edit to add: the 25% number was erroneous. Not sure where they got that. Maybe just a typo. The source they cite does specifically say 20%, though with a confidence interval of 12% to 30%. Also, that source was published in mid August 2020, so is probably based on data from very early in the pandemic when a different variant was prevalent and different tests were being used. In other words, likely not representative of what's being used today.
    – reirab
    Dec 13, 2021 at 22:09

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