PCR test for travel - false positives after recovery?
Discussion
I've also posted this in holidays & travel so apols if anyone reads this twice.
My daughter has just tested positive for Covid.
We have a holiday booked in a month, for which we all need negative PCR tests.
I'm aware that PCR sometimes gives a false positive after recovery when it detects dead virus RNA fragments.
Does anyone know how frequently this happens?
My daughter has just tested positive for Covid.
We have a holiday booked in a month, for which we all need negative PCR tests.
I'm aware that PCR sometimes gives a false positive after recovery when it detects dead virus RNA fragments.
Does anyone know how frequently this happens?
I recieved a positive PCR test when I was due to travel abroad for work and had to self isolate. Also asymptomatic. Because I know of people who have been badly affected (several hospitalisations including one death) at work I was going to take another PCR the day before returning to work but was advised by track and trace not to as it would likely return positive for the same reason you have mentioned. So I reckon it happens a lot. The graph on the form below shows detectable levels after six weeks, but I'm not sure if it's for illustration purposes only.
Not technically a false positive as it either finds the material or it doesn't, but like your daughter I was fine and past the most transmissible stage.
So I can't answer your question, but I'd be pessimistic and check into who's doing the test. I got the impression that the guys I went to were some pop-up clinic who were just throwing swabs through the process all day to cash in. I'm not sure if a more nuanced answer is possible from a better lab or if it's really a case of Computer Says No. My experience of travel bureaucracy leaves me suspecting the latter.
An issue seems to be that positive/negative is a binary outcome, but PCR tests multiply several times (usually 40 cycles). If you've loads of genetic material after 20 cycles you're obviously more infected than if it takes 40, but both are positive. Also if you've waited two weeks and have low levels of genetic material you're probably fine, but that takes context to know that you're getting better and haven't recently caught it, something a snapshot PCR can't do in isolation.
As ever, I'm just a guy on the internet who got booted from a work jolly so no authority implied. Govt advice on PCR testing seems to imply that different labs may have different yes/no thresholds and some are more nuanced than others. Perhaps it is possible to take several PCRs over a period of weeks showing the decline of Covid RNA?
Not technically a false positive as it either finds the material or it doesn't, but like your daughter I was fine and past the most transmissible stage.
So I can't answer your question, but I'd be pessimistic and check into who's doing the test. I got the impression that the guys I went to were some pop-up clinic who were just throwing swabs through the process all day to cash in. I'm not sure if a more nuanced answer is possible from a better lab or if it's really a case of Computer Says No. My experience of travel bureaucracy leaves me suspecting the latter.
An issue seems to be that positive/negative is a binary outcome, but PCR tests multiply several times (usually 40 cycles). If you've loads of genetic material after 20 cycles you're obviously more infected than if it takes 40, but both are positive. Also if you've waited two weeks and have low levels of genetic material you're probably fine, but that takes context to know that you're getting better and haven't recently caught it, something a snapshot PCR can't do in isolation.
As ever, I'm just a guy on the internet who got booted from a work jolly so no authority implied. Govt advice on PCR testing seems to imply that different labs may have different yes/no thresholds and some are more nuanced than others. Perhaps it is possible to take several PCRs over a period of weeks showing the decline of Covid RNA?
gov said:
A single Ct value in the absence of clinical context cannot be relied upon for decision
making about a person’s infectivity.
Ct values cannot be directly compared between assays of different types – not all
laboratories use the same assay, and some may use more than one.
The clinical significance of positive results with high Ct are difficult to interpret in the
absence of clinical history and context. Positive results with low viral load (high Ct) can
be seen in the early stages of infection (before the person becomes capable of
transmission of the infection) or late in infection when the risk of transmission is low
(periods indicated by the dotted red line). Thresholds (Ct cutoffs) are established to
ensure specific results, but in some assays an indeterminate zone is established which
may require a follow up sample to determine the significance of the result. This will be
the case in laboratories that do not have clinical information to assist guidance on
interpretation, and when repeat analyses may not be possible
Understanding cycle threshold (Ct) in SARS-CoV-2 RT-PCRmaking about a person’s infectivity.
Ct values cannot be directly compared between assays of different types – not all
laboratories use the same assay, and some may use more than one.
The clinical significance of positive results with high Ct are difficult to interpret in the
absence of clinical history and context. Positive results with low viral load (high Ct) can
be seen in the early stages of infection (before the person becomes capable of
transmission of the infection) or late in infection when the risk of transmission is low
(periods indicated by the dotted red line). Thresholds (Ct cutoffs) are established to
ensure specific results, but in some assays an indeterminate zone is established which
may require a follow up sample to determine the significance of the result. This will be
the case in laboratories that do not have clinical information to assist guidance on
interpretation, and when repeat analyses may not be possible
Edited by glazbagun on Saturday 31st July 00:15
I took a PCR along with 3 others when I was trying to leave India in April, I and a friend both were positive. No symptoms or illness in either of us. The other 2 people who were negative, live with each of us.
As soon as we got the result (6 hrs after the test), we both went to take another which returned negative the next day. In my my case the reading from the first test was 30 cycles. We'd both had some kind of cold/flu around January time but no positive cv case as far as we knew.
As soon as we got the result (6 hrs after the test), we both went to take another which returned negative the next day. In my my case the reading from the first test was 30 cycles. We'd both had some kind of cold/flu around January time but no positive cv case as far as we knew.
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