Positive PCR test v Negative LFT test
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I work for a disability adult social care organisation. We have been completing whole home testing for a few months now. We have had a few staff test positive on a PCR test but then a negative LFT , I am trying to clarify what we should do in this situation. Is it pos or neg, do they self isolate or not?
There are a number of things that appear to contradict each other e.g. don’t do a PCR test within 90 days of a positive result, concerns regarding the reliability of LFT’s, if anyone has any words of wisdom that makes this clearer than mud?
Thanks
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johnkov Kimfidget65
Posted
PCR is good at identifying small quantities of the virus. If you get a positive PCR result, you should self-isolate as it means that while the amount of the virus is low, technically speaking you're sick and the disease will progress. LFT detects COVID antigen, and is less sensitive than PCR. The advantage of LTF is that it does not require any full laboratory testing, and can be carried out in the field. So yes... it is a trade off. LFT may give a false negative result when the person got infected recently and the virus did not have the time to replicate. PCR can give a false positive result if the swab is somehow contaminated, but if there is no viral RNA to start with, it won't be amplified by the PCR reaction. Positive PCR means that the virus was present in the swab.
From the purely biological point of view, positive PCR means that you should self-isolate, as does a positive LFT result. PCR will detect the virus sooner (but is way more difficult to process), LFT may miss the infection in the early days (but can be done easily).
'Don't do PCR within 90 days of another positive test' - is a total rubbish. Anyone who makes it through no longer has any virus particles in his/her body. Antibodies bind to the virus particles, and 'tag' them so our immune system can recognise them and eliminate the virus. Followng recovery, there are no longer COVID particles in the body, so any PCR result should come back as negative. If a person has two positive results within 90 days, it probably means that either one of the swabs (or both?) was contaminated, or... it was a different strain, and a sample should be forwarded for full sequencing.
The only "positive" result that is a good news (sort of) is the antibody test, but these are not widely available yet (at least not in the UK). They cannot detect active infection, but can confirm with close to 100% accuracy whether someone had COVID in the past, and has a degree of immunity. The result of an antibody test should also be positive following vaccination.