RAT Discussion on Limitations Diagnostic testing is a mixture of art and science, and intepreting results is best left to the doctors. There is a lot of grey area when it comes to test results that most people aren't comfortable with, most people like a pos to be actually pos and a neg to be actually neg, but when dealing with people, medical complexities and biochemistry, life often isn't like that.
There is such a thing as a weak positive that may have been identified by the lab, but it is unlikely that you would get this info. Note too that at the moment there can be up to a 7 day delay between testing and the PCR result being confirmed. PCR can pick up historical cases whereas RATs only pick up active infections.
RATs have been designed to be as robust as possible, and they have been extremely well tested, and Medsafe have gone over all the data. If your people are following the instructions correctly, and reporting the results correctly, that is all the business is responsible for. There honestly is no need to burn through your precious and expensive tests to decide if it meets standard. They are hardly doing that in the Emergency Department, so you don't need to either.
Deciding if this person is a covid case, and needs to isolate or not, is not the job of the business. Best leave that to the health system, they have protocols to work to. Your person above is presumably in the medical system in order to have had three PCR tests already, so that is good, and they will presumably let you know if RAT testing is useful from here.