I am happy I did PCR with metramex just before going to possibly the world leading expert on diagnostics of a series of parasites.....PCR test missed it...The doctor found it with the very gold standard way of testing....old well known diagnostic method described in the best books, ignored by doctors and labs.
explentaion has been there all the time, in several even old books it is well knan and described that some of these parasites lives only in bowel wall....only in early stage before they digg into bowel wall they are found on such tests....in chronic stage when person really are getting ill they are often missed on such tests.This has been well known from the best books for at least 30 years.
but never be 100% certain...possibly also many of these PCR tests and even antigen tests could be made to detect genetics material (or antigens) based of certain sub-types....but yes some of these parasites have many many known subtypes....but yes I think the best books already have good explanation, they live in bowel wall...to find them one need to look there, not where they not are. You can have the most sensitive test in the world if you use it on sample where there is nothing to be found it will not find it...this even if the parasite can be near by in the bowel wall.
I do not say these labs not can find any parasites, many of these labs can find several of them, I say some gastro paasites they will not find, they should just be used as initial screening, not to say they have ruled out parasites. And if one carefully read the best books it seems like this has been well known for very very long time...but ignored by almost ever medical doctor. Luckily there are still a few exceptional doctors around. They have no secret ticks, they did not necessary come up with any new fancy new method, if doctors just bother reading the books AND following the books...reading is not enough if not turned into practice!