My two had it, one gave to another. Test came positive for one and negative for another and we went ahead and treated both. The reason being is that the parasites shed every few days so hard to catch when they do as mine went from slightly firmer poop to runny. But I knew something has happened. They even soiled themselves and this never happened. Also, you need to get the poop to your vet and they need to speed up sending to the lab as parasites die within 24hours I think and you may get false negative result. Sadly, you have to go through the vet, at least I did few years ago as the lab would not let me send it myself.
I had to find out about it and tell my vet and be firm with the vet as they will give you useless antibiotics which will do nothing. If you are in the UK, antibiotics will need to be ordered from USA. It's called ronidazole. My cats went on it (has to be done as per weight) and we had a problem pilling one of them so I extended the course when I saw runny poop again. Once they went on it, they got better and never had a problem since. I gave them slippery elm in food after the treatment to help recuperate.
Chances are your vet will not know or be interested in finding out how to deal with it. I had to research myself and tell mine. Also sometimes this gets passed by a mother to kittens and gets triggered by stress so your breeder may not have known anything was wrong as cats can have it but show no symptoms too.
This may help
http://vetmed.tamu.edu/gilab/service/as ... richomonas