How were you diagnosed?

Bit of background, my daughter (16) randomly passes out. She will get a metallic taste in her mouth, feel extremely tired and is out cold within seconds. It is terrifying. Takes hours for her to recover and during that time she is in and out of consciousness. We have been to emerg and they say everything is fine. ??? Overall she suffers from extreme fatigue, when she exercises it gets worse and she gets super tired and either passes out or is on the verge of passing out. She also cannot sleep and has trouble concentrating. I can see the exhaustion on her.

Referral to a cardiologist was confusing to say the least. They did a bunch of tests, all came back as "normal". Yet he said it is probably something we call POTS. Then he listed the 4 things we need to do and left the room abruptly. Nurse came in and I asked my questions - she said testing doesn't indicate POTS but sometimes it won't (fair enough - I get it).

Sorry this is long, I really appreciate everyone who shares their story as I have learned a lot.

How were you finally properly diagnosed and how do we know it is POTS?