Doctors will likely be familiar with POTS (more so than with EDS or MCAS), but may have not learned correct testing protocols. I (and others I know) have been incorrectly tested on multiple occasions. According to experts like Dr. Peter Rowe, here’s how it should go:
Lay down for a few minutes to get your body regulated. Then put on a blood pressure cuff and record your BP and HR while horizontal.
Stand up and record your new BP and HR. **It is normal for these numbers to change as your body finds equilibrium from the sudden change.
Continue standing up for 10 minutes. Do not wiggle your legs, slouch, lean against a counter, etc. If you have POTS, you will likely feel a desperate need to do one of these things. Record your symptoms and your need to do these actions, but do your best to stand upright and still. Record your BP/HR every minute or two for the 10 minutes.
If your HR went up by more than 30 bpm while standing (or 40 for kids), you have POTS. Your BP changes and symptoms can help give information of what’s happening for you, likely treatment options, and can help figure out what type of POTS you may have, but is not important for the diagnosis of POTS.
If your HR did not go up 30, that doesn’t necessarily mean you don’t have POTS. I tested myself for fun every day for awhile. I only tested positively about half the time. My results were much more significant when I was feeling more symptomatic. So, if you can, test yourself when you feel horrible, or when having frequent head-rushes. Feel free to buy a BP cuff to use at home and then bring in the data you record to show your doctor.