Go to another location. Be well hydrated, I've had my taken at 19, and myself and a friend had ours done at RC with it unable to read which is above 19 but blood pressure was good, took cialis prior and they said wow we can't get a reading on either of you buys your bp is good so they did it.
I've had bp to high before and they lied and did it anyway, sometimes they are in a crunch and have a goal to reach. Go later in the day, they might be desperate but make sure you are hydrated, makes a big difference.
It's nothing to play with, that's what happens when you run non stop cycles or not even a cycle just mon stop, donate on a regular basis.