I have blood taken every three months. The techs nearly always complain that they cannot find a vein. I am always told to clench my fist.
But not only is this bad practice which "has been known since the 1960s" - but have you tried talking to a tech at a commercial lab? All the labs in this region are operated by the same company so there is literally no choice in the matter, though I have to admit that in recent months they seem to have increased their staffing. There were not long line ups on the last two occasions - unlike the long established practice hitherto.
I have never been told of high potassium levels.