From a recent newspaper article in the USA Today:
“If you’ve ever had blood drawn from your arm, you know the drill: Tourniquet in place around your upper arm, you clench your fist a few times to get the blood flow going.
That might not be such a good idea, says a new study in the Annals of Clinical Biochemistry, Fist clenching seems to be one of the main reasons for falsely high potassium levels, a situation called pseudohypercalemia.
Fist clenching does improve blood flow and make veins stick out, lead author Vanessa Thurlow told the BBC. But a lot of good that does if it results in a high potassium level that really isn’t, because that finding could mistakenly lead to further tests or treatment for impaired kidney function.
A phlebotomist, someone who draws blood, told the BBC that a better technique is to ask patients to gently close their hand when the needle is being inserted and then open it. Oh, and tourniquet should be loosened once the blood starts flowing.”