From BMJ 2 Feb 19 by J Ian Robertson and M Gary Nicholls
Sometimes neither patient or clinician pay attention to what happens during venepuncture. But clenching the hand before or during the procedure can cause blood potassium levels to rise markedly. This effect is increased if a tourniquet is used.
Therefore if possible it is best to keep the hand relaxed especially if potassium levels are considered critical. Other chemistry measurements that are adversely affected include calcium, aspartate amino transferase, chloride, creatine kinase, magnesium, sodium and phosphate.
These effects are widely unrecognised. Since no one wants a repeat blood test, remember this at your next appointment.