Making a fist of blood taking

Researchers noticed that if patients having blood taken were asked to clench and relax their fist during the procedure (a common practice in blood taking techniques), there was a significant increase in the likelihood of the test showing an apparently raised potassium level. This can be an indicator of quite serious conditions, such as kidney or heart problems. However, the researchers found that if they asked the patient to give another blood sample, this time without their contracting and relaxing the fist, the sample indicated a normal level of potassium.

The findings were reported by Vanessa Thurlow, consultant clinical scientist in biochemistry, and Dr Ian R Bailey, consultant chemical pathologist, in a paper titled: Is suboptimal phlebotomy technique impacting on potassium results for primary care?, Annals of Clinical Biochemistry.