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Fist clenching 'overestimates blood potassium'

GPs have been warned to ensure phlebotomy is done properly at their practice after a study showed poor technique is landing some patients in hospital unnecessarily.

Asking patients to clench their fist to make finding a vein easier while taking blood can lead to falsely high potassium readings, researchers say.

They found that after a phlebotomy training programme designed to weed out ‘fist-clenching' reduced the proportion of tests breaching the 5.2 mmol/l potassium threshold from 9% beforehand to 6% afterwards.

Fist clenching can lead to ‘serious overestimation' of serum potassium, the researchers concluded.

Study author Dr Ian Bailey, consultant chemical pathologist at the Princess Royal University Hospital in Farnborough, said it was known that patients should avoid clenching their fist when having a blood sample, but a lack of understanding about the implications meant many slipped into bad habits.

‘It's a really important issue because a lot of testing that goes on. The ones who are not genuine will be visited by the emergency doctor or will get sent to A&E,' he said.

Dr Terry McCormack, a GP in Whitby, North Yorkshire and chair of the Primary Care Cardiovascular Society agreed. He said: ‘Some people over-react to a high potassium level and admit people to hospital so anything that would help to reduce unscheduled hospital admissions is important.

‘If you do get a high potassium in a routine test it is important not to panic. Simply repeat the test at the next reasonable opportunity taking these findings into account,' he added.

How technique can affect results

Fist clenched
9% of patients over 5.2 mmol/L
0.9% breached upper telephoning threshold of 5.8 mmol/l

Hand relaxed
6% of patients over 5.2 mmol/L
0.5% breached upper telephoning threshold of 5.8 mmol/l

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