Talking about her analysis of 200,000 blood test results in the Annals of Clinical Biochemistry, biochemistry expert Vanessa Thurlow said that clenching was an outdated practice, and that staff taking blood should not ask patients to do so.
She said that the results on which her study was based belonged to blood tests requested by GPs between 2002 and 2005.
The updated training was offered in September 2003, she said.
According to her, the percentage of tests showing worryingly high potassium levels fell significantly after the change.
Vanessa said that, though the impact of the fist-clenching while giving blood has been known since the '60s, the procedure was being passed on from generation to generation of phlebotomists.
"It seems to be hit or miss whether they get trained to avoid using this procedure," the BBC quoted her as saying.
She believes that the impact of hand-gripping has been underestimated.
"We think that as a result patients might have to have their medication adjusted. We don't know how widespread a problem this is in other parts of the country," she said.
Vanessa, however, accepted the fact that it was very difficult to get blood samples from some patients.
"The pressure on the phlebotomist to obtain some blood somehow, particularly with very nervous patients, can be high. Clenching and relaxing the fist does improve the blood flow and makes veins stand out, making it easier to get a sample," she added.
Jackie Hough, president of the National Association of Phlebotomists (NAP), agreed that clenching the fist could alter the blood test results.
"But best practice advocates that patients don't tightly clench but gently close their hand during needle insertion and that the hand is loosened prior to the collection of blood. Also the tourniquet should not be tightened on the arm for longer than 60 seconds or during the collection of the sample," Hough said.