Unexpectedly high levels of potassium were found in the blood
People who take blood have been warned not to get the patient to tightly clench their fist during the procedure.
UK researchers who examined 200,000 blood test results found clenching then relaxing the fist could raise potassium levels, potentially confusing results.
Raised potassium levels can indicate kidney or heart problems, the Annals of Clinical Biochemistry study said.
A UK expert said clenching was an outdated practice and staff taking blood should not ask patients to do so.
'Hit or miss'
Vanessa Thurlow, a biochemistry expert and co-author of the report, first noticed a problem when GPs were questioning test results for a small number of patients.
When she retested them, their potassium levels were normal - but they had all started to clench their fists before the test.
It led Ms Thurlow to speak to managers of the phlebotomy - blood-taking - services in her trust, which then instructed staff not to use the clenching method.
Her study looks at 200,000 results from blood tests requested by GPs between 2002 and 2005. The updated training was offered in September 2003.
The percentage of tests showing worryingly high potassium levels fell significantly after the change.
The impact of fist-clenching while giving blood has been known since the 1960s, but she said 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," she said.
She said other factors such as exposure to cooler temperatures have been known to cause false raised-potassium levels, but she believes the impact of this hand-gripping is 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."
Ms Thurlow admitted some patients were very difficult to bleed.
"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."
Jackie Hough, president of the National Association of Phlebotomists (NAP), agreed 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."