The study found that measurements using forearm-drawn blood often overestimated blood sugar levels when levels were low. Traditional fingersticks and blood sugar readings done on the palm gave more accurate readings when a person had low blood sugar.
Researchers conclude that blood drawn from the forearms should not be used for glucose testing. Experts interviewed by WebMD did not agree, however.
They note that forearm testing gives accurate readings in most clinical situations but probably should be avoided when patients think their blood sugar levels are dropping.
"These findings confirm what we have seen clinically," says Om Ganda, MD, of Harvard University's Joslin Diabetes Center. "We have been telling our patients for a long time now that if they have symptoms (indicating low blood sugar) they should test their finger, not their forearm."
Blood sugar monitoring using the forearm is now common. The products are marketed as less painful alternatives to traditional testing because they can read blood sugar from the forearm instead of having to prick a fingertip.
To compare the accuracy of blood drawn from the forearm to that taken from the fingertip or the palm, Shu Meguro, MD, PhD, and colleagues from Tokyo's Saiseikai Central Hospital gave 10 healthy volunteers insulin injections to lower their blood sugar levels.
The researchers then took blood sugar readings every five minutes for 70 minutes from a fingertip, forearm, and the fleshy part of a palm.
At the lowest point, blood sugar measurements were 37 mg/dL at the fingertip, 38 at the palm, and 50 at the forearm.
Forearm blood sugar readings agreed with fingertip readings only about 25% of the time during hypoglycemia. Palm measurements agreed about 85% of the time.
American Diabetes Association spokesman Nathaniel Clark, MD, says he doesn't believe that the palm will become a popular testing site because, like the fingertips, the fleshy underside of the hand is extremely sensitive to pain.
"I just can't see many people testing there," he says.
Clark is concerned that fewer diabetic people will test their blood sugar levels as regularly as they should if they are told not to use the forearm.
Though he says most diabetic patients can test using forearm blood in most situations, people who have frequent low blood sugar episodes or who don't experience symptoms when their blood sugar drops should stick to blood drawn from their fingertips.
Diabetic patients who think they need to treat their low blood sugar or who want to see if treatment has worked to raise blood sugar levels should probably do the same.
"This study supports a concern that has been out there that the forearm is not as accurate as the finger when it comes to testing," Clark tells WebMD. "But that doesn't mean that it should not be used."