Accurate Blood Sugar Readings Are at Your Fingertips

Medically Reviewed by Gary D. Vogin, MD
From the WebMD Archives

July 9, 2001 -- Pricking your finger every day to test your blood for its sugar content is the painful reality of people with diabetes. Recently, relatively pain free devices that take blood from the forearm have become available, but are they accurate?

People with diabetes don't produce enough of or respond appropriately to a hormone called insulin, which is needed to control the level of sugar in the blood. As a result, many diabetics must routinely check their blood sugar levels to make sure they're within a healthy range.

Checking blood sugar levels has, until recently, involved taking a painful finger prick. Now manufacturers are answering the need for a less painful way of testing for sugar in the blood by developing devices that require only a very tiny amount of blood from an alternate site on the body, namely the forearm.

New research, however, has called into question the accuracy of blood sugar testing from the forearm. Author of the study, Theodor Koschinsky, MD, PhD, tells WebMD that during rapid blood sugar changes "clinically relevant differences" occurred in blood sugar readings taken from the forearm and the fingertip. He is from the German Diabetes Research Institute and an associate professor at the University of Dusseldorf in Germany.

Koschinsky and his colleague gave men with diabetes a high sugar breakfast followed by a strong insulin treatment in order to make their blood sugar levels go very high then very low. They used both a finger prick device and a forearm device to check their blood sugar levels at several points during the study.

When the amount of sugar in the blood was rising or dropping rapidly, only the finger prick testing accurately caught these rapid changes. It took about 30 minutes for the forearm values to catch up to those reported by the finger prick tests. This research was presented recently in Philadelphia at the annual meeting of the American Diabetes Association.

C. Kurt Alexander, MD, CDE, FACP, has also performed research on the accuracy of forearm vs. finger prick blood sugar testing for Roche Diagnostics, the makers of blood sugar testing devices. He also found that, "a drop of blood out of your forearm is not [always] the same as a drop of blood out of your fingertip."

In Alexander's research, forearm testing was most likely to be inaccurate during the two hours after eating a meal, and there was no consistency as to whether the forearm reading was lower or higher than the finger prick reading. He is an assistant clinical professor of medicine at Indiana University School of Medicine in Indianapolis.

So, should you throw away your forearm blood testing device? Absolutely not! Both Alexander and Koschinsky agree that they can be used as long as it's not a potential emergency situation. So, you might want to stick to a finger prick device if you're about to drive a long distance or if you feel you are developing low blood sugar, which is a potentially dangerous condition called hypoglycemia. Alexander also says you might also want to use a finger prick device during the two hours after a meal.

Furthermore, Claresa S. Levetan, MD, director of diabetes education at MedStar Health/Washington (D.C.) Hospital Center says that encouraging people with diabetes to take regular readings, which is more likely to happen with a less painful forearm device than a painful finger prick one, is more important than ensuring that every reading is 100% accurate. She explains that the real goal of testing blood sugar is to look at general trends, not detect potentially emergency situations like hypoglycemia.

"If you suspect that you're dramatically low, none of the meters are very good at reading the very low end," she says. So, someone who suspects they have hypoglycemia should eat some food containing sugar to be on the safe side.

No matter what device you choose, the most important thing is to use it properly. Ask your doctor or nurse to show you each step exactly. According to Levetan, "most studies have shown that people who are doing home glucose monitoring don't do everything properly."