The US Food and Drug Administration (FDA) has granted marketing approval to Cygnus' GlucoWatch Biographer, a non-invasive blood sugar monitoring device designed to be worn like a wristwatch by diabetic patients aged 18 and older.

The device, which works by extracting fluid through the skin using tiny electric currents, is "one of the first steps" on the road to technology that could eventually replace the traditional finger-stick method of monitoring blood sugar, or glucose.

For now, the GlucoWatch is intended for use only in combination with finger-stick blood testing because of the device's "potential for error." The FDA noted that the GlucoWatch results differed by more than 30% from those yielded by finger-stick tests up to 25% of the time.

Moreover, the wristwatch device is less reliable at detecting very low blood sugar levels than very high levels and will not work at all if the user's arm is "too sweaty," the agency cautioned.

For these reasons, physicians and patients should never base changes in insulin doses on a single GlucoWatch reading, the FDA advised. "Instead, the GlucoWatch results should be interpreted with several sequential readings over time and then confirmed with a finger-prick test."

High blood sugar levels -- a complication of diabetes -- can lead to nerve damage, kidney disease and blindness. But regular monitoring and regulating of blood sugar levels with insulin injections and other treatments can lower these risks.

The agency noted that the wristwatch device caused mild to moderate skin irritation in about half of patients tested and has not been studied in children.

The GlucoWatch measures blood sugar levels in the blood every 20 minutes for 12 hours "even during sleep," the FDA said. An alarm is triggered if the patient's blood sugar levels exceed acceptable levels.

Washington, D.C, FDA Report March 22, 2001