Though it does not test blood, the accuracy of the GlucoWatch monitor compares well with existing blood glucose meters, researchers have concluded.

Satish K. Garg, MD, evaluated the accuracy of the GlucoWatch monitor in comparison to blood sugar measurements obtained using a HemoCue blood glucose analyzer and a One Touch Profile meter. The results of Garg's study were published in the October issue of Diabetes Care.

Glucose Readings From Interstitial Fluid

Up until now, all home blood glucose testing has relied on extracting a drop of blood from the body. The GlucoWatch monitor uses a process called reverse iontophoresis, which causes interstitial fluid to migrate across the skin's surface. This is the clear fluid just underneath the surface of the skin that surrounds cells. You might see interstitial fluid ooze from a scrape on the skin surface. Both interstitial fluid and blood contain glucose. The GlucoWatch monitor also uses an electro-chemical sensor to obtain glucose readings every 20 minutes.

A total of 28 people with type 1 diabetes were evaluated in a clinical setting and at home. Subjects wore the GlucoWatch monitor on the forearm in an area three inches above the wrist. The subjects wore the monitor 15 hours per day for three consecutive days. Short-acting insulin doses were varied by 10 to 20 percent from normal so that a range of glucose values between 40 and 400 mg/dl could be evaluated. The GlucoWatch monitor performed well at both the high level (240 mg/dl) and the low level (70 mg/dl) as well as all points in between.

People who were pregnant and had skin rashes were excluded from the study. Also, the researchers excluded people taking acetaminophen (Tylenol), citing that it may be a source of "potential interference" for people who tested their blood sugars with the GlucoWatch monitor.

The GlucoWatch monitor measures interstitial glucose sugar values up to three times per hour. Fingerstick samples (using the One Touch Profile meter at home) were collected once per hour.

In this trial, the GlucoWatch monitor did not display glucose readings to the user. They were later downloaded to a computer for analysis. To calibrate the GlucoWatch monitor, a single blood measurement using a conventional blood glucose meter is used. The calibration is used to account for skin thickness among subjects.

The researchers note that rapid temperature changes, excessive sweat, electrical noise, high background currents and open or short circuits can cause the GlucoWatch monitor to "skip" tests. In the home test, 26 percent of all tests were skipped by the device.

Accuracy in Blood Sugar Measurement

Garg reports that the results of the study show a similar accuracy between the GlucoWatch monitor and the blood glucose meters. There is, however, a 20-minute lag between the GlucoWatch monitor and the actual blood glucose value. Garg says these delays are a result of the monitor and its extraction process.

26 Percent of BG Readings Were Skipped

Four of the GlucoWatch monitor devices shut off during the study period: two because of excessive sweating, the others because of computer downloading and blood sugar synchronization problems. According to Garg, "...the glucose in sweat can confound the measurements if the skin conductance [which increases with sweat] exceeds a predetermined threshold." If the measurement for that cycle is skipped, and if six or more skips occur in a sequence, the GlucoWatch monitor is shut off. The study reports that "spurious data is skipped removing aberrant glucose measurements."

Under home-use conditions, 19.7 percent of the points were skipped and 7.2 of the points were removed because of early shut off. Garg points out, "even if half of the glucose measurements are skipped, the patient and the caregiver are likely to receive three to four times the number of glucose values compared to traditional finger sticks." He adds that the large number of glucose values per day should be valuable in adjusting the subjects' insulin dosages and in avoiding extremes of blood glucose values.

Side Effects

According to Garg, mild skin irritation, including itching and tingling, was present in all subjects during the first three hours of operation, "but was not a major deterrent in accepting the GlucoWatch monitor," and usually subsided after three hours. Mild skin edema (accumulation of fluid) and erythema (redness of the skin) were also discovered in all subjects.

There were no more skin reactions after the GlucoWatch monitor was removed.

4 out of 5 Patients Recommend the GlucoWatch

Garg writes, "The increased frequency of glucose measurements possible with the [GlucoWatch monitor] will likely help in recognition of 'glucose trends' and provide a better indication of hypo- or hyperglycemia as compared with the current standard of three to four blood glucose measurements per day."

When asked later, 80 percent of the subjects stated that they would like to have the GlucoWatch monitor as a tool to assist them in the management of their blood sugars.