Accuracy Study of the Medtronic Minimed Continuous Glucose

Monitoring System (CGMS) and GlucoWatch® G2TM Biographer (GW2B) in Children with Type 1

Diabetes – A GCRC-based study

Abstract #156

Background: DirecNet

• Diabetes Research in Children Network

• NIH funded collaborative study group

• 5 clinical centers, central laboratory, coordinating center, and representatives from NICHD and NIDDK

• Objective: to critically evaluate the clinical usefulness of current and future glucose sensors in children and adolescents

Goals of Present Study

To determine the accuracy of the CGMS and G2B – During hypoglycemia and hyperglycemia– Over a broad age range in pediatrics– Comparing nighttime to daytime readings– Determine if accuracy changes as sensors

age

Devices used – GW2B

• GlucoWatch 2 - Cygnus• Two hour calibration• Provides a glucose reading

every 10 minutes • Data can be generated for up

to 13 hours• GlucoWatch glucose lags

serum glucose by 17.5 minutes and this lag was adjusted for in assessing accuracy

• For details of study see Poster #432

Devices used - CGMS

• Medtronic Minimed• Functions up to 72 hours• Data from subcutaneous glucose

is stored and retrospectively reviewed

• In 11/02 the sensor manufacturing process was modified– We studied 115 “original” and 25

“modified” sensors

• For details of study see Poster #387

Methods

• All subjects admitted to a GCRC for ≈ 24 hours• Sensors calibrated to a home glucose meter• Serum glucose levels sent to a central lab• Samples every hour during the day, and every ½

hour from 9 PM to 7 AM• Samples every 5 minutes for up to 90 minutes

following insulin induced hypoglycemia• Samples every 5 minutes for 1 hr following meal

induced hyperglycemia

Subject Demographics

• 91 Children and Adolescents

• 51% Female• 43% Pumpers• Mean HbA1c = 7.8%

Ages of Subjects

3 to < 7

7 to < 12

12 to < 18

Young Subject Wearing Devices

Example of data generated from CGMS use

Subject Graph

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6:00 PM 8:24 PM 10:48 PM 1:12 AM 3:36 AM 6:00 AM 8:24 AM 10:48 AM 1:12 PM 3:36 PM 6:00 PM

Glu

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(m

g/d

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Reference CGMS

IV test Meal test

Graphic example of data generated from GlucoWatch 2

Subject Graph

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6:00 PM 8:24 PM 10:48 PM 1:12 AM 3:36 AM 6:00 AM 8:24 AM 10:48 AM 1:12 PM 3:36 PM 6:00 PM

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Reference GW2B

IV test Meal test

Measures of Accuracy

• Pearson’s correlation

• Relative absolute difference (RAD) %

• |(sensor – reference)/reference| x 100%

• ISO criteria (proposed)

• Reference glucose ≤75 mg/dL• Sensor value within ±15 mg/dL

• Reference glucose value >75 mg/dL• Sensor value within ±20%

Overall Accuracy Results

n r Mean RAD

Median RAD

within ISO

GW2B 3,672 0.86 22% 16% 60%

CGMS (original)

5,658 0.77 26% 19% 53%

CGMS (modified)

1,120 0.90 16% 11% 72%

Ultra 2,068 0.97 9% 6% 94%

ISO Criteria: If reference glucose ≤ 75 mg/dL, sensor glucose within ± 15 mg/dL; if reference glucose > 75 mg/dL, sensor glucose within ± 20%.

Factors NOT Impacting Accuracy For Either the GW2B or CGMS

• Age of the Subject

• Time of Day (Nighttime vs. Daytime)

• BMI (body mass index)

• Location of GW2B placement– Upper vs lower arm– Inner vs outer arm

Effect of Glucose Concentration on Accuracy

Assessed by Median RAD%

0%

5%

10%

15%

20%

25%

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40%

0 50 100 150 200 250 300 350 400

Reference Glucose (mg/dL)

Median RAD

GWB

Original CGMS

Modified CGMS

Ultra

GW2B Function By Hours Of Use

Hours < 4 4 to <8 ≥ 8 p

Median RAD

15% 15% 19% 0.002

Sensor age had no effect on CGMS function

Sensitivity and False Positive rate for detection of hypoglycemia (<60 mg/dl)

“Alarm” Setting

(mg/dL)

Sensitivity False Positive Rate

GWB CGMS Original

CGMS Modified

GWB CGMS Original

CGMS

Modified

60 23% 36% 49% 51% 63% 58%

80 59% 67% 84% 67% 68% 64%

100 84% 85% 100% 80% 76% 75%

120 92% 91% 100% 85% 83% 84%

Adverse Events

• GlucoWatch– Minor skin irritation in 29% of GW2B uses– No severe or persistent skin reactions

• CGMS– No significant skin irritation or site infections

Summary

• GW2B accuracy was unaffected by site of wear

• The GW2B was less accurate in the last 4-5 hours of use, whereas CGMS accuracy was unaffected by sensor age

• When compared to the original CGMS, the modified CGMS was more accurate in all measures

Summary

• For both the GW2B and the CGMS:– Subject age had no effect on accuracy– Time of the day had no effect on accuracy– Accuracy was greater for hyperglycemia than

for hypoglycemia– Neither device is as accurate as currently

available meters

• Barbara Davis Center

– H. Peter Chase– Rosanna Fiallo-Scharer– Jennifer Fisher

• University of Iowa– Eva Tsalikian– Michael Tansey– Linda Larson

• Nemours Children’s Clinic– Tim Wysocki– Nelly Mauras– Kristen Gagnon

• Stanford University– Bruce Buckingham– Darrell Wilson– Jennifer Block

• Yale University– William Tamborlane– Stuart Weinzimer– Elizabeth Boland

• University of Minnesota Central Laboratory:– Michael Steffes– Jean Bucksa– Maren  Nowicki

• Jaeb Center for Health Research– Roy Beck– Katrina Ruedy– Craig Kollman– Andrea Booth– Gladys Bernett