Evaluation of factors affecting CGMS calibration.
BACKGROUND
The optimal number/timing of calibrations entered into the CGMS (Medtronic MiniMed, Northridge, CA) continuous glucose monitoring system have not been previously described.


METHODS
Fifty subjects with Type 1 diabetes mellitus (10-18 years old) were hospitalized in a clinical research center for approximately 24 h on two separate days. CGMS and OneTouch Ultra meter (LifeScan, Milpitas, CA) data were obtained. The CGMS was retrospectively recalibrated using the Ultra data varying the number and timing of calibrations. Resulting CGMS values were compared against laboratory reference values.


RESULTS
There was a modest improvement in accuracy with increasing number of calibrations. The median relative absolute deviation (RAD) was 14%, 15%, 13%, and 13% when using three, four, five, and seven calibration values, respectively (P < 0.001). Corresponding percentages of CGMS-reference pairs meeting the International Organisation for Standardisation criteria were 66%, 67%, 71%, and 72% (P < 0.001). Nighttime accuracy improved when daytime calibrations (pre-lunch and pre-dinner) were removed leaving only two calibrations at 9 p.m. and 6 a.m. (median difference, -2 vs. -9 mg/dL, P < 0.001; median RAD, 12% vs. 15%, P = 0.001). Accuracy was better on visits where the average absolute rate of glucose change at the times of calibration was lower. On visits with average absolute rates <0.5, 0.5 to <1.0, 1.0 to <1.5, and >or=1.5 mg/dL/min, median RAD values were 13% versus 14% versus 17% versus 19%, respectively (P = 0.05).


CONCLUSIONS
Although accuracy is slightly improved with more calibrations, the timing of the calibrations appears more important. Modifying the algorithm to put less weight on daytime calibrations for nighttime values and calibrating during times of relative glucose stability may have greater impact on accuracy.
DirecNet Coordinating Center, Jaeb Center for Health Research, 15310 Amberly Drive, Suite 350, Tampa, FL 33647, USA. direcnet@jaeb.org
Buckingham
Bruce A
BA
Kollman
Craig
C
Beck
Roy
R
Kalajian
Andrea
A
Fiallo-Scharer
Rosanna
R
Tansey
Michael J
MJ
Fox
Larry A
LA
Wilson
Darrell M
DM
Weinzimer
Stuart A
SA
Ruedy
Katrina J
KJ
Tamborlane
William V
WV
D000293:Adolescent; D001786:Blood Glucose / Q000032:analysis*; D002138:Calibration; D002648:Child; D002940:Circadian Rhythm; D003922:Diabetes Mellitus, Type 1 / Q000097:blood*; D005080:Exercise Test; D008401:Gas Chromatography-Mass Spectrometry / Q000379:methods*; D006801:Humans; D007297:Inpatients; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
D001786:Blood Glucose