title: Rise in background current over time in a subcutaneous glucose sensor in the rabbit: relevance to calibration and accuracy.
abstract: In order to calibrate a continuous glucose monitor, accurate determination of the background current (I0) is necessary, in part because I0 could change over time. We compared two methods of I0 measurement: (1), extrapolation of sensor output data (as a function of glucose level) to the intercept at zero glucose and (2) direct measurement of the output of a blank anode with no enzyme coat. We implanted telemetric sensors subcutaneously in rabbits and measured their outputs during tri-level glucose clamps once per week for 5 weeks. The two methods yielded similar results. I0 rose substantially over time and this increase reached significance during week 3 by the direct method but not until week 5 by the extrapolation method. Using the direct method, I0 rose from 3.41 (0.60-8.48 nanoamperes (nA), median and range) during week 1 to 13.42 (9.1-14.3) during week 5. Using the extrapolation method, I0 rose from 0.57 (0-16.7) during week 1 to 15.3 (12.2-21.6) during week 5. We conclude that I0 can rise over time. If this rise went undetected and was assumed to be stable, a one-point calibration procedure would overestimate glycemia in the hypoglycemic range, i.e. fail to appreciate the severity of hypoglycemia. It is recommended that during validation of a chronic glucose sensor, I0 be measured sequentially over time.
authors.affiliation: Legacy Health System, Holladay Park Research Center, Portland, OR 97232, USA. wkward@lhs.org
authors.forename: W K
authors.initials: WK
authors.lastname: Ward
authors.forename: M D
authors.initials: MD
authors.lastname: Wood
authors.forename: J E
authors.initials: JE
authors.lastname: Troupe
mesh_terms: D000818:Animals; D015374:Biosensing Techniques; D002138:Calibration; D004563:Electrochemistry; D005260:Female; D005947:Glucose / Q000032:analysis*; D011817:Rabbits; D013997:Time Factors
chemical_list: D005947:Glucose