title: The effect of rising vs. falling glucose level on amperometric glucose sensor lag and accuracy in Type 1 diabetes.
abstract: BACKGROUND
  Because declining glucose levels should be detected quickly in persons with Type 1 diabetes, a lag between blood glucose and subcutaneous sensor glucose can be problematic. It is unclear whether the magnitude of sensor lag is lower during falling glucose than during rising glucose.
  
  
  METHODS
  Initially, we analysed 95 data segments during which glucose changed and during which very frequent reference blood glucose monitoring was performed. However, to minimize confounding effects of noise and calibration error, we excluded data segments in which there was substantial sensor error. After these exclusions, and combination of data from duplicate sensors, there were 72 analysable data segments (36 for rising glucose, 36 for falling). We measured lag in two ways: (1) the time delay at the vertical mid-point of the glucose change (regression delay); and (2) determination of the optimal time shift required to minimize the difference between glucose sensor signals and blood glucose values drawn concurrently.
  
  
  RESULTS
  Using the regression delay method, the mean sensor lag for rising vs. falling glucose segments was 8.9 min (95%CI 6.1-11.6) vs. 1.5 min (95%CI -2.6 to 5.5, P<0.005). Using the time shift optimization method, results were similar, with a lag that was higher for rising than for falling segments [8.3 (95%CI 5.8-10.7) vs. 1.5 min (95% CI -2.2 to 5.2), P<0.001]. Commensurate with the lag results, sensor accuracy was greater during falling than during rising glucose segments.
  
  
  CONCLUSIONS
  In Type 1 diabetes, when noise and calibration error are minimized to reduce effects that confound delay measurement, subcutaneous glucose sensors demonstrate a shorter lag duration and greater accuracy when glucose is falling than when rising.
authors.affiliation: Legacy Health System, Legacy Research Institute, Portland, OR 97232, USA. wardk@ohsu.edu
authors.forename: W K
authors.initials: WK
authors.lastname: Ward
authors.forename: J M
authors.initials: JM
authors.lastname: Engle
authors.forename: D
authors.initials: D
authors.lastname: Branigan
authors.forename: J
authors.initials: J
authors.lastname: El Youssef
authors.forename: R G
authors.initials: RG
authors.lastname: Massoud
authors.forename: J R
authors.initials: JR
authors.lastname: Castle
mesh_terms: D015374:Biosensing Techniques / Q000295:instrumentation*; D001786:Blood Glucose / Q000378:metabolism*; D003922:Diabetes Mellitus, Type 1 / Q000097:blood*; D005260:Female; D006801:Humans; D007004:Hypoglycemic Agents / Q000008:administration & dosage; D007328:Insulin / Q000008:administration & dosage; D008297:Male; D008875:Middle Aged; D008991:Monitoring, Physiologic / Q000295:instrumentation
chemical_list: D001786:Blood Glucose; D007004:Hypoglycemic Agents; D007328:Insulin