Precision and Accuracy of Two Blood Glucose Meters: Methods

 

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Methods

Study Design

Laboratory sheets of adult patients coming to the phlebotomy area of an ambulatory care endocrinology practice were reviewed to identify patients scheduled for a venipuncture for glucose measurement. After explanation of the study’s purpose and testing procedure, patient demographic information (age, sex, and absence or presence of diabetes) and time elapsed since the patient’s most recent meal were obtained. Informed consent was obtained from all patients. The study was approved by the Albany College of Pharmacy’s institutional review board. Pregnant women and children (under 18 years of age) were excluded from the study.

Venipuncture blood samples were obtained, centrifuged within 15 minutes after being drawn, and analyzed for glucose concentration within two hours of collection. Within 5 minutes after venipuncture, duplicate finger and forearm glucose measurements were taken with the plasma-calibrated FreeStyle Flash and One Touch Ultra monitors using the same forearm and finger side used for the venipuncture. Blood samples from the same lance were used for each set of duplicate tests. A minimum of four lances were performed, with additional sticks allowed for the forearm if an adequate blood sample for a single test was not initially obtained. Forearm samples were obtained from the lateral aspects of the distal forearm, as indicated in the package labeling. The determination of meter sequence was randomly assigned. One investigator performed all measurements in accordance with the manufacturers’ instructions.[22,23] All finger and forearm blood samples were obtained using manufacturer-recommended lancets, lancet devices, and caps. Lancet devices were set at the middepth setting for finger sampling and at the maximum depth setting for forearm sampling. Test strips with identical lot numbers (FreeStyle Flash lot 0416212 and One Touch Ultra lot 2563238) were used for the study. Results of finger and forearm glucose measurements were compared with the laboratory reference value obtained via venipuncture (hexokinase and glucose-6-phosphate dehydrogenase method, Cobas Integra, Roche Diagnostics, Indianapolis, IN). A control solution check was performed on each meter before each day of testing.

Study supplies for the FreeStyle Flash were donated by the manufacturer. The One Touch Ultra meter was donated by the manufacturer; all other study supplies for the One Touch Ultra were purchased at a local pharmacy.

Clinical and Statistical Analyses

Accuracy was assessed by comparing the glucose results of the first finger and forearm stick obtained by the two meters with the laboratory reference value. Two methods were used for this analysis: (1) error-grid analysis[12,24] of meter results compared with venipuncture results and (2) comparison of the percentage of results obtained with each meter falling within 10% of the laboratory value (chi-square analysis).[15]

The precision of the meters was determined by calculating the absolute mean percent differences in glucose values between the first and second finger or forearm test results of patients for whom two glucose values were available (paired t test).

The effect of time since the patient’s most recent meal on meter accuracy was assessed by comparing the average difference in blood glucose values from the forearm and finger samples to the laboratory values at predetermined time intervals (analysis of variance [ANOVA]). The predetermined intervals were 0–2, 2.1–4, 4.1–6, and >6 hours since the previous meal. These time intervals are consistent with those used by other investigators.[25,26]

The success rate of forearm blood sample attainment was evaluated by comparing the number of patients for whom an adequate forearm sample could be obtained with one lance using the FreeStyle Flash versus the One Touch Ultra (chi-square analysis), with the resulting forearm blood glucose value falling in zone A or B of the error-grid analysis (considered clinically accurate) or within 10% of the laboratory reference value. The error grid is divided into five zones of clinical accuracy, indicating the appropriateness of the therapeutic decision based on the results of the SMBG system compared with that made on the basis of the laboratory result.[12,24] The error grid sets the x-axis as the laboratory reference values and the y-axis as the values determined by the blood glucose meter.[12] Values in zones A and B are considered to be clinically accurate; values in zone A do not vary by more than 20% from the laboratory reference values. The difference in zone B is greater than 20% from the reference. While decisions based on zone B results would not be entirely appropriate, they would not lead to serious consequences for the patient. Values in zone C could result in an overcorrection of blood glucose values, resulting in hyperglycemia or hypoglycemia. Zone D represents a dangerous failure to detect and treat errors; the reference values are in the hyperglycemic or hypoglycemic range, while the blood glucose meter displays values that are in the normal range. Zone E is defined as the erroneous treatment zone; glucose values generated by the glucose meter are opposite the reference values. Treatment decisions based on these results would be dangerous.[12,24]

The a priori level of significance for all analyses was set at 0.05.

 
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References

  1. American Diabetes Association. Self-monitoring of blood glucose (Consensus Statement). Diabetes Care. 1994; 17:81–6.
  2. The Diabetes Control and Complications Trial Research Group. The effect of intensive treatment of diabetes on the development and progression of long-term complications in insulin-dependent diabetes mellitus. N Engl J Med. 1993; 329:977–86.
  3. Stades AM, Hoekstra JB, van Den Tweel I et al., for the STABILITY Study Group. Additional lunchtime basal insulin during insulin lispro intensive therapy in a randomized, multicenter, crossover study in adults: a real-life design. Diabetes Care. 2002; 25:712–7.
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  9. Bergenstal RM, Gavin JR III, on behalf of the Global Consensus Conference on Glucose Monitoring Panel. The role of self-monitoring of blood glucose in the care of people with diabetes: report of a Global Consensus Conference. Am J Med. 2005; 118(suppl 9A):1S–6S.
  10. American Diabetes Association. Tests of glycemia in diabetes (position statement). Diabetes Care. 2002; 25(suppl 1):S97–9.
  11. McGarraugh G, Schwartz S, Weinstein R. Glucose measurements using blood extracted from the forearm and finger. www.abbottdiabetescare.ca/download/GlucoseMeasurements.pdf (accessed 2006 May 2).
  12. Harris MI, Cowie CC, Howie LJ. Self-monitoring of blood glucose by adults with diabetes in the United States population. Diabetes Care. 1993; 16:1116–23.
  13. Clarke WL, Cox D, Gonder-Frederick LA et al. Evaluating clinical accuracy of systems for self-monitoring of blood glucose. Diabetes Care. 1987; 10:622–8.
  14. Rheney CC, Kirk JK. Performance of three blood glucose meters. Ann Pharmacother. 2000; 34:317–21.
  15. In-vitro diagnostic test systems—requirements for blood-glucose monitoring systems for self-testing in managing diabetes mellitus. ISO 15197:2003(E). Geneva, Switzerland: International Organization for Standardization; 2003.
  16. Poirier JY, Le Prieur N, Campion L et al. Clinical and statistical evaluation of self-monitoring blood glucose meters. Diabetes Care. 1998; 21:1919–24.
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  18. Solnica B, Naskalski JW, Sieradzki J. Analytical performance of glucometers used for routine glucose self-monitoring of diabetic patients. Clin Chim Acta. 2003; 331:29–35.
  19. Aboezz R, Miller DR. Accuracy of portable glucose monitors. J Am Pharm Assoc. 2005; 45:514–6.
  20. Demers J, Kane MP, Bakst G et al. Accuracy of home blood glucose monitors using forearm blood samples: FreeStyle versus One Touch Ultra. Am J Health-Syst Pharm. 2003; 60:1130–5.
  21. Lucidarme N, Alberti C, Zaccaria I et al. Alternate-site testing is reliable in children and adolescents with type 1 diabetes, except at the forearm for hypoglycemia detection. Diabetes Care. 2005; 28: 710–1.
  22. One Touch Ultra product information. Milpitas, CA: LifeScan Inc.; 2001.
  23. Freestyle Flash product information. Alameda, CA: Abbott Diabetes Care; 2005.
  24. Cox DJ, Gonder-Frederick LA, Kovatchev BP et al. Understanding error grid analysis. Diabetes Care. 1997; 20:911–2.
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  26. Ellison JM, Stegmann JM, Colner SL et al. Rapid changes in postprandial blood glucose produce concentration differences at finger, forearm, and thigh sampling sites. Diabetes Care. 2002; 25:961–4.
  27. Food and Drug Administration. Summary minutes. Meeting of the Clinical Chemistry and Clinical Toxicology Devices Panel. www.fda.gov/ohrms/dockets/ac/01/minutes/3801ml.htm (accessed 2005 Oct 1).
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Table 1. Accuracy of the Tested Glucose Meters

Meter No. Measurements
Finger Sticka Forearm
Error-Grid Analysisb Within 10% of Laboratory Reference Valuec (% CI) Error-Grid Analysisb Within 10% of Laboratory Reference Valuec (% CI)
Zone A Zone B Zone A Zone B
FreeStyle Flash 96 4 72d (63–81) 89e 9e 64f (54–74)
One Touch Ultra 81 19 57 (47–67) 62g 12g 36 (37–60)

a n = 100 for each meter.
bValues in zone A do not vary by >20% from the laboratory reference values. Values in zone B vary by >20% from the laboratory reference values. Values in both zones are considered to be clinically accurate.
cBlood glucose obtained via venipuncture. CI = confidence interval.
d p = 0.027.
e n = 99.
f p = 0.035.
g n = 74.

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Authors and Disclosures

Shannon M. Rivers, Pharm.D., is Assistant Professor; Michael P. Kane, Pharm.D., FCCP, BCPS, is Associate Professor, Department of Pharmacy Practice, Albany College of Pharmacy, Albany, NY; Gary Bakst, M.D., is Attending Physician; Robert S. Busch, M.D., FACE, is Attending Physician, The Endocrine Group, Albany; Robert A. Hamilton, Pharm.D., is Professor, Department of Pharmacy Practice, Albany College of Pharmacy.

Disclosure: Dr. Kane has received grant funding and speaking honoraria from Abbott Diabetes Care. Dr. Busch has received speaking honoraria from Abbott Diabetes Care.

 
 
 
 
 
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