Accu Chek Comparison
Jayme Chouinard
Finger prick blood glucose readings and sensor glucose reading won't always match and in fact are likely to be different. That's because sensor glucose readings come from the interstitial fluid (ISF), a thin layer of fluid that surrounds the cells of the tissues below your skin, not from your blood. There is a 5 to 10 minute delay in ISF glucose response to changes in blood glucose.4 Glucose readings on ISF have been proven to reliably reflect glucose levels.
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The FreeStyle Libre system is accurate, stable and consistent over 14 days [1] without the need for fingerprick calibrations.
To assess the accuracy of the FreeStyle Libre sensor, the glucose readings the sensor provides are compared to a known independent reference. In this case the reference is a finger prick blood glucose reading which is taken at the same time as the sensor scan. The comparison between the 2 different readings is plotted on a graph called a Consensus Error grid.
The closer the sensor reading to the reference blood glucose meter reading, the better the accuracy. This is reflected by the Consensus Error Grid by various areas on the graph labelled A to E. The higher the percentage of readings in Zones A and B, the more accurate the sensor is. The consensus Error Grid below shows 99.7% in Zone A and 99.0% in Zone B of the Consensus Error Grid [1].
This website and the information contained herein is intended for outside of the US only.
For any product related information and further details on Abbott Products in Australia please visit www.myfreestyle.com.au. The FreeStyle Libre and FreeStyle Libre 2 Flash Glucose Monitoring System is indicated for measuring interstitial fluid glucose levels in people (aged 4 and older) with insulin-requiring diabetes. The indication for children (age 4 - 17) is limited to those who are supervised by a caregiver who is at least 18 years of age. Always read the label and use as directed.
Abbott Diabetes Care, 666 Doncaster Road, Doncaster VIC 3108 Australia. ABN 95 000 180 389. ADC-29573 V75.0
For more information call Customer Service on 1800 801 478.
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The POCG1 used in the Tauk et al1 study and the POCGb (POCG2) assessed in the study reported here are similar in that they use test strips in which pyrroloquinoline quinone glucose dehydrogenase is the primary reaction enzyme. Pyrroloquinoline quinone glucose dehydrogenase converts glucose to gluconic acid and creates an electric current that is quantified and is proportional to the glucose concentration.9 Pyrroloquinoline quinone glucose dehydrogenase has a high catalytic efficiency and is not affected by changes in blood oxygen concentration, but it is not substrate specific, and in addition to glucose, it also catalyzes the oxidation of allose, 3-O-methyl-glucose, lactose, cellobiose, and maltose.10 Therefore, a modified pyrroloquinoline quinone glucose dehydrogenase that is more substrate specific has been engineered for use in the POCG test strips. The POCG1 evaluated in the Tauk et al1 study uses test strips with unmodified pyrroloquinoline quinone glucose dehydrogenase, whereas the POCG2 assessed in the study reported here uses test strips with a glucose-specific modified pyrroloquinoline quinone glucose dehydrogenase.10
Consequent to the results of the Tauk et al1 study, the protocols for use of the POCG1 with canine and feline blood samples were changed at the University of Pennsylvania Ryan Veterinary Hospital. Specifically, measurement of glucose concentration by the POCG1 was limited to serum samples. Soon after implementation of the new protocol, a new POCG2 was introduced to the hospital, and use of the POCG1 evaluated in the Tauk et al1 study was discontinued.
All study protocols were reviewed and approved by the University of Pennsylvania Institutional Animal Care and Use Committee. Any dog or cat examined at the University of Pennsylvania Ryan Veterinary Hospital that underwent venipuncture was eligible for study enrollment. None of the animals underwent additional venipuncture owing to study enrollment. All blood samples were obtained between September 2016 and December 2018 during a designated research period for one of the investigators (MJL).
A sample size calculation was performed as described.1 Briefly, a 2-sided paired t test was used to determine the number of samples required to detect a difference of at least 15 mg/dL between the glucose concentrations determined by the POCG2 and ABA with a power of 0.8 and type I error rate (α) of 0.05. The calculation was made on the basis of the mean SD serum glucose concentration (117 25 mg/dL) used to establish the reference interval for the ABA. Other assumptions included a correlation of 0.7 and a ratio of 1 between POCG2 and ABA measurements. Results of the calculation indicated that 44 blood samples were required from both dogs and cats; however, samples continued to be obtained from eligible animals until the end of the allocated research time. Additionally, 44 blood samples from hyperglycemic dogs and cats were obtained to ensure that the study had sufficient power to detect a glucose concentration difference of at least 15 mg/dL between the POCG2 and ABA measurements for such animals.
The experimental protocol was performed as described.1 Briefly, for each blood sample obtained, 1 drop of blood was analyzed by the POCG2 immediately after collection, and the remainder of the sample was placed into 2 heparinized microhematocrit tubes,c 2 nonheparinized microhematocrit tubes,d and an evacuated serum separator tube.e Two microhematocrit tubes of each type were filled in case 1 of the 2 was damaged during centrifugation. Immediately after the microhematocrit tubes were filled, they were centrifuged for 3 minutes, after which the PCV was measured and plasma and serum were harvested from the heparinized and nonheparinized microhematocrit tubes, respectively, for measurement of the glucose concentration by the POCG2. Blood samples placed in serum separator tubes were submitted to the on-site clinical pathology laboratory, and serum was harvested from each sample within 15 minutes after blood sample collection for measurement of the glucose concentration by an ABA. Serum samples that were not harvested within 15 minutes after blood sample collection were excluded from the study.
One POCG2b was used for all measurements. Manufacturer-provided control solutions, test strips, and code chips were used for all calibrations and measurements. The POCG2 was calibrated and used in accordance with the manufacturer's directions except that serum, plasma, and venous blood (rather than capillary blood) were used for measurement of glucose concentration. The code chip was changed, and control and calibration tests were performed once monthly and each time a new box of 50 test strips was opened. For each measurement, a test strip was inserted in the POCG2, and 1 drop (approx 0.6 μL) of plasma, serum, or blood was placed on the test strip. The POCG2 provided an automated reading of the measured glucose concentration after 5 seconds. Blood samples for which 1 or more POCG2 glucose concentration measurements were > 600 mg/dL were excluded from analyses. All POCG2 measurements were performed by the same investigator (MJL).
The ABAf measured the glucose concentration in serum by means of hexokinase-mediated conversion of glucose to glucose-6-phosphate that was further oxidized by glucose-6-phosphate dehydrogenase to the reduced form of nicotinamide adenine dinucleotide, which was then quantified spectrophotometrically. The analyzer required 10 μL of serum for analysis and was operated by trained laboratory technologists. Reference intervals established by the clinical pathology laboratory for the ABA were used to categorize samples as hypoglycemic, euglycemic, and hyperglycemic. The serum glucose concentration reference interval was 65 to 112 mg/dL for dogs and 67 to 168 mg/dL for cats. Hypoglycemia was defined as an ABA-measured serum glucose concentration < 65 mg/dL for dogs and < 67 mg/dL for cats. Hyperglycemia was defined as an ABA-measured serum glucose concentration > 112 mg/dL for dogs and > 168 mg/dL for cats. Pronounced hyperglycemia was defined as an ABA-measured serum glucose concentration > 250 mg/dL for both dogs and cats. The serum glucose concentration determined by the ABA was considered the gold standard against which all POCG2 measurements were compared.
Multiple blood samples were analyzed for some animals, and all blood samples included in the study were considered independent observations. Dog and cat data were analyzed separately. For each species, descriptive statistics were calculated to summarize glucose concentrations in serum, plasma, and blood measured by the POCG2 and the serum glucose concentrations measured by the ABA. For each sample, the respective differences between the POCG2-measured glucose concentrations in serum, plasma, and blood and ABA-measured serum glucose concentration were calculated. The data distributions for those differences were then assessed for normality visually and by calculation of the skewness and kurtosis. None of the differences were normally distributed; therefore, the Mann-Whitney test was used to compare median differences among the 3 sample types (serum, plasma, and blood).
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