Blood glucose concentrations can provide important information about a patient's metabolic status. Because veterinarians ultimately make clinical decisions regarding a patient's glucose status on the basis of these values, it is important that we validate the methods used for evaluation of blood glucose in veterinary species to ensure appropriate treatment.
Various methods are used to assess blood glucose concentrations, including POC blood glucose meters, color test strips, portable chemistry analyzers, and standard laboratory chemistry analyzers. The technological means by which glucose concentration is determined and the type of sample required vary according to the method used. Treatment of animals with glucose disorders may require serial measurements of blood glucose concentrations and rapidly reported results; therefore, POC devices that require a minimal amount of blood should be ideal for this purpose. The introduction of reliable, inexpensive, and accurate handheld POC blood glucose meters has greatly facilitated the diagnosis and treatment of humans,1 cats, and dogs2 that have abnormalities in blood glucose concentrations. The routine use of such meters has been made possible through studies3–5 in which investigators determined the accuracy of specific devices in the species of interest. Unfortunately, these devices have not been validated in many vertebrate species, including cervids.
Point-of-care blood glucose meters commonly rely on amperometric or colorimetric technologies.6 When an amperometric device is used, a reaction between glucose in the blood sample and glucose oxidase or glucose dehydrogenase produces an anodic current. The meter measures the resulting current and converts it into a glucose concentration value revealed on the unit's liquid crystal display. In this type of device, blood is applied to a test strip and the correct sample volume is automatically drawn into the hollow portion of a test strip where the reaction takes place. Similarly, when a colorimetric device is used to evaluate blood glucose concentration, the correct sample volume is automatically drawn into the hollow portion of a test strip; oxidation of glucose in the blood results in a concentration-dependent color change, which is read by use of reflectance photometry. This information is then converted into a blood glucose concentration value shown on the unit's liquid crystal display.
The purpose of the study reported here was to evaluate agreement of blood glucose concentrations measured in juvenile white-tailed deer (Odocoileus virginianus) by use of 2 POC blood glucose meters and 1 portable chemistry analyzer with values obtained in serum by use of a standard laboratory chemistry analyzer, and to evaluate agreement between results obtained with the 2 POC meters. We hypothesized that there would be good agreement between devices for each of the comparisons performed.
Materials and Methods
Animals—White-tailed deer fawns evaluated at the University of Illinois Wildlife Medical Clinic between July 2–17, 2007, were used in the study. Fawns were approximately 1 to 12 weeks old as estimated via physical characteristics and were considered healthy on the basis of results of gross physical examination and PCV within the reference interval (20% to 45%) established at the Wildlife Medical Clinic. Fawns were bottle-fed a commercially available milk replacera and were offered free-choice hay or sweet feed as appropriate for their developmental stages with additional water provided in a bowl. Fawns were individually housed in standard stainless steel cages or pens. All samples were obtained during the course of routine clinical evaluation.
Sample collection—A 25-gauge needle and 3-mL syringe were used to collect approximately 1.0 mL of blood from a cephalic or jugular vein. A portion of each blood sample was used to determine glucose concentration with the 2 POC meters, and the remainder of the blood sample was immediately divided between 2 microtubesb (1 that contained lithium heparin and 1 with no anticoagulant). Samples in serum separator microtubes were immediately centrifuged at 5,000 × g for 5 minutes. Serum was stored in a freezer overnight at −30°C.
Measurement of glucose concentrations—All devices used to measure blood and serum glucose were calibrated before use according to manufacturers' recommendations. The 2 POC amperometric blood glucose meters used in the study (meters Ac and Bd) were commercially available devices designed for use in human patients. Samples were processed by the attending clinician (SB, MAM, JN, and BH). Immediately after blood collection, glucose concentration was assessed with the 2 POC meters. The order in which blood (approx 1.5 μL/test) was applied to each POC meter was alternated between samples, and each sample was tested 3 times with each meter to determine precision. Heparinized whole blood (0.1 mL) was then processed by use of a portable chemistry analyzere with a comprehensive diagnostic rotor.f Within 24 hours after collection, frozen serum samples were thawed and serum glucose concentrations were determined by use of a standard laboratory chemistry analyzer.g Samples analyzed with the portable chemistry analyzer and standard laboratory chemistry analyzer were tested only once.
Statistical analysis—The distribution of values for glucose concentration in blood and serum were evaluated by use of the Kolmogorov-Smirnov test. A Friedman nonparametric test for repeated measures was used to evaluate precision of the POC blood glucose meters. The Bland-Altman method7 was used to assess agreement between each POC meter and the laboratory analyzer, between the portable analyzer and the laboratory analyzer, and between the 2 POC meters. Bias was defined as the mean difference between the 2 methods. The 95% LOA were defined as the 95% CI of the mean difference between the 2 methods. Standards in human medicine recommend that the accuracy of a POC meter be within 15% of the reference value8; therefore, results for the POC meters were considered to be in good agreement with results for the laboratory analyzer when the 95% LOA of the POC meter were within 15% of the laboratory analyzer values. The same standard was used when comparing the POC meters with each other and when comparing the standard laboratory chemistry analyzer with the portable chemistry analyzer. Statistical analysis was performed by use of commercially available statistical software.h,i
Results
Blood samples collected from 14 healthy white-tailed deer fawns were used in the study. Two samples could not be run on the portable chemistry analyzer or laboratory analyzer because of insufficient volume. The median PCV of samples (n = 14) was 37.1% (range, 24% to 44%).
The data were normally distributed, and mean ± SD glucose values were as follows: POC blood glucose meter A, 92.6 ± 35.9 mg/dL (range, 44 to 200 mg/dL); POC blood glucose meter B, 125.9 ± 45.2 mg/dL (range, 56 to 235 mg/dL); portable chemistry analyzer, 98.4 ± 29.0 mg/dL (range, 56 to 150 mg/dL); and laboratory chemistry analyzer, 97.3 ± 29.0 mg/dL (range, 56 to 150 mg/dL). Results of precision evaluation revealed no significant within-sample differences in blood glucose for POC meter A (P = 0.6) or POC meter B (P = 0.5).
Agreement was good between the laboratory chemistry analyzer and portable chemistry analyzer (bias, −1.6 mg/dL; 95% LOA, −15.3 to 12.1 mg/dL; Figure 1). Agreement was poor between the laboratory analyzer and POC blood glucose meter A (bias, 2.9 mg/dL; 95% LOA, −70.2 to 76.0 mg/dL; Figure 2) and between the laboratory analyzer and POC blood glucose meter B (bias, −30.8 mg/dL; 95% LOA, −111.6 to 49.9 mg/dL; Figure 3). Agreement between the 2 POC meters was also poor (bias, 31.0 mg/dL; 95% LOA, −47.2 to 109.2 mg/dL; Figure 4).
Bland-Altman plot of the differences between glucose concentrations measured in 12 white-tailed deer (Odocoileus virginianus) fawns by use of 2 devices (a standard laboratory chemistry analyzerg and a portable chemistry analyzere) versus the mean of values obtained with the 2 devices. Laboratory analyzer values were generated from serum samples and portable analyzer values were generated from whole blood samples. Horizontal dotted lines represent 95% LOA; the solid line represents mean difference between the 2 devices (ie, bias).
Citation: Journal of the American Veterinary Medical Association 240, 5; 10.2460/javma.240.5.596
Bland-Altman plot of the differences between glucose concentrations measured in 12 white-tailed deer fawns by use of 2 devices (a laboratory chemistry analyzerg and a POC blood glucose meter [meter Ac]) versus the mean of values obtained with the 2 devices. Laboratory analyzer values were generated from serum samples, and POC meter values were generated from whole blood samples. See Figure 1 for remainder of key.
Citation: Journal of the American Veterinary Medical Association 240, 5; 10.2460/javma.240.5.596
Bland-Altman plot of the differences between glucose concentrations measured in 12 white-tailed deer fawns by use of 2 devices (a laboratory chemistry analyzerg and a POC blood glucose meter [meter Bd]) versus the mean of values obtained with the 2 devices. Laboratory analyzer values were generated from serum samples, and POC meter values were generated from whole blood samples. See Figure 1 for remainder of key.
Citation: Journal of the American Veterinary Medical Association 240, 5; 10.2460/javma.240.5.596
Bland-Altman plot of the differences between blood glucose concentrations measured in 14 white-tailed deer fawns by use of 2 POC blood glucose meters (meter Ac and meter Bd) versus the mean of values obtained with the 2 devices. See Figure 1 for remainder of key.
Citation: Journal of the American Veterinary Medical Association 240, 5; 10.2460/javma.240.5.596
Discussion
In the present study, agreement of glucose values between the standard laboratory chemistry analyzer and portable chemistry analyzer was good, whereas that between the laboratory analyzer and each of the 2 POC blood glucose meters was poor, suggesting that the POC meters do not provide an accurate means for measurement of blood glucose concentrations in juvenile white-tailed deer. Surprisingly, agreement between the 2 POC meters was also poor. Considering the similar technologies by which the 2 POC meters operate, the low degree of agreement in the measurement of blood glucose between instruments also suggests that accuracy may be an issue. Because there were no significant within-sample differences detected for the POC meters, precision for each was considered adequate. However, precision should not be considered to compensate for poor agreement because blood glucose sample interpretation would be biased.
In a previous study,9 readings from POC blood glucose meters were reported to have a negative bias when these devices were used to evaluate human blood samples with high (58.3%) Hct values. Although highly concentrated samples could have influenced results of the present study, PCV values of fawns were within the published specifications for each meter.
An alternative explanation for lack of agreement between the laboratory analyzer and POC blood glucose meters used in this study is a potential for inaccuracy at low blood glucose concentrations, as has been demonstrated in some POC meters.10 In dogs, this inaccuracy resulted in lower-than-expected measurements.3 None of the samples in the present study had glucose values < 56 mg/dL as determined by use of the laboratory analyzer. If low blood glucose concentrations contributed to POC meter inaccuracy in our study, it is likely that lower-than-expected values would have been consistently reported for a subset of samples within the lower glycemic range. However, although the number of samples was too small to allow for analysis of this variable, inaccurate results yielded by POC meters in the present study did not appear to follow any consistent patterns.
Studies have shown that POC blood glucose meters can consistently underestimate blood glucose concentrations in some nonhuman species.6,11 Results of a study11 in which investigators assessed the use of POC meters in seabirds found that these meters underestimated blood glucose concentrations by approximately 33%, compared with laboratory analyzer values. In the present study, POC meter values were both higher and lower than those determined by use of the laboratory analyzer.
Although the cause of poor agreement between the POC blood glucose meters and laboratory analyzer is unclear, a potential weakness of the present study is that both POC meters used determined blood glucose concentrations via amperometric methods. It is possible that a reflectance photometry unit would have produced results that were in better agreement with the laboratory analyzer values. Further studies are needed to evaluate the use of reflectance photometry units for measurement of blood glucose in white-tailed deer.
Results of the present study suggest that the POC blood glucose meters evaluated are not appropriate tools for assessment of blood glucose concentrations in juvenile white-tailed deer. However, the portable chemistry analyzer used in the present study provided results that were in good agreement with laboratory analyzer values.
ABBREVIATIONS
| LOA | Limits of agreement |
| POC | Point of care |
Fox Valley Deer Milk Replacer, Fox Valley Animal Nutrition Inc, Lake Zurich, Ill.
Becton-Dickinson, Franklin Lakes, NJ.
Albertsons IQ Prestige Smart System Handheld Glucometer, HDI Home Diagnostics Inc, Fort Lauderdale, Fla.
Walgreens Prestige Smart System Glucometer, HDI Home Diagnostics Inc, Fort Lauderdale, Fla.
VetScan Classic, Abaxis, Union City, Calif.
Comprehensive Diagnostic Profile, Abaxis, Union City, Calif.
Hitachi 917 chemistry analyzer, Roche, Indianapolis, Ind.
MedCalc, version 9.3.9.0, MedCalc Software, Mariakerke, Belgium.
GraphPad Prism, version 5.0 for Macintosh, GraphPad Software Inc, San Diego, Calif.
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