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Performance of precision G blood glucose analyzer with a new test strip G2b on neonatal samples.

In newborns, hypoglycemia must be detected in venous specimens with a wide range of hematocrits and oxygen tensions. The need for a simple and rapid test for glucose to meet special applications such as those found in pediatric wards has been suggested frequently (1-4). Simple tests are available, but their use in neonatal units has usually not been recommended, reflecting, at least in part, the influence on results of sample hematocrit and oxygen tension (5, 6).

The Precision G System is a new instrument with a new electrode strip (G2b), which is purported to address many of the shortcomings found in previous generations of biosensors. The bioactive component of Precision G System is glucose oxidase. This is also used in many other glucose measuring systems (7). The component that makes the Precision G System unique is the signal transducer and mediator ferrocene (8).

We performed laboratory and clinical evaluations of the Precision G System with the G2b test strips to determine its suitability for use in a neonatal unit. We compared the performance of the Precision G System with a laboratory method using venous specimens with a wide range of hematocrits taken from patients in a neonatal intensive care unit.

After approval from the Institutional Review Board of the Izmir Social Security Institute (SSK) Tepecik Education Hospital, the instrument was permitted to be used in the neonatal unit of the hospital. After informed consent was received from the parents, 3.5 [micro]L of the 1.5-mL venous blood samples, taken for the necessary routine biochemical analysis during the treatment period of each neonate, were used for studies described in this report. The samples were kept at room temperature in heparinized tubes before use. The samples were applied to the Precision G system within 5 min after they were taken from the neonates. Venous samples from 100 patients had whole-blood glucose measured on the Precision G System and plasma glucose measured on the Hitachi 911 reference system. The hematocrit of each sample was also measured (Bayer Advia 120 Hematology System).

In the Precision G System, glucose reacts with glucose oxidase on the test strip. The chemical reaction releases electrons, which are transferred from the enzyme to the electrodes by [ferricinium.sup.+], the oxidized form of the mediator ferrocene. These electrons form a small current. The electrical current, detected by the electrodes on the test strip, is proportional to the concentration of glucose in the specimen. The analysis time is 20 s.

The Precision G test strip contains three electrodes (reference, working, and background compensation electrodes). The background compensation electrode, containing no glucose oxidase, measures the nonspecific current from potentially interfering substances such as ascorbic acid and urea. This background current is subtracted from the current measured on the working electrode.

The Precision G System starts testing when it detects that a sample has been applied to the test strip. If the test fails to start because of an insufficient sample amount, the user may apply a second drop of blood to the same test strip within 30 s.

Unlike photometric (reflectance) analyzers, blood does not enter the sensor during testing with the Precision G System. Each Precision G2b is sealed in an individual foil packet, which has a barcode on the exterior. The barcode contains lot-specific information, including calibration data. To perform a test, the system reads the barcode on the test strip, thus calibrating the sensor for that specific lot of test strips. The test strip is then inserted into the system's port, which can be extended up to 6 feet from the system. To avoid loss of glucose from glycolysis, fresh venous blood from the neonates must be measured within 15 min (9). In this study, the samples were applied to the target area of the test strip by micropipet after 5 min. This duration of 5 min was necessary for the sample to reach room temperature (25.0 [+ or -] 0.5[degrees]C).

[FIGURE 1 OMITTED]

For comparison, we measured glucose by an automated glucose oxidase method (Hitachi 911). After the measurement of whole-blood glucose on the Precision G system, the samples were centrifuged (1000g for 5 min) to obtain plasma for analysis on the Hitachi 911 system. The time delay between sample collection and analysis on the Hitachi 911 reference system was ~15 min.

The within-run imprecision (CV) of the system was assessed in each of three samples of heparinized venous blood (n = 20) analyzed in a period of ~15 min to avoid effects of glycolysis (9). At mean glucose concentrations of 13.4 mmol/L (2420 mg/L), 4.8 mmol/L (860 mg/L), and 2.4 mmol/L (440 mg/L), the CVs were 2.5%, 3.4%, and 4.5%, respectively.

Patient samples for the comparison study exhibited a wide range of hematocrits (median, 44%; range, 25-60%) and oxygen tensions (median, 60 mmHg; range, 50-70 mmHg). The Precision G and Hitachi 911 results are compared in Fig. 1. When we compared the results obtained with the two methods, 30%, 55%, 76%, and 87% of Precision G results agreed within 5%, 10%, 15%, and 20%, respectively, of the laboratory plasma values.

The oxygen sensitivity of the G2b was investigated with venous whole-blood samples at three glucose concentrations. A 1-mL blood sample was taken from a hospital staff member with informed consent; blood glucose was measured by the Precision G system, and [Po.sub.2] was measured immediately by the Nova Stat Profile 9 Blood Gas Analyzer. The glucose was 2.5 mmol/L (450 mg/L) and [Po.sub.2] 30 mmHg. This [Po.sub.2] was used as the control condition. This sample was divided into three heparinized tubes. Two of these were supplemented to 4 mmol/L (720 mg/L) and 15.3 mmol/L (2750 mg/L) glucose, respectively.

Each of the three new samples was further divided into three heparinized tubes. These tubes were put in a closed chamber and exposed to oxygen for 1, 3, and 5 min, respectively, and the samples were analyzed by the Precision G system and the Nova Stat Profile 9 Blood Gas Analyzer at the same time in triplicate. The [Po.sub.2] did not affect measured glucose (Table 1).

The most commonly observed [Po.sub.2] in neonates is 50-70 mmHg; [Po.sub.2] >150 mmHg can be found only in patients receiving oxygen therapy (10). In our oxygen sensitivity assessment, the oxygen tension was examined in a wide range from 70 to 380 mmHg to verify the applicability of Precision G system.

The data appear to corroborate the manufacturer's claims that this new biosensor test strip represents an advance over previous generations of whole-blood glucose sensors. The effect of sample oxygen tension on this test strip appears negligible, whereas the effect of sample hematocrit is substantially less than with earlier strips (11, 12). These features are advantages of the Precision G System for neonatal use.

B. M. acknowledges a PhD scholarship from the Munir Birsel Foundation-Turkish Technical and Scientific Research Council (TUBITAK). We acknowledge technical support from Abbott Laboratories, Istanbul, Turkey.

References

(1.) Costrini NV, Ganeshappa KP, Whalen GE, Soergel KHA. A simple method for regulating and monitoring circulating glucose levels. J Lab Clin Med 1973; 82:179-82.

(2.) Drury MI, Sweeney EC, UaConaill D. Blood glucose determination by Dextrostix Reflectance Meter System. Br J Med Sci 1972;141:181-4.

(3.) Joffe BI, Settel HC. Comparison of Dextrostix/Reflectance Meter and Autoanalyzer methods of blood glucose determination. S Afr Med J 1971; 45:1200-5.

(4.) Haworth JC, Dilling LA, Van Woert M. Blood glucose determination in infants and children. Am J Dis Child 1972;123:69-72.

(5.) Barreau P, Buttery J. Effect of haematocrit value on the determination of glucose levels by reagent-strip methods. Med J Aust 1987;147:286-8.

(6.) Barreau P, Buttery J. Effect of haematocrit concentration on blood glucose value determined on Glucometer II. Diabetes Care 1988;11: 116- 8.

(7.) Buch-Rasmussen T, Olsen BR, Kulys J, Bechgaard K, Christensen JB, Wang J, Ozsoz MS, Colin F, Garcia O, inventors. Use of benzene derivatives as charge transfer mediators. World patent WO9207263, 1992.

(8.) Cass AEG, Davis G, Francis GD, Hill HAO, Aston JW, Higgins IJ, et al. Ferrocene mediated enzyme electrode for amperometric determination of glucose. Anal Chem 1984;56:667-71.

(9.) Nair KS, Karki SD. Carbohydrate metabolism. In: South CM, Reynard AM, eds. Textbook of pharmacology. Philadelphia: WB Saunders, 1992:741-71.

(10.) Pruden EL, Siggard Anderson O, Tietz NW. Blood gases and pH. In: Burtis CA, Ashwood ER, eds. Tietz textbook of clinical chemistry, 2nd ed. Philadelphia: WB Saunders, 1994:1394.

(11.) Braidwood J, Warnook L. Effect of variation in the hematocrit on whole blood glucose monitors. Can J Med Technol 1990;52:104-6.

(12.) Lewis BD. Laboratory evaluation of the Glucocard[TM] Blood Glucose Test Meter. Clin Chem 1992;38:2093-5.

Burcu Meric, [1] Nazife Kilicaslan, [2] Kagan Kerman, [1] Dilsat Ozkan, [1] Umran Kurun, [3] Nejat Aksu, [3] and Mehmet Ozsoz [1] *

[1] Department of Analytical Chemistry, Faculty of Pharmacy, Ege University, 35100 Bornova, Izmir, Turkey;

[2] Departments of Biochemistry and

[3] Neonatal Unit, Izmir Social Security Institution Tepecik Educational Hospital, 35120 Tepecik, Izmir, Turkey;

* author for correspondence: e-mail ozsozs@pharm.ege.edu.tr
Table 1. Effect of oxygen on Precision G System results. (a)

P[o.sub.2], mmHg Measured glucose, mmol/L

30 (b) 2.5 4.0 15.3
70 2.3 3.8 14.7
250 2.3 3.7 14.3
380 2.2 3.6 13.9
RSD, (c) % 5.4 4.6 4.2

(a) Values are the means of triplicate determinations.

(b) The P[o.sub.2] in the collected samples.

(c) RSD, relative SD.
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Title Annotation:Technical Briefs
Author:Meric, Burcu; Kilicaslan, Nazife; Kerman, Kagan; Ozkan, Dilsat; Kurun, Umran; Aksu, Nejat; Ozsoz, Me
Publication:Clinical Chemistry
Date:Jan 1, 2002
Words:1618
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