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At the Bedside:
Reducing Errors in Glucose Measurements
Advance For Administration of the Laboratory, August 2000, Vol.9, No.8, Pg.10-11 by Zuping Tang, MD and Gerald Kost, MD, PhD

The potential source of errors in glucose measurement for point-of-care testing have been investigated and this article summarizes the findings:

Oxygen Levels

After evaluating the oxygen effects on glucose meter measurements using both glucose oxidase and glucose dehydrogenase based biosensors, Kost et al demonstrated that glucose oxidase method of glucose testing is sensitive to oxygen. Amperometric and photometric test strips were also tested and again glucose oxidase based amperometric test strips caused a decrease in glucose measurements when blood oxygen tension (PO₂) was increased.

High PO₂ (>100 torr) levels are frequently found in patients on oxygen ventilation and it is imperative that glucose results be valid so physicians can make sound therapeutic decisions. Changes in PO₂ do not affect glucose dehydrogenase based test strips because they do not rely on oxygen for oxidation of glucose. 

Hematocrit Levels

Changes in Hematocrit can affect glucose results. Low Hematocrit (19.1 ±0.7 percent mean ± SD) can result in overestimates and high Hematocrit (58.3 ± 0.7 percent) can result in underestimates of glucose measurements. Manufacturers of glucose meters attempt to minimize hematocrit effects through internal calibration algorithms. The sample matrix and accessibility of glucose in the plasma fraction to reagents on the test strips may be different from conditions during calibration; therefore high and low hematocrits can affect glucose meter results.

Drug Therapy

Ascorbic acid, acetaminophen, dopamine and mannitol can alter glucose measurements. Spurious oxidation with electrochemical test strips or spurious color development with photometric test strips may be the reason glucose measurements are affected by drug interference. Mannitol effects on glucose results may result from the chemistry of the test strip or from an osmotic effect. High doses of these drugs are used in a variety of diseases, hence understanding drug interferences with glucose measurements is important to POCT.

PH Changes

In contrast to PO₂, hematocrit and drugs, it has been found that changes in pH generally do not affect glucose measurements significantly. Dr Kost et al makes the following recommendations to glucose meter users:

• Study limitations of glucose meters and test strip measurements

• Understand changes in blood composition in critically ill patients

• Use oxygen-insensitive test strips in patients undergoing oxygen ventilation and other interventions

• Avoid using hand-held glucose meters in patients receiving high doses of interferant drugs, such as ascorbic acid or acetaminophen.

Clinical Laboratory News, September 2000, Volume 26, Number 9, Pg 19

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