Which of the following glucose methods should not be used during the administration of an oral xylose absorption test?

• A. Glucose oxidase—colorimetric
• B. Glucose oxidase—polarographic
• C. Glucose dehydrogenase
• D. Hexokinase

Answer: (C)

During an oral xylose absorption test, you want a glucose measurement method that won’t be confounded by the presence of xylose given in the test. Glucose dehydrogenase-based assays use cofactors and generally have broader substrate specificity, so they can oxidize other sugars such as xylose as well as glucose. That means xylose present in the sample can produce a signal in a GDH assay and falsely elevate the apparent glucose level, muddling the interpretation of the test.

In contrast, glucose oxidase–based methods and the hexokinase method are more specific for glucose and show minimal interference from xylose. Glucose oxidase primarily targets glucose and doesn’t react efficiently with xylose, and hexokinase is highly specific for glucose, phosphorylating it to glucose-6-phosphate with little to no activity on xylose.

So, the glucose dehydrogenase method should not be used during the administration of an oral xylose absorption test because its broader substrate range can cause erroneous readings in the presence of xylose.