Which substance is primarily tested when conducting a copper reduction test?

The copper reduction test primarily targets monosaccharides and certain disaccharides, with glucose being the primary substance tested in this assay. This test is based on the ability of reducing sugars to convert copper ions in an alkaline solution from their cupric form (Cu^2+) to cuprous form (Cu^1+), resulting in a color change that indicates the presence of sugar.

When glucose is present in the sample being tested, it reduces the copper ions, leading to a color change that can be observed visually or measured spectrophotometrically. The test is often used as a screening tool for diabetes mellitus and to assess glycosuria, which occurs when glucose spills into the urine due to elevated blood sugar levels.

The other substances listed have different testing methods and are not the primary focus of the copper reduction test. Proteins, for example, are assessed using methods like the Biuret test, while urobilinogen is evaluated through tests specific for bilirubin and its breakdown products. Ketones are measured with tests that specifically look for acetone and related compounds, not through the copper reduction mechanism. Thus, it is the unique reaction of glucose with copper ions during this test that makes it the primary substance examined.