REDUCING SUBSTANCES - urine

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Specimen: Random urine.

Method: Reduction of cupric to cuprous ions in alkaline solution. This may be done with a commercial tablet or Benedict’s solution.

Reference Interval: Normal urine is negative.

Application: This test is little used now as, for the detection of glucosuria, it has been superseded by specific tests for glucose. Its current application is the detection of non-glucose reducing substances that are not detected by specific tests for glucose. These substances are mostly sugars. The test has been used to screen for galactosaemia (galactosuria), but this is unreliable. The test is positive in alkaptonuria, due to the presence of homogentisic acid.

Interpretation: The commonest cause of a positive result is glucosuria. If the test is positive in a baby, but a specific test for glucosuria is negative, further testing for galactosaemia should be done and lactose removed immediately from the diet, pending the test results. A number of drugs and unusual dietary sugars may give positive results.

Reference: Free A and Free H. Urinalysis in Clinical Laboratory Practice. CRC Press 1975.

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