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A young infant, who was nourished with a synthetic formula, had a sugar in the blood and urine. This compound gave a positive reducing sugar test but was negative when measured with glucose oxidase (specific test for detection or estimation of Glucose). Treatment of blood and urine with acid (which cleaves glycosidic bonds) did not increase the amount of reducing sugar measured. Which of the following compounds is most likely to be present in this infant’s blood and urine?
A. Glucose
B. Fructose
C. Maltose
D. Sorbitol
E. Lactose

The right answer is fructose.

Reducing sugars are usually detected by Benedict’s reagent, which contains copper sulphate, sodium citrate and sodium carbonate. Sodium carbonate makes the medium alkaline. Copper sulphate furnishes Cu2+ ions and sodium citrate prevents the precipitation of cupric ions as cupric hydroxide by forming a loosely bound cupric- sodium –citrate complex which on dissociation gives a continuous supply of cupric ions.

Benedict’s test


Carbohydrates with free aldehyde or ketone groups have the ability to reduce solutions of various metallic ions. Reducing sugars under alkaline conditions tautomerise and form enediols. Enediols are powerful reducing agents. They reduce cupric ions to cuprous form and are themselves converted to sugar acids. The cuprous ions combine with OH- ions to form yellow cuprous hydroxide which upon heating is converted to red cuprous oxide.


Take 5 ml of Benedict’s reagent. Add 8 drops of carbohydrate solution. Boil over a flame or in a boiling water bath for 2 minutes. Let the solution cool down.


Benedict’s test is a semi quantitative test. The color of the precipitate gives a rough estimate of a reducing sugar present in the sample (figure-1)

Green color- Up to 0.5 g %(+)

Green precipitate -0.5-1.0 g %(++)

Yellow precipitate -1.0-1.5 g %(+++)

Brick red precipitate- >2.0 G% (++++)

Negative benedict's testPositive benedict's test

(-ve)                (+ve)

Figure– The positive test is given by reducing sugars. The color of the precipitate determines the rough estimate of the reducing sugar present in the given sample.

Fehling test is an alternative to Benedict’s test. It differs from Benedict’s test in that it contains sodium potassium tartrate in place of Sodium citrate and potassium hydroxide as an alkali in place of sodium carbonate in Benedict’s reagent. It is not a preferred test over Benedict’s test since the strong alkali present causes caramelisation of the sugars; hence it is less sensitive than Benedict’s reagent.

Positive Benedict’s test for urine signifies Glycosuria.

Glycosuria is a non-specific term. Glucosuria, lactosuria, galactosuria, pentosuria and fructosuria denote the presence of specific sugars in urine.

Causes of Glycosuria are:

a. Renal glycosuria

b. Diabetes mellitus

c. Alimentary glucosuria

d. Hyperthyroidism, hyperpituitarism and hyperadrenalism

e. Stress, severe infections, increased intracranial pressure

Lactosuria– in lactose intolerance

Galactosuria– in galactosemia

Fructosuria– in hereditary fructose intolerance

Pentosuria – in essential pentosuria

Examples of non-carbohydrate substances which give a positive Benedict’s reaction are:

a) Creatinine

b) Ascorbic acid

c) Glucuronates

d) Drugs: Salicylates, PAS and Isoniazid.

Glucose oxidase test is a specific enzymatic method for the determination and estimation of glucose present in a given sample. True glucose can be estimated by this method.

As regards other options

Glucose cannot be present since specific test is negative.

Sorbitol is non reactive to reduction test.

Maltose and lactose would have caused increase in the amount of reducing sugar upon acid hydrolysis.

Hence it is fructose which is reducing in nature but non reactive to glucose oxidase.


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