Test for Presence of Sugar in Urine

Excretion is an important characteristic of living organisms. The liquid excretory product in humans is urine. It contains different chemical components like urea, uric acid, enzyme traces, hormones, carbohydrates and large amounts of water. Urine analysis is carried over to understand the physiological condition in certain conditions.

Glycosuria is one such condition with a detected presence of sugars in urine.  The urine sugar test detects the sugar levels, which are compared to the urine glucose levels chart by the doctor to understand the diseased stage of that patient.

Table of Contents

The following article contains

• Aim

• Apparatus required

• Theory

• Procedure 

• Observations

• Result 

• Precautions

Aim 

To perform the test for detecting the presence of sugar in the given urine sample.

Apparatus required

• Test tubes

• Test tube holders

• Burners

• Measuring cylinders

• Urine sample

• Benedict’s solution ( For carrying out Benedict’s test)

• Fehling’s solution A and Fehling’s solution B ( For carrying out Fehling’s test)

Theory

Humans excrete the waste products like urea, creatinine, uric acid, sodium, potassium, phosphate, chloride and sulphate in the form of liquid. Water accounts for a maximum of 95%. There are trace amounts of certain compounds like sugars, blood, bile, proteins and ketone bodies under some abnormal conditions. Glucose is not the general component of human urine. However, it is not rare that glucose is present in urine. This condition of urine containing glucose is glucosuria. This is diagnosed by performing the urine sugar test.

The presence of sugar in the urine is detected by performing the following tests.

1. Benedict’s Test

2. Fehling’s Test

Benedict’s Test

For testing the presence of sugar in the given urine sample by Benedict’s test, Benedict’s reagent is used.  It is a blend of sodium citrate, copper sulphate pentahydrate and sodium carbonate.

Fehling’s Test

For testing the presence of sugar in the given urine sample by Fehling’s test, two solutions, namely Fehling’s A and Fehling’s B, are used.

Fehling’s A - It is an aqueous solution of copper II sulphate and is blue.

Fehling’s B - It is an aqueous solution of sodium potassium tartrate and is a colourless solution.

The copper sulphate present in Fehling’s and Benedict’s reagent is reduced by the reducing sugar in the test sample and forming the cuprous oxide precipitate. Depending on the glucose concentration in the urine sample, the colour of the precipitate varies.

\[{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}\left( {{\rm{CHO}}{{\rm{H}}_{\rm{4}}}} \right){\rm{CHO + 2C}}{{\rm{u}}^{{\rm{ +  + }}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}^{ - \, - } \to {\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}{\left( {{\rm{CHOH}}} \right)_{\rm{4}}}{\rm{COOH + C}}{{\rm{u}}_{\rm{2}}}{\rm{O + }}\,\,{\rm{4}}{{\rm{H}}^{\rm{ + }}}{\rm{ + SO}}_{\rm{4}}^ - \].

Table: Colour Sequence Based on Glucose Concentration

Procedure

For Benedict’s Test: 

• Take 5mL of Benedict’s reagent in a clean and dry test tube and to it add 0.5mL of freshly collected urine. 

• Hold the test tube carefully using a test tube holder and boil it for 2 minutes by keeping it inclined. 

• Formation of the light green, green, yellow and brick red coloured precipitate indicates the presence of sugar in the given urine sample.

For Fehling’s Test:

• Take a clean and dry test tube and add 2mL of each Fehling’s A and Fehling’s B solutions.

• To the same test tube, add 2mL of the urine sample and shake the tube contents well.

• Using a test tube holder, hold the tube carefully and boil the contents for 2 minutes.

Observations

A yellow-coloured precipitate is formed at the bottom of the test tube, indicating the presence of sugar in the given urine sample. Different coloured precipitates that are green, yellow and brick red are formed in the test tubes based on the sugar concentration in the given test sample. 

Result

The formation of the coloured precipitate in both Benedict’s and Fehling’s tests indicates the presence of sugars in the given urine sample.

Precautions

• Measuring the urine sample and reagents should be done accurately using the measuring cylinder.

• Use the test tube holder to hold the test tubes while boiling.

• The test tubes must be kept in an inclined position and should be stirred well to avoid spilling while boiling.

• Care must be taken while handling burners.

Lab Manual Questions 

1. What is Fehling’s test? 

Ans: Fehling’s test is done for detecting the presence of reducing sugars. Fehling’s A and Fehling’s B solutions are the reagents in the test. If the sample under analysis contains reducing sugars, they reduce the reagent and form a coloured precipitate. 

2. How does Benedict’s reagent react with sugar in the urine?

Ans: Benedict’s reagent is an aqueous solution of copper sulphate and is added to the reaction tube containing the test sample. If the test sample contains the reducing sugar, the copper sulphate is reduced to form a visible coloured precipitate. The formation of a coloured precipitate indicates the presence of reducing sugars.

3. Why is a coloured precipitate formed in the test for sugar presence?

Ans: In the test for the presence of sugar, the reagents used contain copper sulphate. The reducing sugar or glucose in the test sample, reduces the copper sulphate and forms the precipitate of cuprous oxide, which appears coloured to the naked eye.  

Viva Questions

1. Can humans excrete sugars?

Ans: Human urine contains urea and uric acid with traces of enzymes and a large amount of water. Sugars are exceptionally excreted in the glycosuria condition.

2. What is the colour of human urine?

Ans: Human urine is pale yellow due to the presence of a pigment called urochrome.

3. What is the nitrogenous waste that humans excrete?

Ans: Humans excrete a nitrogenous waste compound called urea and hence are ureotelic organisms.

4. Why is testing the presence of sugars in urine important?

Ans: Excreting sugars in urine is confined to abnormal physiological conditions like glucosuria. It is important to detect the presence of these abnormal metabolites to proceed with treatments.

5. Does Benedict’s and Fehling’s test confirm the presence of glucose in the urine?

Ans: Benedict’s and Fehling’s tests confirmed the presence of reducing sugars. Reducing sugars can be glucose, galactose, lactose or fructose also.

6. What is meant by reducing sugar?

Ans: Reducing sugars contain a free functional group which is either an aldehyde or ketone group and is capable of acting as a reducing agent.

7. Does Benedict’s test confirm the presence of all sugars?

Ans: Benedict’s test detects the presence of reducing sugars only. Reducing sugars, including monosaccharides (like glucose and fructose) and disaccharides (like lactose and maltose), are detected by Benedict’s test.

8. Why does Fehling’s test detect only reducing sugars?

Ans: The reducing sugars contain free ketone or aldehyde functional groups, which react with Fehling’s reagent to form a coloured precipitate. These free functional groups are absent in non-reducing sugars; hence, they are not detected by Fehling’s test.

9. What are the sugars whose presence is detected by Benedict’s test?

Ans: Other than glucose, simple sugars like ribose, fructose, galactose and disaccharides like lactose and maltose are also detected by Benedict's test.

10. Why is glucose a reducing sugar?

Ans: The free aldehyde group of glucose can reduce the reactive reagent, and itself undergo oxidation into a carboxylic acid. Because of its ability to act as a reducing agent, glucose is called reducing sugar.

Practical Questions

1. An example of reducing sugar is

1. Sucrose

2. Trehalose

3. Fructose

4. Cellulose

Ans: C) Fructose

2. The pigment responsible for the yellow colour of urine is 

1. Melanin

2. Chlorophyll

3. Phycocyanin

4. Urochrome 

Ans: D) Urochrome

3. Which test can detect the presence of sugars 

1. Biuret test

2. Fehling’s test

3. Sudan test

4. Millon's test

Ans: B) Fehling’s test

4. Condition of glucose in the urine is

1. Glycosuria

2. Ketonuria

3. Alkaptonuria

4. Homocystinuria

Ans: A) Glycosuria

5. Fehling’s test 

1. Confirms glucose presence

2. Confirms fructose presence

3. Confirms reducing sugar presence

4. Confirms non-reducing sugar presence.

Ans: C) Confirms reducing sugar presence

6. A negative result in Benedict’s test appears as 

1. Green coloured precipitate

2. Black coloured precipitate

3. Blue colour solution

4. Violet coloured precipitate

Ans: C) Blue colour solution

7. Which is not a normal component of urine 

1. Blood

2. Urea

3. Uric acid

4. Water 

Ans: A) Blood 

8. If the glucose concentration in the test sample is high, 

1. Green coloured precipitate is formed.

2. Brick red precipitate is formed.

3. Blue-coloured precipitate is formed.

4. Yellow-coloured precipitate is formed.

Ans: Brick red precipitate is formed. 

9. Which among the following show negative results for Benedict’s test?

1. Glucose

2. Fructose

3. Lactose

4. Starch 

Ans: D) Starch

10. Non-reducing sugar is

1. Cellulose 

2. Dextro- Glucose

3. Levo-fructose

4. Maltose

Ans: A) Cellulose 

Conclusion

The given urine sample formed coloured precipitate with both the Benedict’s and Fehling’s reagents showing the presence of sugars.