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Abnormal Urine

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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Q.1- What are the conditions in which urine gives a positive reaction with Benedict’s qualitative reagent?

Answer- Benedict’s test gives a positive reaction with all reducing agents, which may be sugars or other agents. In urine, the commonly found reducing sugars are- Glucose, Fructose, Galactose, Lactose and Pentoses. The other non sugar reducing agents are Ascorbates,Urates, Glucuronates, Homogentisic acid, Salicylates etc which also give positive reaction with Benedict’s reagent. Hence positive Benedict’s test should be further explored to know the exact identity of the substance present in urine.

Q.2- A 23- year-old male was denied a job of a sales man on the ground of positive Benedict’s test. Further investigations revealed normal fasting blood glucose and normal oral glucose tolerance test. The specific test for glucose in urine was negative. There was no abnormality detected on physical examination of the patient. There was no family history of diabetes mellitus. What are the conditions in which such like findings can be observed?

Answer- There are two main possibilities-

1) It might be case of renal glycosuria due to transporter deficiency (SGLT-2) required for the reabsorption of Glucose from the filtrate. But such patients generally have polyuria as a result of glucosuria (depending upon the severity of defect) and Polydipsia also due to excessive polyuria. In this patient there are no such symptoms, so it can be ruled out.

2) The second possibility is of Pentosuria, which seems to be the most likely cause in the given patient. Pentosuria goes undiagnosed and it is always a chance finding.There are no symptoms and no treatment is required for this.  Essential Pentosuria occurs due to deficiency of Xylitol dehydrogenase, the enzyme of uronic acid pathway. The confirmation is done by Bial’s test and chromatography.

Q.3- In case of non availability of Benedict’s reagent, is there any alternative test for detection of reducing sugars in urine?

Answer- Fehling’s test can be undertaken,but that is less sensitive than Benedict’s test. The strong alkali (KOH) of Fehling test causes caramalizaton of sugars hence they can be left undetected.

Q.4- Enlist some of the common conditions of Glycosuria

Answer- Glycosuria (presence of detectable amount of any sugar in urine) includes the following:

1) Glucosuria: (presence of detectable amount of glucose in urine).

a) Uncontrolled DM: The concentration of glucose in plasma exceeds renal threshold.

b) Renal Glucosuria: Normal plasma glucose concentration with proximal tubular malfunction leads to decreased renal threshold (gestational diabetes, transported defect and Fanconi’s syndrome).

2).Fructosuria: (Presence of fructose in urine)

a) Alimentary: due to increased fructose intake.

b) Metabolic:deficiency of fructokinase or aldolase B enzyme in the liver.

3). Galactosuria: (Presence of galactose in urine)

a) Alimentary: increased galactose intake

b) Metabolic: deficiency of Galactokinase or galactose -1-phosphate   uridyl transferase in theliver.

4). Lactosuria: (Presence oflactose in urine)

a) Lactation

b) Lactose intolerance

5) Pentosuria :( Presence ofpentose in urine)

a) Alimentary- Excessive ingestion of fruits containing pentoses like Raspberries

b) Metabolic defect- Essential Pentosuria- Deficient Xylitol dehydrogenase

Q.5- What are the conditions causing Ketonuria?

Answer-(Presence of ketones”Acetone, acetoacetic acid and β- Hydroxybutyric acid” in urine)

a) Diabetic ketoacidosis.

b) Glycogen storage diseases

c) Starvation

d) Prolongedvomiting

e) Unbalanced diet:high fat and low carbohydrate diet.

Q.6- What is the possibility if Rothera’s test for ketone bodies in urine is negative but blood ketone level is high ?

Answer- Rothera test is given by acetone and Acetoacetate. Beta hydroxy butyrate, which is the main ketone body of urine does not give this test positive. Hence it is possible despite a good concentration of ketone bodies in urine the test is still negative. The indirect test is performed, by converting beta hydroxy butyrate to Aceto acetate to get the positive result.

Q.7- What are the common causes of proteinuria?

Answer- Normally less than 200 mg protein is excreted in the urine daily; More than this level leads to a condition called “proteinuria”.
Proteinuria is either glomerular or tubular-

1) Glomerular proteinuria is due to increased glomerular permeability leading to filtration of high molecular weight proteins (e.g. glomerulonephritis)

2) Tubular proteinuria occurs as a result of decreased reabsorption with normal glomerular permeability leading to excretion of low molecular weight proteins (e.g. chronic nephritis).

Proteinuria is classified into prerenal,renal and post renal.

1) Pre-renal proteinuria:

a) Dehydration

b) Congestive Heart failure with passive congestion of kidneys

c) Increased intra abdominal pressure as in ascites, tumor

d)  Severe anaemia, fever

e) Bence-Jones protein: This abnormal gamma globulin (light chains only)is synthesized by malignant plasma cells (multiple myeloma). It precipitates at60oC, redissolves on boiling and reprecipitates on cooling to 60oC.

2. Renal proteinuria:

a) Nephrotic syndrome


c)Gestational (in the 3rd trimester of pregnancy)

d) Glomerulonephritis

e) Diabetic nephropathy 

f)  Renalischemia or neoplasia

g) Nephrotoxins(aminoglycosides; gentamicin, streptomycin…etc.)

h) Overflow proteinuria (haemoglobin due to intravascular haemorrhage, microglobulin in leukaemia and lymphoma, and myoglobin in rhabdomyolysis

3)- Post-renal proteinuria

a)  Lower urinary tract infection, tumors or stones

b) Vaginal bleeding

c) Mixing of urine with semen or vaginal secretions (False proteinuria)

Besides these cause functional  proteinuria may be there in conditions like prolonged standing, exposure to cold, severe exercise and in extreme stress.

Q.8- What is Acholuric jaundice?

Answer- Acholuric jaundice means- jaundice without bilirubin in urine Haemolytic jaundice or Prehepatic jaundice is called Acholuric jaundice, since there is Unconjugated hyperbilirubinemia. Unconjugated bilirubin can not appear in urine due to two reasons –

1) Water insoluble,only water soluble substances are excreted in urine.

2) Unconjugated bilirubin is bound to albumin making it a macro molecule; hence can not be filtered.

Thus In haemolytic jaundice test for bilirubin (Fauchet test) is negative for urine but test for urobilinogen(Ehrlich’s test) is positive since urobilinogen is there in excess.Dark color of urine in haemolytic jaundice is due to Urobilinogen and not due to Bilirubin.

Q.9- In which type of jaundice both Bilirubin and urobilinogen are there in urine ?

Answer- In hepatic jaundice both conjugated and unconjugated hyperbilirubinemia is there, hence urine contains both urobilinogen and bilirubin.

Q.10- A patient has reported to Medical OPD with jaundice. There is a previous history of several episodes of biliary colic.  The patient is in acute distress and there is pain and tenderness in the right hypochondrium. His blood and urine samples have been sent for biochemical investigations. What should be the urinary findings for this patient in terms of Bilirubin and urobilinogen?

Answer- The patient has obstructive jaundice due to gall stones. Hence there should be conjugated hyperbilirubinemia. Urine will be positive for bilirubin while negative for urobilinogen.

Q.11- What are the causes of Hematuria and hemoglobinuria?

Answer- 1) Hematuria (Presence of detectable amount of blood in urine):

a) Acute and chronic glomerulonephritis.

b) Local disorders of kidney and genitourinary tract (trauma, cystitis, renal calculi,  tumors etc). 

c) Bleeding disorders(haemophilia). 

d) Malignant hypertension

2) Hemoglobinuria(Presence of hemolysed blood in urine):

a)Hemoglobinopathies (sickle-cell anaemia and Thalassemia)

b) Transfusion reaction (blood incompatibility).

c) Malaria(plasmodium falciparum)

d) Snake bite

Q.12- What is chyluria? 

Answer- Chyluria (Presence of lymph/ fat in urine):

a)The urine acquires a milky appearance which disappears on shaking with ether.  

b)Due to abnormal connection between the intestinal lymphatic system and urinary   tract 

c) It  may be congenital or acquired.

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