glucose

Glucose

Most common carbohydrate found in urine

Normal urine glucose

≤15 mg/dL

Normal glucose excretion

Glycosuria

Presence of glucose in urine

Mellituria

Presence of any sugar in urine

Most common cause of glycosuria

Diabetes mellitus

Renal threshold for glucose

160–180 mg/dL plasma glucose

Why does glycosuria occur?

Blood glucose exceeds the renal threshold or renal tubular reabsorption is impaired.

Maximum tubular reabsorption (TmG)

Approximately 375 mg/min

Hyperglycemic glycosuria

Glucose in urine due to elevated blood glucose.

Most common cause of hyperglycemic glycosuria

Diabetes mellitus

Endocrine disorders causing glycosuria

Acromegaly, Cushing syndrome, hyperthyroidism, pheochromocytoma

Drug-induced glycosuria

Thiazides, corticosteroids, ACTH, oral contraceptives

Alimentary glycosuria

Temporary glycosuria after consuming a large amount of carbohydrates.

Renal glycosuria

Glucose in urine despite normal blood glucose.

Cause of renal glycosuria

Defective renal tubular reabsorption.

Diseases causing renal glycosuria

Fanconi syndrome, cystinosis, tubular necrosis, pregnancy, lead poisoning, multiple myeloma

Pregnancy and glycosuria

Occurs because the renal threshold is lowered.

Fanconi syndrome

Generalized proximal tubular dysfunction causing glycosuria.

Copper reduction test principle

Reducing sugars convert cupric ions (Cu²⁺) to cuprous oxide (Cu⁺) in an alkaline solution with heat.

Copper reduction test detects

Reducing sugars

Reducing sugars

Glucose, galactose, lactose, fructose, pentoses

Copper reduction reagent

Copper sulfate

Copper reduction requires

Heat and an alkaline medium

Copper reduction color change

Blue → Green → Yellow → Orange → Red

Blue color

Negative

Green color

Approximately 0.5% reducing sugar

Yellow color

Approximately 1.0% reducing sugar

Orange color

Approximately 1.5% reducing sugar

Brick-red color

Approximately 2.0% or more reducing sugar

Limitation of copper reduction test

Cannot distinguish glucose from other reducing sugars.

Benedict's test

Qualitative test for reducing sugars

Principle of Benedict's test

Copper reduction reaction

Benedict's test detects

Glucose and other reducing sugars

Advantage of Benedict's test

Detects non-glucose reducing sugars.

Clinitest

Tablet copper reduction test for reducing sugars

Principle of Clinitest

Benedict's copper reduction reaction

Clinitest detects

All reducing sugars

Main use of Clinitest

Detect reducing substances in infant urine.

Pass-through phenomenon

Very high glucose causes the color to return from orange/red to green-brown, producing a falsely low result.

Cause of pass-through phenomenon

Excessive heat from very high glucose concentration.

How to avoid pass-through phenomenon

Use the two-drop Clinitest procedure.

Routine Clinitest method

Five-drop method

Modified Clinitest method

Two-drop method

False-positive copper reduction test

Other reducing sugars or reducing substances.

Reducing substances causing false positives

Galactose, lactose, fructose, pentoses, ascorbic acid, homogentisic acid

False-negative copper reduction test

Very high glucose due to pass-through phenomenon.

Glucose reagent strip principle

Double sequential enzyme reaction.

First enzyme in glucose strip

Glucose oxidase

Second enzyme in glucose strip

Peroxidase

Product of glucose oxidase reaction

Gluconic acid and hydrogen peroxide

Chromogen in glucose strip

Potassium iodide

Glucose strip color change

Yellow → Green → Brown

Negative glucose strip

Yellow

Positive glucose strip

Green to brown

Glucose strip detects

Glucose only

Advantage of glucose oxidase strip

Specific for glucose

False-positive glucose strip

Strong oxidizing agents (bleach, peroxide)

False-negative glucose strip

High ascorbic acid (vitamin C)

Another false-negative glucose strip

High ketones

Another false-negative glucose strip

High specific gravity

Clinical significance of glycosuria

Indicates abnormal carbohydrate metabolism or renal tubular dysfunction.

Most specific urine glucose test

Glucose oxidase reagent strip

Most sensitive screening test for reducing sugars

Clinitest

Reducing sugars not detected by glucose oxidase

Galactose, lactose, fructose, pentoses

Why is Clinitest important in infants?

It detects galactosemia and other inherited carbohydrate disorders.

Disease screened using Clinitest in infants

Galactosemia