Normal Constituents of Urine

Introduction

• Clinical laboratory examination of urine provides important information about both renal diseases and systemic diseases that affect the urinary system.
• Urine analysis helps detect disorders related to the kidneys, ureters, urinary bladder, and urethra.
• Both structural abnormalities and functional disturbances of the kidneys and lower urinary tract can be identified through urine examination.
• Laboratory examination of urine also provides sequential information about the nature of disease, possible cause, and prognosis.
• Careful urine analysis helps narrow the clinical differential diagnosis of many renal disorders.
• It is one of the simplest and most valuable laboratory investigations used in routine clinical practice.
• Proper collection, examination, and interpretation of urine remain essential for accurate diagnosis in clinical medicine.

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Normal Constituents of Urine in a 24-Hour Sample

A 24-hour urine sample gives a more accurate picture of the total amount of substances excreted by the body in one day. It is commonly used for biochemical estimation because many urinary constituents vary during different times of the day.

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Explanation of Important Constituents

Water

• Water forms the major part of urine.
• It helps dissolve waste materials and remove them from the body.

Urea

• Urea is the major nitrogenous waste product formed in the liver.
• It is produced from protein metabolism.

Uric Acid

• Uric acid is formed from purine metabolism.
• It is excreted in small amounts under normal conditions.

Creatinine

• Creatinine is produced from muscle metabolism.
• It is excreted at a relatively constant rate and is useful for kidney function assessment.

Electrolytes

• Sodium, potassium, calcium, phosphate, and chloride are normally excreted in urine.
• Their amount depends on diet, hormones, and kidney function.

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Urine Collection and Preservation 

Proper urine collection and preservation are essential for valid laboratory reporting.

Types of Urine Collection

1. Random Specimen

• Random urine specimen is the most commonly collected sample.
• It can be collected at any time during the day.
• It is easy to obtain and immediately available.

Common Uses

• Glucose detection
• Ketone bodies
• Bile pigments
• Blood pigments

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2. Midstream First Morning Specimen

• This is the preferred sample for routine urinalysis.
• Overnight urine remains in the bladder for several hours, so analytes become concentrated.
• It gives better laboratory accuracy.

Method of Collection

• The patient should discard the first portion of urine.
• The middle portion of urine is collected in a clean container.
• Usually 15–20 mL urine is sufficient.

Advantages

• Concentrated sample
• Less contamination
• Better for microscopic examination

Storage

• Urine should be examined immediately.
• If delayed, it should be refrigerated for one or two days.

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3. 24-Hour Urine Collection

• 24-hour urine collection is required when substances show daily variation in excretion.

Used For Estimation Of

• Creatinine
• Urea nitrogen
• Calcium
• Proteins
• Glucose
• Sodium
• Potassium
• Hormones such as catecholamines and hydroxysteroids

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Method of 24-Hour Collection

• Start collection at 6 AM.
• Ask the patient to empty the bladder and discard the first sample.
• Collect all urine passed after that into a container.
• Continue collection for the next 24 hours.
• Include the urine passed exactly at 6 AM next morning as the final sample.

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Preservatives Used for 24-Hour Urine Collection

Common Preservatives

• 50 mL of 2N HCl
• 10 mL concentrated HCl
• Thymol crystals
• 5 mL thymol solution in isopropanol

These preservatives prevent bacterial growth and chemical changes in urine.

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Routine Examination of Urine

Routine urine examination is usually divided into three sections:

a. Physical Examination

• Color
• Volume
• Appearance
• Odor
• Specific gravity

b. Chemical Examination

• Protein
• Glucose
• Ketone bodies
• Bile salts
• Blood

c. Microscopic Examination

• Cells
• Crystals
• Casts
• Microorganisms

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Physical Examination

1. Color of Urine

• Urine color depends mainly on its degree of concentration and the pigments present in urine.
• Normal urine is usually pale yellow because of the pigment Urochrome.
• Dilute urine appears lighter, while concentrated urine appears darker.
• Abnormal urine color may indicate presence of blood pigments, bile pigments, metabolic products, or disease.

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Normal and Abnormal Urine Colors

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Important Abnormal Colors

Red Urine

• Red urine usually indicates blood pigments.

Causes

• Red blood cells in urine (hematuria)
• Hemoglobin in urine (hemoglobinuria)
• Menstrual contamination

Clinical Importance

• May indicate urinary tract bleeding, kidney disease, or hemolysis.

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Red-Brown Urine

• This occurs when muscle pigments or porphyrins are present.

Causes

• Myoglobinuria
• Porphyrinuria

Clinical Importance

• Seen in muscle injury or certain metabolic disorders.

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Yellow-Orange Urine

• Dark yellow urine indicates concentrated urine.

Causes

• Dehydration
• Jaundice

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Yellow-Green Urine

• Occurs due to bile pigments.

Causes

• Bilirubin
• Biliverdin

Clinical Importance

• Suggests liver disease or obstructive jaundice.

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Brown-Black Urine

• Urine may become dark on standing.

Causes

• Homogentisic acid
• Methemoglobin
• Porphyrins

Clinical Importance

• Seen in Alkaptonuria

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Milky White Urine

• Urine becomes cloudy white when fat is present.

Causes

• Chyle

Clinical Importance

• Seen in lymphatic obstruction or chyluria.

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2. Volume of Urine

• Urine volume depends on:
  • Fluid intake
  • Solutes excreted such as sodium and urea
  • Loss of water through skin, lungs, and bowel
  • Cardiovascular status
  • Kidney function
• Normally, a healthy adult excretes 1000 to 2500 mL urine per day depending on fluid intake and environmental conditions.

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Abnormal Changes in Urine Volume

Polyuria

• Polyuria means urine output greater than 2.5 liters per day.
• It occurs when the kidneys excrete excessive water.

Causes of Polyuria

• Diabetes Mellitus
• Diabetes Insipidus
• Later stages of chronic renal failure
• Use of diuretic drugs

Clinical Importance

• Persistent polyuria may indicate endocrine or renal disease.

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Oliguria

• Oliguria means urine output less than 300 mL per day.
• It usually indicates reduced kidney filtration or fluid loss.

Causes of Oliguria

• Fever
• Acute nephritis
• Early chronic glomerulonephritis
• Diarrhea
• Cardiac failure

Clinical Importance

• Oliguria is often an early warning sign of dehydration or kidney injury.

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Anuria

• Anuria means complete absence of urine output for 24 hours or extremely minimal urine formation.

Causes of Anuria

• Shock
• Acute tubular necrosis
• Mercury poisoning
• Incompatible blood transfusion

Clinical Importance

• Anuria is a medical emergency and requires immediate evaluation.

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3. Appearance of Urine

• Freshly passed normal urine is usually clear and transparent.
• Normal urine may become slightly cloudy on standing because crystals can form.

• Phosphate crystals
• Urate crystals
• Oxalate crystals

Abnormal Turbidity

Urine becomes abnormally turbid due to:

• White blood cells
• Red blood cells
• Bacteria
• Pus cells

Clinical Importance

• Turbid urine may indicate urinary tract infection or inflammation.

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4. Odor of Urine

• Normal urine has a slightly ammoniacal odor due to breakdown of urea.

Abnormal Odors and Their Causes

Fruity Odor

• Fruity smell occurs because of acetone in urine.

Cause

• Ketoacidosis, especially in uncontrolled diabetes

Foul Smell

• Foul odor usually suggests bacterial infection.

Cause

• Urinary tract infection

Mousy Smell

• A characteristic mousy odor is found in certain metabolic disorders.

Cause

• Phenylketonuria

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5. Specific Gravity of Urine

• Specific gravity indicates the concentration of dissolved substances in urine.
• It reflects the kidney’s ability to concentrate or dilute urine.
• Specific gravity is directly proportional to the amount of solutes excreted.

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Normal Range

• Normal urine specific gravity ranges from 1.012 to 1.024.
• Dilute urine has lower specific gravity.
• Concentrated urine has higher specific gravity.

Instrument Used

• Specific gravity is measured using Urinometer.

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High Specific Gravity

• High specific gravity indicates concentrated urine.

Causes

• Restricted water intake
• Dehydration
• Presence of glucose in urine
• Diabetes Mellitus
• Proteinuria
• Adrenal insufficiency

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Low Specific Gravity

• Low specific gravity indicates dilute urine.

Causes

• Polyuria
• High fluid intake
• Diabetes Insipidus
• Hypothermia

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Fixed Specific Gravity (1.010)

• Specific gravity fixed at 1.010 suggests severe kidney damage.

Clinical Importance

• Seen in chronic renal failure
• Indicates loss of concentrating ability of kidney

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Correction of Specific Gravity

Glucose Correction

• For every 1 g/dL glucose, subtract 0.004

Protein Correction

• For every 1 g/dL protein, subtract 0.003

Temperature Correction

• Urinometer is calibrated at 15°C or 20°C
• For every 3°C above calibration, add 0.001
• For every 3°C below calibration, subtract 0.001

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6. Reaction of Urine (pH)

• Freshly voided urine is usually slightly acidic.
• It may also be neutral or faintly alkaline.
• On standing, urine becomes alkaline because bacteria convert urea into ammonia.

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Normal Urine pH

• Normal urine pH ranges from 4.6 to 8.0

Alkaline Tide

• After meals, urine may become alkaline.
• This happens because hydrochloric acid is secreted in the stomach.
• Blood temporarily becomes alkaline.
• This is called alkaline tide.

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Effect of Diet on Urine pH

Acidic Urine

• High protein diet
• Low carbohydrate diet

Alkaline Urine

• Diet rich in vegetables and fruits

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Causes of Acidic Urine

• High protein diet
• Starvation
• Dehydration
• Diarrhea
• Diabetic ketoacidosis
• Metabolic acidosis
• Respiratory acidosis

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Causes of Alkaline Urine

• Vegetable diet
• Vomiting
• Renal tubular acidosis
• Respiratory alkalosis
• Metabolic alkalosis
• Ammonia-producing bacteria
• Chronic renal failure
• Low carbohydrate diet

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Chemical Examination

A. Organic Constituents

1. Urea in Urine

Principle (Urease Test)

• Urease enzyme decomposes urea into ammonia and carbon dioxide.
• Ammonia makes the solution alkaline, changing the indicator color from yellow to pink.

Reaction

CO(NH2)2+H2O→2NH3+CO2

Procedure, Observation and Interpretation

Procedure	Observation	Interpretation
Take 2 mL urine in one test tube and 2 mL water in another tube	—	—
Add one drop of phenol red indicator to both tubes	—	—
Add sodium carbonate till pink color appears, then add acetic acid till pink just disappears	—	Slightly acidic medium prepared
Add a pinch of soybean powder or horse gram powder	Pink color reappears in urine tube only	Urea present in urine

​Principle (Hypobromite Test)

• Urea reacts with sodium hypobromite and releases nitrogen gas causing effervescence.

CO(NH2)2+3NaOBr+2NaOH→NaBr+N2+Na2CO3+3H2O

Procedure, Observation and Interpretation

Procedure	Observation	Interpretation
Take 5 mL urine in a test tube	—	—
Add 1 mL alkaline sodium hypobromite	Effervescence occurs	Urea present in urine

Clinical Interpretation

• Normal urinary urea: 25–30 g/day

Increased Urinary Urea

• High protein diet
• Fever
• Tissue breakdown

Decreased Urinary Urea

• Renal Failure
• Severe liver disease
• Low protein diet

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2.Test for Uric Acid 

Principle (Phosphotungstic Acid Reduction Test)

Uric acid reduces phosphotungstic acid in alkaline medium to form tungsten blue.

Procedure, Observation and Interpretation

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Clinical Interpretation

• Normal urinary uric acid: 250–750 mg/day

Increased Urinary Uric Acid

• High purine diet
• Gout
• Leukemia
• Cortisone or ACTH therapy

Decreased Urinary Uric Acid

• Chronic renal failure

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3. Creatinine in Urine

Principle (Jaffe’s Test)

Creatinine reacts with picric acid in alkaline medium to form creatinine picrate, which gives a reddish-orange color.

Procedure, Observation and Interpretation

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Clinical Interpretation

• Normal urinary creatinine: 1–2 g/day

Increased Urinary Creatinine

• Fever
• Myasthenia Gravis
• Muscular atrophy
• Myositis
• Hyperthyroidism
• Starvation

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4. Ammonia in Urine

Principle

Ammonia is liberated when urine is boiled in alkaline medium.

Procedure, Observation and Interpretation

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Clinical Interpretation

• Normal urinary ammonia: 0.4–1.0 g/day

Increased Urinary Ammonia

• Acidosis
• Diabetes Mellitus
• Starvation
• Hepatic disease

Decreased Urinary Ammonia

• Alkalosis
• Renal failure
• Glomerulonephritis
• Addison’s Disease

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B. Inorganic Constituents 

1. Chloride

Principle

Chloride reacts with silver nitrate in presence of nitric acid to form white silver chloride precipitate.

Procedure, Observation and Interpretation

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Clinical Interpretation

• Normal: 10–12 g/day

Increased

• Addison’s Disease
• Polyuria
• Diabetes

Decreased

• Sweating
• Diarrhea
• Vomiting
• Burns

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2. Inorganic Phosphorus

Principle

Inorganic phosphate reacts with ammonium molybdate in acidic medium to form canary yellow ammonium phosphomolybdate.

Procedure, Observation and Interpretation

• Normal: 0.8–1.2 g/day

Increased

• Hyperparathyroidism

Decreased

• Rickets
• Pregnancy
• Nephritis

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3. Calcium

Principle

Calcium reacts with potassium oxalate in acidic medium to form calcium oxalate precipitate.

Procedure, Observation and Interpretation

• Normal: 0.1–0.3 g/day

Increased

• Hyperparathyroidism
• Hyperthyroidism
• Multiple myeloma

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4. Sulfates

Principle

Sulfates react with barium chloride to form barium sulfate precipitate.

Procedure, Observation and Interpretation

• Normal inorganic sulfate: 0.7–1 g/day

Increased

• High protein diet

Decreased

• Renal dysfunction