Introduction
Urine analysis (or urinalysis) is one of the oldest and most common laboratory diagnostic tests. It provides vital information about the health of the kidneys and other organs. The test involves physical, chemical, and microscopic examination of urine and helps detect a wide range of conditions, including kidney disease, diabetes, and urinary tract infections (UTIs).
Modern urinalysis combines traditional observations like color, odor, and sediment with advanced methods such as dipstick testing for multiple analytes and osmolality measurement for solute concentration.
Types of Urine Examination
- Macroscopic and Physical Examination
- Chemical (Qualitative or Semi-Quantitative) Tests
- Microscopic Examination of Urinary Sediment
A routine urinalysis is typically performed on the first-morning urine sample, as it is most concentrated and provides reliable results.
1. Macroscopic and Physical Examination
a) Volume
The normal urine output of an adult is about 1–1.5 liters per day (approximately 1 mL/min). It varies with fluid intake, activity, temperature, diet, and hormonal control.
- Oliguria: Decreased urine output (< 400 mL/day). Causes – dehydration, low blood pressure, shock, acute tubular necrosis, or urinary obstruction.
- Polyuria: Excessive urine output (> 3 L/day). Causes – diabetes mellitus, diabetes insipidus, or excessive fluid intake.
b) Colour
Normal urine is pale yellow to amber due to the pigment urochrome. Abnormal colors can indicate pathology:
- Red or smoky: Presence of blood or hemoglobin (bleeding in urinary tract).
- Brownish-green or deep yellow: Presence of bile pigments (liver disease).
- Dark brown or black: Homogentisic acid in alkaptonuria.
- Orange or blue tinge: Due to certain drugs or dyes.
c) Odour
- Fresh urine has a characteristic mild odor.
- Fruity smell: Suggests ketones (as in diabetic ketoacidosis).
- Ammoniacal odor: Indicates bacterial decomposition or old urine sample.
- Maple syrup smell: Seen in rare genetic condition Maple Syrup Urine Disease.
d) Specific Gravity
Specific gravity (SG) measures the urine’s density compared to water. It reflects the kidney’s concentrating and diluting ability. The normal range for a 24-hour specimen is 1.015–1.025.
- Low SG (<1.010): Indicates dilute urine (renal tubular damage or overhydration).
- High SG (>1.030): Suggests dehydration or presence of glucose/protein in urine.
Methods to Measure Specific Gravity:
1) Using a Urinometer:
- Fill a cylinder three-fourths with urine.
- Insert the urinometer carefully so it floats freely.
- Read the SG at the lowest point of the meniscus.
- Apply temperature correction: add/subtract 0.001 for every 3°C deviation from calibration temperature.
2) By Refractometry:
- Place a drop of clear urine on the refractometer prism.
- View towards light and read where the light-dark boundary crosses the scale.
3) By Dipstick:
Uses a polyelectrolyte-indicator system that changes color based on ionic concentration. It gives a rough estimate but is less accurate than other methods.
e) Osmolality
Osmolality measures the total solute concentration in urine. It is expressed in milliosmoles per kilogram (mOsm/kg) of water and provides an accurate reflection of urine concentration.
Normal Reference Values:
- Serum: 278–305 mOsm/kg
- Random urine: 40–1350 mOsm/kg
- 24-hour urine: 500–800 mOsm/kg
- After water deprivation: 850–1350 mOsm/kg
Osmolality is more reliable than SG because it is not significantly affected by large molecules like glucose or protein.
2. Qualitative or Semi-Quantitative Tests (Chemical Examination)
Performed using dipsticks impregnated with specific reagents that change color upon reacting with target substances. The most commonly tested parameters are:
- pH
- Protein
- Glucose
- Ketone bodies
- Blood
- Bilirubin
- Urobilinogen
- Nitrite
- Leukocyte esterase
a) pH
Normal urine pH ranges from 4.6 to 8.0 (average 6.0). It depends on diet and metabolic state:
- Acidic urine: Seen in starvation, high-protein diet, or diabetes.
- Alkaline urine: Occurs in urinary tract infections or after prolonged alkali intake.
b) Protein
Normal urine contains less than 150 mg/day of protein, mostly albumin and glycoproteins. Proteinuria indicates kidney damage, particularly glomerular injury.
1) Dipstick Method:
Based on the protein error of indicators—the reagent pad changes color (yellow to green-blue) as proteins bind to the dye (bromophenol blue). Results range from trace to 4+.
2) Sulfosalicylic Acid Test:
- Mix 3 mL of centrifuged urine with 3 drops of 25% sulfosalicylic acid.
- Observe turbidity at the interface after 1 minute.
- Grade as trace (+/-) to 4+ depending on cloudiness.
c) Glucose
Normally, urine contains no detectable glucose. Glycosuria usually indicates diabetes mellitus or renal glycosuria.
Dipstick Principle:
Glucose oxidase converts glucose to gluconic acid and hydrogen peroxide (H₂O₂). In the presence of peroxidase and a chromogen, H₂O₂ forms a color compound (brown or blue). The intensity corresponds to glucose concentration.
d) Ketone Bodies
Ketonuria occurs in uncontrolled diabetes, starvation, or fasting. Dipsticks contain sodium nitroprusside, which reacts with acetoacetate and acetone to produce a lavender color.
e) Blood
Dipsticks detect even trace hemoglobin by its peroxidase-like activity, producing a blue color. Presence indicates bleeding, hemolysis, or renal injury.
f) Bilirubin, Urobilinogen, Nitrite, and Leukocyte Esterase
- Bilirubin/Urobilinogen: Detect liver or biliary diseases.
- Nitrite: Indicates bacterial infection (conversion of nitrate to nitrite).
- Leukocyte esterase: Detects white blood cells—suggestive of urinary tract infection.
3. Microscopic Examination (Optional)
Performed on centrifuged urine sediment to detect cells, crystals, casts, bacteria, or parasites. This test confirms dipstick findings and helps identify renal or urinary tract pathologies.
Summary Table: Normal Urine Findings
| Parameter | Normal Range / Observation |
|---|---|
| Volume | 1–1.5 L/day |
| Colour | Pale yellow to amber |
| Specific Gravity | 1.015–1.025 |
| pH | 4.6–8.0 (avg 6.0) |
| Protein | < 150 mg/day |
| Glucose | Absent |
| Ketones | Absent |
| Bilirubin | Absent |
| Osmolality | 500–800 mOsm/kg (24 hr) |
Detailed Notes:
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