INTRODUCTION

Urine analysis (urinalysis) reflects the state of function of the kidneys and urinary tract. It provides information about metabolic or systemic (nonrenal) disorders. Urine examination should precede all other invasive/noninvasive diagnostic investigations for renal function.

Indications for urinalysis :

•• Renal system:

–– To detect urinary tract infections
–– Suspicion of renal diseases

•• Detection and management of:

–– Metabolic diseases, e.g. diabetes mellitus
–– Plasma cell dyscrasias

•• Diagnosis of pregnancy

•• Differential diagnosis of jaundice

COLLECTION OF URINE SPECIMEN

Time of Collection

Urine should be collected in a clean, dry and preferably sterilized container.
•• A single specimen:

It may be first morning voiding sample, random sample or a postprandial sample.

  1. –– First morning sample: For routine examination, the first-morning (8 hours concentrated) sample is preferred but any fresh random urine specimen is satisfactory for chemical analysis. Demonstration of casts and crystals is easy with first morning samples.

  2. –– Random sample: It is the sample that is collected any time and is used for routine urine examination.

  3. –– Post-prandial sample: It is the urine sample collected exactly 2 hours after a meal.

•• 24-hour sample: 

This is necessary for quantitative estimation of protein (e.g. nephrotic syndrome) and hormones in urine.


Methods of Collection

•• Midstream specimen: 

This is used for all types of urine examination. After voiding first half of urine into the toilet, a part of the next voided urine is collected as midstream sample.


•• Clean-catch specimen: 

This is the method of collection for culture and sensitivity of urine. The external genitalia is cleaned with soap and water and specimen is collected as mentioned in the midstream sample.

•• Cather specimen: 

It is used for culture and sensitivity in bed-ridden patients or patients with urinary tract obstruction.

•• Collection from infants:

It is usually collected by a clean plastic bag attached around the genitalia. For bacteriological examination urine is aspirated from bladder by needle.

PRESERVATION OF URINE

Urine sample should be examined within 2 hours of collection. If delay is likely to occur, it should be preserved by one of the following methods:

•• Refrigeration without any preservative.

•• Use of preservatives like:

  1. toluene (add few drops to form a thin layer on the urine surface),
  2. concentrated HCl (in the ratio of 1 mL for 125 mL of urine), 
  3. thymol (one crystal/100 mL), 
  4. chloroform (5 mL/100 mL) and 
  5. formaldehyde (add 1 drop for 15 mL of urine—preserves cells and casts).


EXAMINATION OF URINE

Urine examination consists of:

(1) physical examination,
(2) chemical examination and
(3) microscopic examination.

PHYSICAL EXAMINATION

Physical properties of normal urine are listed below :

  1. Volume___600–2000 mL/day
  2. Color___Yellow (straw to amber)
  3. Transparency___Clear
  4. Odor___Faintly aromatic
  5. pH___4.6–8
  6. Specific gravity___1.003–1.035 (adult random urine)

Volume

A healthy adult excretes about 600–2000 mL of urine in 24 hours. In infants, the volume is 300–600 mL/day. Volume is measured by collecting 24-hour urine samples in a measuring cylinder.

Polyuria

Increased urine output (more than 2 liters in 24 hours).

Causes

•• Physiological: Increased fluid intake.
•• Pathological
–– Diabetes mellitus (polydipsia—excessive intake of water) due to osmotic diuresis.
–– Chronic renal diseases—due to loss of concentrating power of kidney.
–– Diuretic therapy.
–– Diabetes insipidus—due to failure of secretion of antidiuretic hormone (ADH).
–– Primary aldosteronism.

Nocturia

Excretion of more than 500 mL of urine at night.

Oliguria

Decreased urinary output (less than 500 mL in 24 hours).

Causes

•• Restricted intake of fluid (e.g. fever).
•• Excessive loss of fluid, e.g. in hemorrhage, burns, dehydration and shock.
•• Renal diseases.
–– Acute glomerulonephritis
–– Nephrotic syndrome
–– Acute tubular necrosis as in shock, burns, crush syndrome, incompatible blood transfusion and heavy metal poisoning.
•• Addison’s disease.

Anuria

Markedly diminished urine output, usually less than 125 mL in 24 hours.

Cause

Renal ischemia, acute tubular necrosis (e.g. in shock, hemolytic transfusion reactions), complete urinary tract obstruction, tumors and renal stones.

Color

Normal urine is straw to amber colored due to the presence of urochrome pigment, excretion of which is generally proportional to the metabolic rate.
Conditions associated with color changes in urine:


  1. Colorless___Dilute urine as in polyuria (e.g. diabetes mellitus, diabetesinsipidus)
  2. Dark brown___Oliguria
  3. Smoky (red or redbrown)___Red blood cells (hematuria)
  4. Cola colored___Intravascular hemolysis
  5. Yellow-brown___Bilirubin
  6. Orange brown ___Urobilin/urobilinogen
  7. Dark colored on standing___Alkaptonuria (presence of homogentisic acid)
  8. Milky___Pus or chyle (chyluria)


Chyluria

Chyluria is a rare condition in which the urine contains lymph. Obstruction to lymph flow and rupture of lymphatic vessels into the renal pelvis, ureters, bladder or urethra may be associated with chyluria. The causes include filariasis, abdominal lymphadenopathy and tumors. The amount of lymph determines the color of urine which may range from clear to opaque or milky.

Lipiduria

In nephrotic syndrome, urine shows fat globules which are triglycerides (neutral fat) and cholesterol. It may also be observed in patients with bone fractures.

Transparency and Turbidity

Normal urine is usually clear when passed fresh. It may become cloudy, hazy or turbid with the presence of mucus, phosphates, pus, crystals, blood, casts or bacterial growth.

Odor

Normal urine has a faintly aromatic odor because of volatile acids.
Causes of different odor:

  1. Fruity (sweet)___Presence of acetone (ketonuria)-ketoacidosis
  2. Ammoniacal___Bacterial decomposition
  3. Mousy/musty___Phenylketonuria
  4. Putrid or foul___Severe urinary tract infectionFishy___UTI by Proteus

Reaction (pH)

Normal urine is usually acidic. However, urinary pH ranges from 4.6 to 8 (if pH is <7 = acidic, if pH is 7.0 = neutral and if pH is >7.0 = alkaline). Urinary pH is measured as given below.

•• Litmus paper

Technique: The reaction of urine is determined with blue and red litmus paper. The urine when examined must be fresh as it turns alkaline on standing due to bacterial decomposition.

–– Acidic urine turns blue litmus paper red
–– Alkaline urine turns red litmus paper blue
–– Both blue and red litmus papers turn reddish purple when the urine is faintly on the acid side (pH 7.0) or neutral.

•• pH indicator paper strips/strip multistix method uses a methyl red and bromthymol blue double indicator system which can measure a pH from 5 to 9.

Cause

Causes of alkaline and acidic urine

Alkaline urine

  1. •• Metabolic and respiratory alkalosis
  2. •• Phosphaturia
  3. •• Severe vomiting
  4. •• Infection with ammonia producing urea splitting bacteria like Proteus
  5. •• Potassium deficiency
  6. •• Hyperaldosteronism
  7. •• Intake of fruits of the citrus family (orange)
  8. •• Intake of bicarbonates

Acidic urine

  1. •• High protein diet
  2. •• Febrile illness
  3. •• Ketonuria (e.g. diabetes mellitus, starvation)
  4. •• Leukemia
  5. •• Urinary tract infection by Escherichia coli
  6. •• Starvation
  7. •• Severe diarrhea
  8. •• Diabetes mellitus
  9. •• Respiratory disease

Specific Gravity

Specific gravity depends on the number, density and weight of the solute particles in the urine. It is used as a measure of the concentrating power of the kidney. The specific gravity of urine is its density compared with the density of distilled water that is conveniently fixed as 1.000 at 20°C. 

Normal specific gravity of a 24-hour urine sample is 1.002–1.028, average being 1.018.

Specific gravity (SG) is measured by:

Fig. 1: Urinometer for specific gravity measurement


(1) urinometer,
(2) refractometer and,
(3) dipstick method.


Urinometer and refractometer methods are more accurate as compared to the dipstick method.

Urinometer Method:


Urinometer (Fig.1)  is a specialized hydrometer. It consists of a weighted glass cylinder which floats in urine and has a calibrated stem to measure specific gravity at a given temperature. 

The scale in many urinometers read in small divisions from 1.000 to 1060. It is usually calibrated in the range of 15° to 20°C (depends on the manufacturer). 

As the specific gravity varies with temperature, the reading obtained should be corrected to the room temperature by applying temperature correction.


•• Temperature correction: For every 3°C rise in room temperature beyond the calibrated temperature (15°C), add 0.001 to the recorded reading; and subtract 0.001 for every 3°C fall in temperature.


•• Correction for protein and sugar: Subtract 0.003 for every 1 g/dL of protein and 0.004 for every 1 g/dL of glucose.

Technique: 


The urine is poured into a wide necked vessel (cylinder), and the urinometer is floated so that it does not touch the sides of the container. The specific gravity is read from the graduations which lie at level with the true surface of the urine (lower meniscus). 

Observe the temperature of the urine. Check the temperature at which the urinometer is calibrated.


•• Technique when only small volumes of urine are available: 

The urine may be diluted with equal or double its volume of distilled water.
The specific gravity is calculated as follow:


–– Dilution with equal volume = Last two figures of the observed value × 2
–– Dilution with double volume = Last two figures of the observed value × 3.

Refractometer Method

It is based on refractive index of urine and has the advantage of requiring only a few drops of urine.

Dipstick Method:

Specific gravity is recorded by change of color on the strip and compared with the color chart on the multistix bottle.

Principle:

It is an indirect method for measuring specific gravity. 

The reagent area of dipstick has three main ingredients namely: polyelectrolyte, indicator substance and buffer. 

This method is based on the pKa (symbol for the quantitative measure of the strength of an acid in solution) change of the pretreated polyelectrolytes in relation to ionic concentration of the urine. When the ionic concentration is high, the pKa as well as pH is decreased. 

The indicator substance (bromothymol blue) then changes its color relative to ionic concentration and this is translated to specific gravity values. The value is not affected by high amounts of glucose, protein or radiographic contrast material in urine.


Causes

•• Increased specific gravity

–– Glycosuria (e.g. diebetes mellitus), proteinuria (e.g. nephrotic syndrome)
–– Dehydration: Restricted fluid intake, diarrhea, vomiting, fever, and excessive sweating.

•• Decreased specific gravity
–– Excessive fluid intake
–– Diabetes insipidus
–– End stage kidney
◆◆ Chronic glomerulonephritis
◆◆ Chronic pyelonephritis
◆◆ Bilateral polycystic kidneys
◆◆ Hypertension


•• Low and fixed specific gravity: 

When specific gravity is fixed at 1.010, this is known as isosthenuria. It is indicative of severe renal damage (chronic renal failure) with disturbance of both the concentrating and diluting abilities of the kidney.
Interpretation.

•• Specific gravity provides information about the renal status and hydration.

•• Specific gravity indicates the relative proportions of dissolved solid constituents to total volume of the urine. Urine concentration and dilution is modulated by the tubular absorptive and secretory functions thereby affecting the specific gravity.

Chemical constituents in normal urine

  1. Protein, albumin___Negative-trace
  2. Glucose___Negative
  3. Ketone bodies___Negative
  4. Bilirubin___Negative
  5. Urobilinogen___Trace
  6. Bile salts___Negative
  7. Blood___Negative