Urinalysis

 

Routine urinalysis (or urine analysis) is the oldest of all laboratory tests.  We know that the Greeks from at least 500 BC used the examination of urine as an important component of their diagnostic process.  In Europe, from the middle ages, urine testing was a key aspect of a physicians’ art.

Today, routine urinalysis remains an important diagnostic procedure.  It consists of two parts: chemical tests and a microscopic examination. The urine is also examined to assess its colour, appearance and odour.

   Dipstick picture

The chemical tests are performed using a plastic strip which has a series of absorbent paper pads glued to it.  Each pad contains chemicals that react with substances in the urine.  The chemical mixture is cleverly arranged so that the substance being measured will change the color of the pad.  The intensity of the color increases in proportion to the amount of substance.

The results of urinalysis are not totally specific but they provide clues regarding disorders that should be followed up with additional testing.

 

Reference Ranges: 

Specific gravity

1.010 – 1.030

Colour and appearance

Straw, clear

Odour

None, Mild

Dipstick

§         pH

4.8 – 7.5

§         Protein

Trace

§         Glucose

Trace

§         Ketones

None

§         Hemoglobin

None

§         Bilirubin

None

§         Urobilinogen

None

§         Leukocyte esterase

Negative

§         Nitrite

Negative

Urinalysis Interpretation

Specific Gravity
The specific gravity is a measure of the weight of urine compared to an equal amount of water.  The specific gravity will always be greater than 1.00 and will increase as more materials are dissolved in the urine.  The value changes throughout the day depending on fluid intake. A dilute urine will have values less than 1.010.  A concentrated urine will have values over 1.020.

Color and Appearance

Turbidity:        Turbidity is usually due to crystal formation in the bladder (due to high concentration of solutes or unusual pH) or in the collection vessel due to reduction in temperature when the specimen stands. Turbidity is rarely significant.  Turbidity due to infection is, of course, pathologic.

Odour:    Normal urine has a mild, characteristic odour due to urea breaking into ammonia. A pungent odour may be due to a urinary tract infection or an old specimen. Some disorders have characteristic smells: Maple Syrup Urine disease, Phenylketonuria, Isovaleric acidemia, methenamine malabsorption, and eating asparagus.

Colour:  Urine is generally some shade of “straw” due to small amounts of biliary pigment.  Increased yellow can be due to bilirubin or urobilin, carrots, rhubarb, cascara, fluorescein, nitrofurantoin, phenacetin, picric acid, or quinacrine.

Other unusual colours may be observed.

These are tabulated below.

Colour

Cause

Red

Blood

Hemoglobin

Myoglobin

Porphyrins

Indicator dye such as phenolphthalein (Exlax)

Beets

Blackberries

Rhubarb, anthraquinone laxatives, some diagnostic dyes

Other: acetophenetidin, acrolei, aminopyrine, anisindione, antipyrine, rhubarb, benzene, BSP, cascara, chincophen, crysarobin, Congo Red, crayon pigment, danthron, ethoxazene, mercurochrome, phenazopyridine, deferoxamine, indanediones, phenidione, emodin, phenothiazines, PSP, phenytoin, senna

Brown

Bile, bilirubin

Melanin

All causes of black urine

Pyridium

 

Orange

Pyridium, Serenium

Anisindione, gantrisin, azogantrisin, ethoxazone, indanediones, mannose, phenothiazines, nitrofurantoin, pyridium, rifampin, rhubarb, carrots, senna

Blue (blue-green)

Methylene blue, other dyes and medications, Doan’s pills, Evans Blue, dithiazine, nitrofurantoin

Pseudomonas

Green-Blue-green-blue

Acrifavine, amitriptyline, methocarbamol, anthraquinone, azuresin, creosote, Doan’s pills, Clorets, methylene blue, nitrofurantoins, phenols, phenylsalicylate, resorcinol, tetralin, thymol, tolonium, triamterene, Vitamin B complex

Bilirubin

Green

Methylene blue, other dyes

Food additives

Black

Homogentisic Acid (Alkaptonuria)

Melanin (Malignant melanoma)

Porphyrin (Acute intermittent porphyria)

Myoglobin (Rhabdomyolysis)

Urobilinogen

Hemoglobin

Aloin, cascara, chloroquine, cresol, furazolidone, gentisic Acid iron salts, metronidazole, nitobenzene, nitrofurantoin, phenols, rhubarb, sulphamethoxazole, sulphonamides

Indican, metronidazole, cascara, iron-sorbitol –citric acid complex, methyldopa (converted to melanin), levodopa, methocarbamol, naphthol, phenols, pyrogallol

pH   Usually 4.8 – 7.5 (slightly acid because the body is an acid producer)  

Protein
Normally 50 – 150 mg of protein is produced per day (20-50% is albumin)

Increases: Proteinuria (see separate discussion), orthostatic proteinuria, exercise induced proteinuria

False Positive: Highly alkaline urine, quaternary ammonium compounds, detergents, high specific gravity (>1.030)

False lowering: High salt content, very dilute urine (<1.010)

Glucose
A small amount of glucose may “spill” normally into the urine.

Glucose will increase linearly in urine when the plasma value exceeds 10 mmol/L

“Spill” will occur at a lower threshold in Proximal Tubular disease and pregnancy

False Positives: peroxidase, oxidizing detergents, hypochlorite

False Negatives: ascorbic acid, 5HIAA, homogentisic acid, aspirin, levodopa, ketones, high specific gravity with low pH

Ketones
Ketones are normally not present but can be produced by starvation (prolonged fasting) and Diabetes Mellitus.

False Positives: levodopa, phthalein dyes, phenylketones, phenazopyridine, phenformin, paraldehyde-ethanol combination, aspirin (causes a false reaction and also causes real ketosis)

Hemoglobin and Red Cells
Blood and hemoglobin can be found in the urine due to : intravascular hemolysis, renal glomerular disease, lower urinary tract bleeding (e.g. Menses, Prostate, UTI, calculi, tumour (particularly bladder)). 50% of cases will have no known cause.

False Positives: myoglobin, oxidizing agents, peroxidases

False Negatives: ascorbic acid, nitrite, protein, pH<5.0, high specific gravity

Trace amounts of hemoglobin are considered a normal finding in the urine but may be significant if consistently present.

Bilirubin
A small amount of bilirubin (conjugated) will find its way into the urine

Increases are associated with liver disease and with hemolytic disease

Bilirubin degrades rapidly in the collection vessel

False Positives: Due to medication colour

False Negatives:  Ascorbic acid, nitrite, and old specimens

Urobilinogen
Urobilinogen gets into the urine after bile is metabolized in the gut and reabsorbed. It is therefore increased with most liver diseases and with hemolysis.

It is labile and will disappear from aged specimens

False Positives: due to medication colour, p-aminosalicylic acid, antipyrine, aponalide, chlorpromazine, phenazopyridine, phenothiazine, sulphadiazine, sulphonamide

False Negatives: Aged specimens, nitrite, formalin

Leucocyte esterase
Certain White Blood Cells contain leucocyte esterase enzyme. If this is present in urine then the presence of WBCs and thus infection can be inferred.

False Positives: oxidizing detergents, strong yellow pigments may give false reading

False Negatives: glucose, protein, increased specific gravity, oxalic acid, gentamycin, tetracyclin, cephalexin, cephalothin

Nitrite
Nitrate is excreted in the urine as a byproduct of vegetable ingestion. Certain bacteria convert the nitrate to nitrite which, if present, infers the presence of bactiuria.

Not all bacteria produce nitrite

False Positives: Medication colour, not fresh urine (skin contaminating bacteria multiply in the specimen container)

False Negatives: Bacteria that do not produce nitrite, voided urine that has not remained in bladder long enough to multiply, ascorbic acid, antibiotics, lack of vegetables in the diet (no nitrate substrate)

Microscopic Examination of Sediment
Following the dipstick examination, the urine may be centrifuged and the solid material (sediment) is examined under a microscope.  This is generally not necessary for outpatients with no clinical condition that would warrant further investigation who also have a negative dipstick examination.

The sediment may contain cells (red, white, urinary tract cells), microorganisms, casts, crystals and miscellaneous other solids. The significance of these is discussed below:

Red Blood Cells:
Normally between 0 – 3 RBCs are seem per HPF (HPF means “high power field” or what can be seen in one view of the slide through the microscope)

Above 8 /HPF is certainly abnormal

RBCs are present after passage through the glomerulus (glomerulonephritis),  bleeding of the urinary tract (infections, tumours, calculi, prostatic disease), direct bleeding within the renal parechyma (trauma, intravascular bleeding diatheses), menstrual contamination

The presence of RBCs with a negative dipstick for hemoglobin is occasionally seen and may be due to: failure of RBCs to lyse (or break up the red cells to release hemoglobin-- the pad contains a lysing agent), ascorbic acid or other oxidant.

White Blood Cells
Normally not more than 3 – 5 WBC/HPF are seen.

Greater than 5/HPF may be due to infection or inflammation within the urinary tract

Epithelial Cells

Epithelial cells of the urinary tract may desquamate and be seen in the sediment

There is probably no significance to moderate numbers of squamous or transitional cells, except they often accompany a poorly collected specimen

Renal tubular cells are always abnormal

Microorganisms
Bacteria are not normally present in properly collected urine that is examined without delay.

Urine allowed to stand at room temperature may breed bacteria.

The presence of bacteria in a properly treated specimen correlates strongly with urinary tract infection.

Yeast (Candida albicans) may be seen

Casts
Casts are collections of protein and other debris that have coalesced within the kidney tubule. They are of various kinds. Their size relates to their location of formation.

Crystals
Crystals form as the result of precipitation of inorganic salts.  Small numbers of crystals are normal and large numbers may form after collection. Most do not have clinical significance.

Other
Also seen in urine are mucous strands (normal finding but increased in UTI), sperm, foreign matter