1.
Color
Fresh urine
has an amber yellow color due to the presence of yellow urochorme
and pink uroerythrine. The color intensity depends on
concentration and amount of urine that in turn depends on water intake and its extrarenal
output. The early morning urine is usually more concentrated and hence darker
than later samples. Some pathological conditions or intake of certain exogenous
substances changes the color of urine. In general, acidic urine is more highly
colored than alkaline urine.
Some causes of
abnormal urine coloration
a. Cloudy
(1) Pathologic
causes: phosphaturia, pyuria, chyluria, lipiduria, hyperoxaluria
(2) Food
and drug causes: diet high in purine–rich foods (hyperuricosuria)
b. Brown
(1) Pathologic
causes: Bile pigments, myoglobin
(2) Food
and drug causes: Fava beans, levodopa (larodopa, metronidazole (flagyl),
nitrofurantoin (Furadantin), some antimalarial agents
c. Brownish–black
(1) Pathologic
causes: bile pigments, melanin, methemoglobin
(2) Food
and drug causes: cascara, levodopa, methyldopa (aldomet), senna
d. Green
or blue
(1) Pathologic
causes: Pseudomonal UTI, biliverdi
(2) Food
and drug causes: amitriptyline (Elavil), Indigo carmine, IV cimetidine
(Tagarnet), IV promethazine (Phenergan), methylene blue, triamterene (Dyrenium)
e. Orange
(1) Pathologic
causes: Bile pigments
(2) Food
and drug causes: phenothiazines, phenozopyridine (pyridium)
f.
Red
(1) Pathologic
causes: hematuria, hemoglobinuria, myoglobinuria, porphyria
(2) Food
and drug causes: beets, blackberries, rhubarb, phenolphthalein, rifampin
(rifadin)
g. Yellow
2.
Clarity
Fresh urine is
usually without any turbidity. Cloudiness that develops after a long standing
of a urinary sample is caused by epithelial cells; and is without pathological
significance. Turbidity of fresh urine can occur due to presence of bacteria,
leukocytes, lipids, phosphates, carbonates, uric acid, leucine, tyrosine and
oxalates. Chemical or microscopic examination of urine can differentiate among
these causes of turbidity.
3.
Specific
gravity
Specific
gravity of the urine is technically its weight compared to an equal volume of
water. Urinary specific gravity (USG) correlates with urine osmolality and
gives important insight into the patient’s hydration status. It also reflects
the concentrating ability of the kidneys. Normal USG can range from 1.003 to
1.030; a value of less than 1.010 indicates relative hydration, and a value
greater than 1.020 indicates relative dehydration. Increased USG is associated
with glycosuria and the syndrome of inappropriate antidiuretic hormone;
decreased USG is associated with diuretic use, diabetes insipidus, adrenal
insufficiency, aldosteronism, and impaired renal function. In patients with
intrinsic renal insufficiency, USG is fixed at 1.010 – the specific gravity of
the glomerular filtrate.
Osmolality of
urine depends on the amount of osmotically active particles excreted into
urine, regardless of their mass, size and electric charge. The osmolality is
expressed in mmol/kg. It is only loosely proportional to the specific gravity
of urine. Measurement of osmolality is considered more accurate compared to the
specific gravity and therefore favored.
Comparing both
parameters, it can be stated that osmolality reflects the molar concentrations
of all dissolved substances, whereas the specific gravity is related to their
mass concentrations. Therefore, the osmolality will be much affected by changes
in concentrations of low–molecular–weight substances such as Na+,
glucose and urea. On the other hand, presence of protein in urine will affect
predominantly the specific gravity.
Normal value
of urinary osmolality under condition of ordinary water intake is 300 – 900
mmol/kg.
Ordinarily,
the higher is the volume of urine, the lower is its specific gravity (diluted
urine); and vice versa, i.e. in low diuresis the specific gravity of urine
increases. But, in diabetes mellitus a high volume of urine with a high
specific gravity is produced.
The specific
gravity enables assessment of concentration ability of the kidneys. Values
above 1.020 indicate good renal function and ability of kidney to excrete
excess of solutes. Highly concentrated urine suggests a substantial decrease in
the circulating blood volume.
Inability of
kidney to concentrate urine is called hyposthenuria. The patient
needs more water to excrete the same amount of solutes. Extremely diluted urine
can be a sign of impaired kidney concentration ability, such as in cases of
diabetes insipidus (lack of ADH), or side effects of some drugs. Combination of
hyposthenuria with polyuria indicates damage to the renal tubular system with
relatively intact glomerular filtration. A serious sign of kidney damage is isosthenuria.
The kidneys lose any ability to concentrate or dilute; and excrete urine of the
same specific gravity as the glomerular filtrate. The relative specific gravity
remains permanently rather low, around 1.010. Simultaneous finding of
isosthenuria and oliguria indicates a severe renal insufficiency. Elevation of
the urinary relative specific gravity – hypersthenuria – results
from proteinuria or glycosuria.
Method of
estimation of specific gravity
a.
Urinometer
The widely
used urinometers are calibrated for temperature at 15oC, because
this temperature roughly corresponds to the temperature of urine standing one
hour at room temperature. When used at different temperature, the values must
be corrected: for every 3oC above the calibrated temperature 0.001
is added and vice versa. A urine sample of volume at least 10 – 15 ml is needed
for this examination.
b.
Refractometer
Refractometer
measures the urine density on the basis of the index of light refraction.
Compared to the urinometer, it offers several advantages: only 1 – 2 drops of
urine suffice for the examination; and no correction for temperature is needed.
c.
Diagnostic
strip
The strip
indication zone contains a suitable polyelectrolyte acting as an ion exchanger
and bromthymol blue as an acid–base indicator. The diagnostic strip function is
based on the exchange of urinary cations, especially Na+, K+
and NH4+, for ion H+ of the polyelectrolyte in
the indication zone. The released H+ acidifies weakly buffered acid–base
indicator, originally in alkalilne (unprotonated) form. Acidification changes the
color of the bromthymol blue. A disadvantage of this system is that the strips
are insensitive to presence of urinary substances non–electrolytic in nature
such as glucose, proteins, urea, creatinine and some others.
4.
Odor
The normal
odor of urine is described as urinoid; this odor can be strong in concentrated
specimens but does not imply infection. Diabetic ketoacidosis can cause urine
to have a fruity or sweet odor, and alkaline fermentation can cause an
ammoniacal odor after prolonged bladder retention. Persons with UTIs often have
urine with a pungent odor. Other causes of abnormal odors include
gastrointestinal–bladder fistulas (associate with a fecal smell), cysteine
decomposition (associated with sulfuric smell), and medications and diet (e.g.,
asparagus).
Varieties in
urine odor
a. Ammoniacal
(1) Due
to the presence of bacteria producing urease, an enzyme catalyzing
decomposition of urea to ammonia and water
b. Acetone
(1) Smells
like overripe apples which is due to excretion of acetone in ketoacidosis
c. Maple
syrup or “Maggi” spice
(1) Due
to branched chain carboxylic oxoacids (2–oxoisocapronic, 2– oxoisovaleric
acids)
d. Hydrogen
sulfide or even putrescent
(1) Bacterial
decomposition of proteins that releases H2S from sulfur–containing
amino acids.
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