Page 692 - Small Animal Internal Medicine, 6th Edition

664 PART V Urinary Tract Disorders

GRADUAL WATER DEPRIVATION preferred because it prevents contamination of the sample
Gradual water deprivation can be used to eliminate diagnos- by the urethra or genital tract, it is simple to perform when
VetBooks.ir tic confusion caused by medullary solute washout. The the bladder is palpable, there is negligible risk of introducing
infection, and it is well tolerated by both dogs and cats. In
owner can be instructed to restrict water consumption to
60 mL/lb/day 72 hours before, 45 mL/lb/day 48 hours before,
it may be helpful first to evaluate a sample collected by
and 30 mL/lb/day 24 hours before the scheduled water animals presented for the evaluation of hematuria, however,
deprivation test. In dogs with psychogenic polydipsia, this voiding because other methods of urine collection may add
will promote the endogenous release of ADH, increased per- RBCs to the sample as a result of trauma.
meability of the inner medullary collecting ducts to urea, and When performing the urinalysis, fresh urine should be
restoration of the normal gradient of medullary hypertonic- examined whenever possible. Refrigerated urine should be
ity. An alternate approach is to instruct the owner to decrease warmed to room temperature before performing the uri-
water consumption by approximately 10%/day over a 3- to nalysis. Note how the sample was collected because this may
5-day period (but not < 30 mL/lb/day). Gradual water depri- influence interpretation. Urinalysis is divided into three
vation should only be used in animals that are otherwise parts—physical properties, chemical properties, and sedi-
healthy on initial clinical evaluation, and the owner should ment evaluation.
provide dry food ad lib and weigh the dog daily to monitor
for loss of body weight. During the period of gradual water
restriction, the water provided each day should be divided PHYSICAL PROPERTIES OF URINE
into several aliquots that can be given to the dog throughout Appearance
the 24-hour time period to prevent the dog from drinking Normal urine is yellow because of the presence of urochrome
all of its daily allotment at once. pigment. Very concentrated urine may be deep amber in
color, whereas very dilute urine may be almost colorless. A
FRACTIONAL CLEARANCE red or reddish-brown color usually is caused by RBCs,
OF ELECTROLYTES hemoglobin, or myoglobin, whereas a yellow-brown to
The extent to which electrolytes appear in the urine is the yellow-green color may be caused by bilirubin; urine con-
net result of tubular reabsorption and secretion. The frac- taining bilirubin appears concentrated. Normal urine usually
tional clearance of electrolytes (FC x ) can be used to evaluate is clear. Cloudy urine often contains increased cellular ele-
tubular function. Fractional clearance is defined as the ratio ments, crystals, or mucus. The most common abnormal odor
of the clearance of the electrolyte in question (U x V/P x ) to that is ammoniacal and is caused by the release of ammonia by
of creatinine (U Cr V/P Cr ): urease-producing bacteria.
)
FC x = ( UV P x ) ( U VP Cr = ( UP Cr ) ( UP x ) Specific Gravity
Cr
x
x
Cr
USG is a reflection of the total solute concentration of urine,
This ratio usually is multiplied by 100 and the fractional and the amount of any substance in urine must be inter-
clearance value expressed as a percentage. The advantage of preted in light of the specific gravity. For example, 4+ protein
this measurement is that a timed urine collection is not in 1.010 urine represents more severe proteinuria than 4+
necessary. In normal animals, the fractional clearances of all protein in 1.045 urine. Refractometry is the best technique
electrolytes are much less than 1.0 (100%), implying net con- for estimation of USG in general practice. Dip strip methods
servation, but values are higher for potassium and phospho- to estimate USG are unreliable in dogs and should not be
rus than for sodium and chloride. Unfortunately, fractional used. USG should be determined before any treatment
excretion values calculated from so-called spot urine samples because fluids, diuretics, or glucocorticoids may alter the
are highly variable and do not correlate well with values specific gravity.
calculated using 72-hour urine samples.
The fractional clearance of sodium may be useful in the
differentiation of prerenal and primary renal azotemia. In CHEMICAL PROPERTIES OF URINE
animals with prerenal azotemia and volume depletion, pH
sodium conservation should be avid and the fractional clear- Urine pH varies with diet and acid–base balance. Normal
ance of sodium very low (<1%). On the other hand, in urine pH of dogs and cats is 5.0 to 7.5. Causes of acidic urine
animals with azotemia caused by primary parenchymal renal pH include meat protein–based diet, administration of acidi-
disease, the fractional clearance of sodium will be higher fying agents, metabolic acidosis, respiratory acidosis, para-
than normal (>1%). Normal values for urinary fractional doxic aciduria in metabolic alkalosis, and protein catabolic
clearance of electrolytes are summarized in Table 39.2. states. Causes of alkaline urine pH include urinary tract
infection (UTI) by urease-positive bacteria, plant protein-
URINALYSIS based diet, urine allowed to stand exposed to air at room
temperature, postprandial alkaline tide, administration of
Urine for urinalysis may be collected by voiding (midstream alkalinizing agents, metabolic alkalosis, respiratory alkalosis,
sample), catheterization, or cystocentesis. Cystocentesis is and distal renal tubular acidosis. Dip strip methods to