Page 690 - Small Animal Internal Medicine, 6th Edition

662 PART V Urinary Tract Disorders

urine collection, and the U Pr /U Cr has been shown to be highly addition of RBCs to canine urine. Semiquantitative tests for
correlated with 24-hour urine protein excretion in dogs and microalbuminuria have relatively high specificity (92% in
VetBooks.ir cats. Its utility lies in the fact that whereas both urine creati- dogs, 82% in cats) but low sensitivity (37% in dogs, 43% in
cats) for differentiating healthy animals from those with sys-
nine and protein concentrations are affected by total urine
solute concentration, their ratio is not. According to Inter-
ingly normal dogs and cats with microalbuminuria are at
national Renal Interest Society (IRIS) guidelines, dogs with temic diseases. It remains to be determined whether seem-
U Pr /U Cr < 0.2 are considered nonproteinuric, those with U Pr / increased risk for the development of progressive renal
U Cr 0.2–0.5 are considered borderline proteinuric, and those disease. Sequential monitoring of animals with documented
with U Pr /U Cr > 0.5 are considered proteinuric. In cats the microalbuminuria probably is warranted until its prognostic
guidelines are < 0.2 (nonproteinuric), 0.2–0.4 (borderline value in otherwise normal animals can be determined.
proteinuric) and > 0.4 (proteinuric).
In general, U Pr /U Cr results are not affected by differences BLADDER TUMOR ANTIGEN TEST
in sex, method of urine collection, fasted versus fed states, The first-generation bladder tumor antigen (BTA) test is a
or by time of day of collection. Urine samples collected at qualitative latex agglutination dip strip test run on voided
home may have somewhat lower U Pr /U Cr than samples col- urine that detects a glycoprotein antigen complex associ-
lected in the hospital. The U Pr /U Cr should be determined ated with bladder neoplasia. The BTA test has high sensitiv-
within 2 to 4 hours of sample collection because values may ity but low specificity for the detection of transitional cell
increase after 12 hours at room temperature. Pyuria and carcinoma in dogs. False positives occur in urine samples
marked blood contamination of urine samples have the with marked proteinuria or glucosuria and in those with
potential to affect the U Pr /U Cr . Consequently, the urine pyuria or hematuria. The high sensitivity of the test and
protein concentration should be evaluated in conjunction its less reliable performance in the presence of pyuria and
with urinary sediment findings, and ideally the U Pr /U Cr hematuria suggest that its most appropriate use would be as a
should be evaluated only if the patient does not have pyuria. routine screening test to rule out transitional cell carcinoma
In a recent study, 25% of apparently healthy geriatric dogs in geriatric dogs. Second- and third-generation BTA and
had U Pr /U Cr results in the borderline proteinuric or protein- other tests that use monoclonal antibodies against human
uric range as defined by IRIS guidelines. A high degree of bladder tumor antigens give false-negative results in dogs
overlap is found between dogs with glomerulonephritis and and should not be used.
those with amyloidosis with regard to their 24-hour urine
protein excretion and U Pr /U Cr . Thus renal biopsy remains the
only reliable way to differentiate these two diseases. Values TUBULAR FUNCTION
for 24-hour urine protein excretion and U Pr /U Cr are pre-
sented in Table 39.1. The kidney is an organ of water conservation. Depending on
the needs of the animal, the kidney can produce urine that
MICROALBUMINURIA is highly concentrated or very dilute. Normal urinary con-
In humans, microalbuminuria is defined as excretion of 30 centrating ability is dependent on the ability of hypothalamic
to 300 mg/day of albumin in urine, and its presence may be osmoreceptors to respond to changes in plasma osmolality,
an early indicator of vascular endothelial damage. Microal- release of antidiuretic hormone (ADH) from the neurohy-
buminuria is an established risk factor for progression of pophysis, and response of the distal nephron to ADH. In
renal disease in human patients with diabetes mellitus and addition, medullary hypertonicity must be generated and
may be predictive of progressive renal disease in those with maintained by the countercurrent multiplier and exchanger
essential hypertension. Microalbuminuria in dogs and cats systems of the kidney, and an adequate number of functional
has been defined as a urine albumin concentration of 1 to nephrons must be present to generate the appropriate
30 mg/dL, and urine albumin concentrations in this range response to ADH. Laboratory tests of tubular function are
can be detected by antigen capture enzyme-linked immuno- summarized in Table 39.2.
assay. Microalbuminuria can be detected in 15% to 20% of
otherwise healthy dogs and cats, and its prevalence increases URINE SPECIFIC GRAVITY
with increasing age (e.g., approximately 7% in dogs < 3 years AND OSMOLALITY
but 49% in dogs ≥ 12 years). Its prevalence in a hospital Total urine solute concentration is measured by urine specific
population of dogs was 36%. The effect of pyuria on micro- gravity (USG) or urine osmolality (U Osm ). Urine osmolality
albuminuria is variable. Many urine samples of dogs with depends only on the number of osmotically active particles,
pyuria have a negligible urine albumin concentration regardless of their size. USG is defined as the weight of a
(<1 mg/dL) and a normal U Pr /U Cr ratio (<0.4). The addition solution compared with an equal volume of distilled water.
of blood to canine urine samples does not result in a urine It is dependent on the number and molecular weight of
albumin concentration more than 1 mg/dL until the urine is the solute particles but has the advantage of requiring only
grossly pink or red and more than 250 red blood cells simple, inexpensive equipment for measurement.
(RBCs)/high-power field (hpf) are observed in the urine Normally, urine is composed of solutes of relatively low
sediment. U Pr /U Cr ratios generally do not exceed 0.4 with the molecular weight (e.g., urea, electrolytes), and a roughly