Page 691 - Small Animal Internal Medicine, 6th Edition

CHAPTER 39 Diagnostic Tests for the Urinary System 663

TABLE 39.2 dehydrated but has dilute urine has already failed the test
and should not be subjected to water deprivation. In such
VetBooks.ir Normal Values for Tests of Renal Tubular Function in an animal, failure to concentrate urine likely is caused by
structural damage, renal dysfunction, or administration of
Dogs and Cats
TEST DOG CAT drugs that interfere with urinary concentrating ability (e.g.,
glucocorticoids, diuretics). The water deprivation test also
is contraindicated in animals that are azotemic. It should
Random urine specific 1.001-1.070 1.001-1.080
gravity be performed with extreme caution in animals with severe
Urine specific gravity 1.050-1.076 1.047-1.087 polyuria because they may rapidly become dehydrated
during water deprivation if they have a defective urinary
after 5%
dehydration concentrating ability.
At the beginning of the water deprivation test, the bladder
Urine osmolality after 1787-2791 1581-2984 must be emptied and baseline data collected—body weight,
5% dehydration
(mOsm/kg) hematocrit, plasma proteins, skin turgor, serum osmolality,
Urine-to-plasma 5.7 : 1-8.9 : 1 Not available urine osmolality, and USG. Water is then withheld and these
parameters monitored every 2 to 4 hours. Urine and serum
osmolality ratio after
5% dehydration osmolalities are the best tests to follow, but osmolality results
are often not immediately available to the clinician. Thus
Fractional Electrolyte Excretion (%) USG and body weight assume the greatest importance for
Sodium <1 <1 decision making during performance of the test. An increase
Potassium <20 <24 in the total plasma protein concentration is a relatively reli-
Chloride <1 <1.3 able indicator of progressive dehydration, but increases in
hematocrit and changes in skin turgor are not reliable. SCr
Phosphate <39 <73 and BUN concentrations should not increase during a prop-
erly conducted water deprivation test.
Maximal stimulation of ADH release will be present
linear relationship exists between urine osmolality and after loss of 5% of body weight. The test is concluded when
specific gravity. The range of urine osmolality correspond- the patient demonstrates adequate concentrating ability or
ing to a given USG value, however, may be relatively becomes dehydrated, as evidenced by loss of 5% or more
wide. If the urine contains appreciable amounts of larger of its original body weight. Use the same scale each time
molecular weight solutes, such as glucose, mannitol, or when weighing the animal, and empty the bladder at each
radiographic contrast agents, these substances will have evaluation.
a proportionally greater effect on specific gravity than on The time required for dehydration to develop during
osmolality. water deprivation varies. Dehydration usually becomes
The term isosthenuria (USG, 1.007–1.015; U Osm , evident within 48 hours in normal dogs and cats but, rarely,
300 mOsm/kg) refers to urine of the same total solute con- a longer period of time may be required. Dogs with dia-
centration as unaltered glomerular filtrate. The term hypos- betes insipidus and psychogenic polydipsia usually become
thenuria refers to urine of a lower total solute concentration dehydrated after a much shorter period of water depriva-
than glomerular filtrate (USG < 1.007, U Osm < 300 mOsm/ tion (<12 hours). By the time dehydration is evident, USG
kg). Although rarely used clinically, the term hypersthenuria usually exceeds 1.045 in normal dogs and cats. Failure to
(baruria) refers to urine of a higher total solute concentra- achieve maximal urinary solute concentration does not
tion than glomerular filtrate (USG > 1.015, U Osm > 300 mOsm/ localize the level of the malfunction, and a structural or
kg). The normal range of total urine solute concentration for functional defect may be present anywhere along the
dogs and cats is wide (USG, 1.001–1.080). Samples obtained hypothalamic-pituitary-renal axis. Furthermore, animals
in the morning have higher USG values than those obtained with medullary solute washout may have impaired concen-
in the evening, and urine concentration decreases with age, trating ability, regardless of the underlying cause of polyuria
but there is no effect of sex on USG. Normal young puppies and polydipsia.
have a lower USG than do older dogs. If there has been less than a 5% increase in urine osmolal-
ity or less than a 10% change in USG for three consecutive
WATER DEPRIVATION TEST determinations, or if the animal has lost 5% or more of its
The water deprivation test is a useful test of tubular func- original weight, 0.2 to 0.4 U/kg aqueous vasopressin
tion and is indicated for the evaluation of animals with (Pitressin), up to a total dose of 5 U, or 5 µg desmopressin
confirmed polydipsia and polyuria, the cause of which (DDAVP) may be given subcutaneously and parameters of
remains undetermined after initial diagnostic evaluation. It urinary concentrating ability monitored 2 to 4 hours after
usually is performed in animals with hyposthenuria (USG ADH injection. Any additional increase in urine osmolality
< 1.007) suspected to have central or nephrogenic diabe- after the administration of ADH should not exceed 5% to
tes insipidus or psychogenic polydipsia. An animal that is 10% in normal dogs and cats.