Page 686 - Small Animal Internal Medicine, 6th Edition

658 PART V Urinary Tract Disorders

CHAPTER 39
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Diagnostic Tests for the

Urinary System

GLOMERULAR FUNCTION also can increase BUN concentrations slightly. Some drugs
may increase BUN concentrations by increasing tissue
Evaluation of glomerular function is an essential part of the catabolism (e.g., glucocorticoids, azathioprine) or decreas-
diagnostic approach to patients with suspected renal disease ing protein synthesis (e.g., tetracyclines), but these effects
because the glomerular filtration rate (GFR) is directly are usually minimal. Conversely, BUN concentrations can
related to functional renal mass. Serum creatinine (SCr) and be decreased by low-protein diets, anabolic steroids, severe
blood urea nitrogen (BUN) concentrations are commonly hepatic insufficiency, or portosystemic shunting. These non-
used screening tests; symmetric dimethylarginine (SDMA) renal variables limit the usefulness of BUN as an indicator
and creatinine clearance are useful for patients with sus- of the GFR. Normal BUN concentrations are 8 to 25 mg/
pected renal disease who have normal BUN and SCr. Plasma dL in the dog and 15 to 35 mg/dL in the cat. Reagent test
clearance of radioisotopes and renal scintigraphy are strips (Azostix, Bayer, Elkhart, Ind) can be used to estimate
advanced techniques that may be used to determine GFR BUN levels in whole blood samples from dogs and cats, with
and obtain information about individual kidney function but relatively high sensitivity and specificity.
do not require urine collection. Iohexol clearance provides
an estimate of the GFR without the need of urine samples, SERUM CREATININE
radioisotopes, or specialized equipment. Evaluation of Creatinine is a nonenzymatic breakdown product of phos-
urinary protein excretion allows for the assessment of the phocreatine in muscle; the daily production of creatinine in
patient for glomerular disease (e.g., glomerulonephritis, glo- the body is determined largely by the muscle mass of the
merular amyloidosis). individual. Young animals have lower serum concentrations,
whereas males and well-muscled individuals have higher
BLOOD UREA NITROGEN concentrations. SCr concentration is not affected appreciably
Urea is synthesized in the liver via the ornithine cycle from by diet. Creatinine is not metabolized and is excreted by the
ammonia derived from amino acid catabolism. Amino acids kidneys almost entirely by glomerular filtration. Its rate of
used in the production of urea arise from the catabolism of excretion is relatively constant in the steady state, and SCr
exogenous (i.e., dietary) and endogenous proteins. Renal varies inversely with the GFR. Thus determination of creati-
excretion of urea occurs by glomerular filtration, and BUN nine clearance provides an estimate of the GFR.
concentrations are inversely proportional to the GFR. Urea Creatinine is measured by the alkaline picrate reaction,
is subject to passive reabsorption in the tubules, which which is not entirely specific for creatinine and measures
occurs to a greater extent at slower tubular flow rates during another group of substances collectively known as noncre-
dehydration and volume depletion. Thus urea clearance is atinine chromagens. These substances are found in plasma,
not a reliable estimate of the GFR and, in the face of volume in which they may constitute up to 50% of the measured
depletion, decreased urea clearance may occur without a creatinine at normal serum concentrations but normally do
decrease in GFR. not appear in urine. As SCr increases because of the progres-
The production and excretion of urea do not proceed at a sion of renal disease and decreasing GFR, the amount of
constant rate. Urea production and excretion increase after a noncreatinine chromagens is unchanged and contributes
high-protein meal, and an 8- to 12-hour fast is recommended progressively less to the total measured SCr. Normal SCr in
before measuring BUN concentrations to avoid the effect of the dog and cat are 0.3 to 1.3 mg/dL and 0.8 to 1.8 mg/dL,
feeding on urea production. Gastrointestinal bleeding can respectively. Greyhounds have slightly higher SCr than non-
increase BUN concentrations because blood represents an Greyhounds; this difference is attributable to increased
endogenous protein load. Clinical conditions characterized muscle mass and not to any decrease in the GFR. Relatively
by increased catabolism (e.g., starvation, infection, fever) wide reference intervals and assay imprecision can result in

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