Proteinuria is routinely detected by semi-quantitative methods, including the dipstick colorimetric test and the sulfosalicylic
turbidimetric test. Proteinuria detected by these semi-quantitative methods should always be interpreted in light of the urine
specific gravity. For example, a 2+ proteinuria with a 1.010 urine specific gravity is suggestive of a much greater urine
protein loss on a 24-hour basis than is the same 2+ proteinuria with a 1.040 urine specific gravity. Because the urine protein
concentration is frequently increased in animals with lower urinary tract inflammation or hemorrhage, proteinuria should also
be assessed in the context of urine sediment changes indicative of inflammation or hemorrhage (e.g., bacteria and increased
numbers of white and red blood cells and epithelial cells in the urine sediment). The occurrence of persistent proteinuria
with normal urine sediment analyses (an exception may be the presence of hyaline casts) is suggestive of glomerular disease.
When glomerular proteinuria is suspected, urine protein excretion should be quantified. This helps evaluate the severity of
renal lesions as well as assess the response to treatment or the progression of disease. The urine protein/creatinine ratio
(UP/C) in canine and feline urine samples has been shown to accurately reflect the quantity of protein excreted in the urine
over a 24-hour period and has greatly facilitated the diagnosis of glomerular disease in small animals. A UP/C of less than
1 is considered normal in dogs and cats, although in several studies normal animals have UP/Cs less than 0.2-0.5. A complete
urinalysis should always be performed before or along with determination of the UP/C, because hematuria or pyuria may indicate
the presence of significant nonglomerular proteinuria. If there is evidence of inflammation (e.g., pyuria, bacteriuria or
hematuria), the protein concentration should be measured again after successful treatment of the inflammatory disorder. The
UP/C cannot be used to differentiate glomerular proteinuria from proteinuria associated with lower urinary tract inflammation
Recently an antigen capture ELISA test for the detection of low levels of albumin in canine urine (microalbuminuria) has become
commercially available (E.R.D.-Screen, Heska Corporation).
Microalbuminuria is defined as a urine albumin concentration between 1.0 and 30 mg/dl. These are concentrations too small
to be routinely detected by standard dipstick screening tests. It is interesting to note that the presence of microalbuminuria
has been shown to be an accurate predictor of subsequent renal disease in human beings with both systemic hypertension and
diabetes mellitus and it has also been observed in human beings with systemic diseases that are associated with glomerulopathy.
Studies in dogs have shown the prevalence of microalbuminuria in apparently healthy dogs and those seeking veterinary care
to be 19 percent and 36 percent, respectively. In soft-coated wheaten terriers genetically predisposed to developing glomerular
disease, the prevalence of microalbuminuria was 76 percent. In another study, microalbuminuria developed in 100 percent of
dogs with experimentally induced heartworm disease.
Further study is necessary to determine if microalbuminuria is an accurate predictor of overt proteinuria and renal disease
in dogs and cats. If microalbuminuria does predict overt proteinuria and/or renal disease, this early detection tool should
significantly increase our ability to alter renal disease progression.
Urine concentrating ability
The kidneys maintain body fluid composition and volume by resorbing water and solutes from the glomerular filtrate. The resorption
of solute in excess of water results in the formation of dilute urine.
Conversely, the resorption of water in excess of solute results in the formation of concentrated urine. For concentrated urine
to form, antidiuretic hormone (ADH) must be produced and released, and the renal tubules must be responsive to the ADH. For
the latter to occur, renal medullary hypertonicity must be present and at least one third of the total nephron population
must be functional. The animal's hydration status, serum urea nitrogen and creatinine concentrations, and current medications
must be known in order to correctly interpret random urine specific gravity values. Normal dogs and cats should produce hypersthenuric
urine (> 1.030-1.035) in response to detectable dehydration. Multiple urine specific gravities that are consistently isosthenuric
(1.008 - 1.012) or minimally concentrated (> 1.012 but <1.030-1.035) can be associated with decreased renal function. Water
deprivation testing should not be performed in patients with suspected renal dysfunction, as dehydration may exacerbate existing
Ultrasonography is used to evaluate renal tissue architecture if kidney abnormalities have been revealed by physical examination
(e.g., abnormal kidney size or shape), clinicopathologic findings (e.g., azotemia or proteinuria), or survey radiographs (e.g.,
abnormal kidney size, shape, or opacity or nonvisualization of a kidney). Normally the renal cortex is hypoechoic compared
with the spleen, liver, and the renal medulla. In comparison, the renal pelvis and diverticula are relatively hyperechoic.
Decreased echogenicity of the renal cortices can be observed in patients with acute tubular necrosis, polycystic kidney disease,
abscesses, and the renal edema associated with acute renal failure. Conversely, relatively hyperechoic renal cortices are
associated with end-stage renal failure, nephrocalcinosis, amyloidosis, feline infectious peritonitis, and calcium oxalate
nephrosis secondary to ethylene glycol ingestion. Glomerular and tubulointerstial disease can show a normal or hyperechoic
echotexture depending on chronicity.
Renal lymphoma can make the renal cortices appear hypoechoic or hyperechoic. Hydronephrosis and hydroureter are easily diagnosed
by ultrasonography and in cases of pyelectasia; fluid for culture and cytologic analysis can be aspirated by ultrasound guidance.
Resistance to renal blood flow (resistive index) can be calculated with the use of color flow doppler, and is increased in
several renal diseases.