Diagnostic testing offers early detection of renal disease - DVM
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Diagnostic testing offers early detection of renal disease

DVM Best Practices

Proteinuria 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 or hemorrhage.

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 renal lesions.

Suggested Readings
Ultrasonography 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.


Source: DVM Best Practices,
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