Serum Protein
The serum or plasma proteins consist of albumin and globulin fractions. These are discussed in detail in Chapter 26.
■ BOX 22.23
Causes of Increased Blood Urea Nitrogen
Common Causes
Prerenal azotemia
Reduced renal perfusion
Hypovolemia
Congestive heart failure
Dehydration after endurance exercise
Renal azotemia
Acute renal failure
Chronic renal failure
Postrenal azotemia
Urolithiasis
Renal calculi
Ureteral calculi
Urethral calculi
Ruptured bladder
Uncommon Causes
Gastrointestinal bleeding
Perirenal abscess
Renal carcinoma
Renal dysgenesis
Carcinoma of the bladder
Postexhaustion multisystemic syndrome in horses Severe exertional rhabdomyolysis with myoglobinuria Severe intravascular hemolysis with hemoglobinuria Intoxication or poisoning
Heavy metal poisoning
Nonsteroidal antiinflammatory drug intoxication Aminoglycoside intoxication
Urinalysis
Urinalysis can be an extremely useful diagnostic tool, providing information on many systemic disorders.
Ideally urine is collected at the same time as blood for clinical chemistry and before any treatment is administered. The interpretation of certain abnormalities in urine (e.g., glucosuria, ketonuria) is facilitated by concurrent knowledge of chemistry results. Urinalysis is essential in the evaluation of primary renal disease. Urine normally is collected as a voided sample or after catheterization. Voided urine samples are safe and easy to collect but are easily contaminated. Catheterization is preferred for bacteriologic evaluation, but the resultant mild trauma may result in a slight increase in urine RBCs and protein. Because of the urethral diverticulum at the level of the pelvis, it is difficult to successfully catheterize male ruminants. Urethral obstruction is common in male small ruminants, and percutaneous aspiration may be the only way to obtain a urine sample in affected animals. Urinalysis should be performed as soon after collection as possible (within 30 minutes) to avoid degeneration of cellular elements, changes in pH, or bacterial overgrowth (cytolysis, crystallization). If this is not possible, samples should be refrigerated but brought back to room temperature before tests. Urine volume and composition are influenced by feed and water intake, salt supplementation, environmental factors, exercise, stress, systemic disease, and drug administration. Urine samples collected after administration of a diuretic are dilute and unsuitable for routine urinalysis. The tranquilizer xylazine, which is often used to assist in the catheterization of male horses, may alter the results of the urinalysis because it produces diuresis as a result of glycosuria and specifically also kaliuresis.When urinalysis is performed, the color should be noted along with the organoleptic characteristics of the urine such as the fact that an adult horse's urine has a lot of mucus and becomes cloudier at the end of urination.
Specific Gravity
Specific gravity is defined as the weight of urine relative to the weight of distilled water (=1 at 4° C [60° F]). The normal specific gravity of serum is approximately 1.008 to 1.012. The urine specific gravity is an indicator of renal function because it reflects the action of the renal tubules and collecting ducts on the glomerular filtrate by providing an estimation of the number of particles dissolved in the urine (e.g., whether the urine is concentrated or diluted via serum). It is usually measured by refractometry on urine supernatant. Urine dipsticks with reagent pads for estimating the urine-specific gravity in humans should not be used for this purpose in domestic animals because a poor correlation has been reported compared with refractometry in large animals.68 Normal animals have the capacity to dilute their urine specific gravity to less than 1.010 and to concentrate greater than 1.050. The normal range reported for most adult large animals is 1.020 to 1.050.
Suckling neonatal animals normally produce very dilute urine with a specific gravity below 1.010. Although this could represent immature renal function, it more likely reflects their high-volume fluid intake, because normal neonates can concentrate their urine if fluids are withheld.Failure to produce concentrated urine, as reflected by a specific gravity below 1.020, in the face of dehydration is an indication of altered renal function, which may be caused by primary renal disease, diabetes insipidus, medullary washout, or nephrogenic diabetes insipidus. With severe and chronic sodium depletion, tubular sodium may be insufficient to sustain normal countercurrent mechanisms for water resorption. This process is called medullary washout and has been reported as a cause of polyuria in lactating dairy cattle on a salt-deficient diet. At least 50% of renal function must remain for highly concentrated urine to be produced. Isosthenuria, a urine-specific gravity that remains around 1.010 despite variation in hydration, occurs when renal disease progresses to the point at which renal function is reduced to less than a third of normal. Altered renal function in these animals is usually reflected by a moderate to marked elevation in BUN and creatinine levels and by other changes in the urinalysis. Hyposthenuria occurs when the specific gravity remains below 1.010; this indicates altered release of or response to antidiuretic hormone, which occurs with diabetes insipidus, medullary washout with chronic sodium depletion, nephrogenic diabetes insipidus, psychogenic polydipsia, and chronic liver failure in some horses. Hypersthenuric urine occurs when SG is greater than 1.030 to 1.035, and it occurs commonly in states of dehydration. It is also common in horses fed a high percentage diet of hay and in those living in hot environments.
pH
The normal range of urine pH for adult herbivores is alkaline, with values ranging from 7 to 9. Neonatal foals tend to have slightly acidic urine with a pH below 7.
Aciduria in adults may be seen in postrace samples collected from racehorses, after prolonged fasting, with ketosis in ruminants, or in response to metabolic acidosis. A paradoxic aciduria is seen in ruminants with hypochloremic, hypokalemic metabolic alkalosis and has been recently attributed to K concentration changes in the urine.69 Urine pH is also influenced by the cation-anion balance of the diet. The SID of urine, specifically the potassium concentration, has the strongest influence on urinary pH in dairy cattle and most likely in horses as well.69Protein
Protein is not normally detected in urine, although a falsepositive protein reaction may be noted on urine dipsticks as the sample of herbivores is normally strongly alkaline. Urine with a positive reaction for protein on dipsticks should be checked for protein by a chemical method (urine protein sulfosalicylic acid precipitation test). Proteinuria should be evaluated in relation to the other findings in the urinalysis. Persistent and strongly positive reactions for protein in the absence of leukocytes, RBCs, bacteria, or casts suggest glomerular protein loss, as in glomerulonephritis or amyloidosis. The presence of bacteria and leukocytes with proteinuria suggests sepsis in the urinary tract, whereas hemorrhage or inflammation in the urogenital tract is often associated with proteinuria. A transient postexercise proteinuria is observed in performance horses at submaximal or maximal levels of exertion.
Mild proteinuria is associated with tubular diseases, whereas severe proteinuria is more commonly related to glomerular damage. Urine urea to protein ratio has been used in horses to evaluate more accurately the protein concentration in urine. In horses this ratio is < 1.0 in normal animals.70
Glucose
Glucose is not found in the urine of normal large domestic animals unless the blood glucose level increases above the renal threshold, which is thought to be approximately 100 to 140 mg/dL (5.5 to 7.7 mmol/L) in ruminants and approximately 160 to 180 mg/dL (8.8 to 10 mmol/L) in horses.
The causes of hyperglycemia and glycosuria were described in a previous section; they include Cushing syndrome, stress, and catecholamine or glucocorticoid hormone release. Hyperglycemia and glycosuria can be created iatrogenically when glucose- containing fluids are administered at an excessive rate. Glycosuria is a fairly consistent finding in sheep with enterotoxemia type D (pulpy kidney). The presence of glycosuria without hyperglycemia strongly suggests renal tubular damage resulting from a toxic or ischemic insult.KETONE BODIES. The presence of ketone bodies in urine is abnormal in large animals. In horses, ketonuria is uncommon because they are poorly ketogenic; however, it is common in ruminants that are in negative energy balance, early-lactation cows, and pregnant ewes (pregnancy toxemia). Measurement of these ketone bodies in blood, milk, and urine has been used to diagnose and monitor clinical and subclinical ketosis in cattle.
Occult Blood
Both myoglobin and hemoglobin give a positive reaction on urine occult blood dipsticks. In most instances proteinuria shows a positive result as well. A tentative clinical impression sometimes can be formed to differentiate myoglobin from hemoglobin in dark urine if the sample is shaken vigorously in a closed, transparent container. Myoglobin tends to produce a brown foam, whereas hemoglobin produces a reddish foam. Hemoglobinuria caused by intravascular hemolysis is generally associated with clinical and hematologic evidence of hemolytic anemia. Hematuria may result in a positive occult blood reaction if lysis of some of the intact RBCs has occurred. This is likely to happen if the urine is very dilute or if it is held at room temperature for an extended period before analysis. False-positive reactions can occur if microbial peroxidase or oxidizing contaminants are present. Hematuria should be confirmed by cytologic examination of the urine sediment.
Myoglobin
Myoglobinuria should be associated with clear clinical and clinicopathologic evidence of extensive muscle damage.
The ammonium sulfate precipitation method of differentiating hemoglobin from myoglobin is imprecise and frequently fails to detect myoglobin in the dark, coffee-colored urine of horses with severe rhabdomyolysis. Accurate differentiation of these compounds in urine requires more sophisticated procedures.Cells
Cytologic evaluation of urine is deemed essential in cases of unexplained fever, hyperfibrinogenemia, weight loss, and complicated neurologic disease. The normal range for cells is generally considered to be up to five RBCs or leukocytes per high-power field. An increased number of erythrocytes indicates hematuria, which may be caused by neoplasia, trauma, inflammation, or coagulopathy. Pyuria, an increase in urine leukocytes, indicates an inflammatory process; when associated with bacteriuria, it indicates a septic process in the urinary tract. The presence of sheets or rafts of transitional cells suggests neoplasia. Foals' urine has relatively more epithelial cells compared with urine of adult animals.
Casts
Casts are accumulations of protein (Tamm-Horsfall glycoprotein) and cellular material that form in the renal tubules, and when present in the urine, they indicate renal damage or tubular disease. Casts can consist of erythrocytes, leukocytes, or renal tubular cells. As cellular degeneration proceeds, the cell type is more difficult to determine and the casts become granular and then waxy. Hyaline casts are noncellular and are formed from mucoprotein. They may be seen with glomerulonephritis, fever with passive congestion, or severe dehydration with altered renal blood flow. However, casts are soluble in alkaline pH, which is the usual urine pH of large animals, thus making their documentation in urine sediment difficult. If present, they indicate tubular involvement.
Crystals
The crystalline structures in the urine of large animals are those usually associated with an alkaline urine. Calcium carbonate crystals normally are found in abundance in horse urine, particularly if the horse has been fed alfalfa hay. Triple phosphate and calcium oxalate crystals are frequently observed in relatively small numbers. In foals calcium oxalate crystals are more prevalent. The major crystals involved in urolithiasis of feedlot cattle is struvite (MgNH4PO4 • 6H2O). In the western United States, silicate stones are most common in livestock under range conditions. Carbonate and oxalate stones are common causes of urolithiasis in small flocks of backyard sheep and goats fed alfalfa hay.
Bacteria
Bacteria are sometimes seen in small numbers in voided urine samples and may represent surface contaminants. Bacterial infections of the urinary tract are usually associated with significant pyuria. When this is noted, a catheterized sample (horse) or clean midstream catch (ruminant) should be obtained for Gram stain, culture, and sensitivity testing. Results from ruminant samples must be carefully interpreted because male ruminants routinely urinate within the prepuce, thus heavily contaminating samples. Fortunately, urinary tract infections in male ruminants and horses are rare.
Urine Creatinine Clearance Ratio71
Urinary electrolyte excretion is affected by a variety of factors, including dietary intake, alterations in renal function, and specific hormones that regulate renal electrolyte excretion. Determination of electrolyte concentrations from randomly collected urine samples is easily done and can be clinically useful when these concentrations are compared with serum concentrations. The presence of substantial amounts of sodium in the urine of an animal with hyponatremia suggests excessive renal sodium loss caused by altered renal function or hormonal control. However, marked variation in the rate of urine production can lead to serious problems in the interpretation of urine electrolyte concentrations. The standard physiologic methods of determining renal electrolyte clearance are complicated by the need for quantitative urine collection and are not well suited to most practical clinical situations. One means of overcoming these difficulties is expression of the renal electrolyte clearance relative to the endogenous creatinine clearance. Expression of the renal clearance as a ratio eliminates the need for quantitative urine collection. This derived value is known as the creatinine clearance ratio or the fractional excretion (FE), and it is calculated by the following formula:
FE = Urine∕(X)Serum∕(X) ? Serum∕(C)Urine∕(C) ? 100
where X represents the electrolyte concentration and C represents the creatinine concentration.
The FE fluctuates somewhat during the day in relation to physical activity and feed and water intake. However, under standardized conditions, there is close agreement between the FE determined from single random samples of blood and urine and that based on samples quantitatively collected during a 12- to 24-hour period.31 The FE of electrolytes has a wide range of normal values. The principal sources of this variation are differences in dietary intake and environmental or experimental conditions. The FE of electrolytes has been useful for detecting specific dietary deficiencies or imbalances. Dietary salt deficiency is associated with an extremely low FE of sodium and chloride, whereas the plasma concentration of these ions generally remains within normal limits. In a similar fashion, dietary calcium and phosphorus imbalances seldom are reflected by the serum concentrations of these ions. Calcium deficiency and phosphorus excess are relatively common dietary problems in large animals and result in a low FE for calcium and a high FE for phosphorus. There are technical problems with the determination of the urine calcium concentration in horses because of their normally alkaline urine and the resultant precipitation of calcium carbonate in the urine. Mixed urine samples from horses must be acidified (hydrochloric acid can be used) to solubilize the calcium. It has favored the use of fractional excretion of phosphorus to diagnose nutritional secondary hyperparathyroidism in horses.
Increases in the FE of sodium are noted with renal tubular damage and impaired sodium resorption. The sodium FE increase has been a useful indicator for the differential diagnosis of prerenal azotemia, which almost invariably results in a low sodium FE, from the azotemia caused by primary renal disease, in which the FE for sodium often is markedly increased. Specific criteria of interpretation in horses have been suggested. In the case of the FENa+, the reference range varies between 0.01% and 1.0%. A level lower than 0.02% indicates Na+ conservation by the kidney; an increase may result from excess of dietary intake, Addison disease, and more commonly tubular insufficiency. FEk+ normal values are considered between 15% and 80%; values below 15% have been associated with potentially recurrent myositis and chronic laminitis. FEcl- has not been reported with a diagnostic value. FEca++ normal values are between 0% and 33% when used in association with FEp. The reference values for FEP have been reported up to 0.5% and as a sensitive diagnostic indicator of calcium and phosphorus balance in horses. An increase above 0.5% is mainly considered an indicator of possible nutritional secondary hyperparathyroidism, renal tubular disease, or pseudohyperparathyroidism.