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May/June 2014 Today's Veterinary Practice 87 URinaLysis in coMPanion aniMaLs PaRT 2: eVaLUaTion of URine cheMisTRy & sediMenT Today's Technician | tvpjournal.com nants and horses) result in a higher urine pH (alka- line urine). • Animals consuming milk diets tend to have acidic urine. Artifactual Effects. Artifactual increases in urine pH occur when samples are not examined promptly. For example, carbon dioxide, which is normally present in urine, diffuses into the atmosphere; this loss causes pH to rise because carbon dioxide acts as an acid. Protein Normally, there is little to no protein present in urine. The glomerulus does not typically filter larger plasma proteins, such as albumin and globulins, but it freely filters smaller proteins, which are reabsorbed in the proximal tubules of the kidneys unless there are signif- icantly increased amounts of these proteins, or impair- ment of renal tubule reabsorption is present. Test strip protein pads are more sensitive to albumin compared to globulins, hemoglobin, Bence-Jones pro- teins, and mucoproteins. • A positive reaction on the protein pad is elicited from trace (5–20 mg/dL) to 4+ (> 1000 mg/dL). • However, this test is influenced by the pH of the urine, and, due to the presence of cauxin in feline urine, false–positive reactions are common, espe- cially in mature cats. • The protein pad is also associated with the most error in interpretation because the color changes are slight. • Because this is a sensitive test (but not very specific), a negative reaction is usually reliable, which makes it a good screening test. Table 1 outlines both nonglomerular and glomeru- lar causes of proteinuria. Influence of pH. In alkaline urine, test strips may indicate falsely elevated protein concentrations. A pos- itive protein result with alkaline urine should be re- checked by a separate method, such as the sulfosalicylic acid (SSA) turbidity test, which is performed by adding equal amounts of urine to a 5% solution of sulfosalicylic acid. Presence of protein results in cloudiness and, at higher protein concentrations, a precipitate. Urine Specific Gravity. Urine protein detected on the test strip pad is often considered in light of the urine specific gravity (USG) because the concentration or dilution of any protein present is directly related to the concentration or dilution of the urine. In a urine sample with a USG of 1.008, a 2+ protein reaction represents much more protein being lost in the urine compared to a 2+ protein reaction in urine with a USG of 1.050. Protein:Creatinine Ratio. Persistent urine protein concentrations of 3 to 4+ on the test strip pad—with- out an obvious nonglomerular cause—may be assessed with a urine protein:creatinine ratio (uPr:Cr). • Creatinine clearance is steady in health—comparing the loss of protein to the constant excretion of cre- atinine identifies actual protein loss via the urinary system. • The uPr:Cr eliminates the need to collect a 24-hour urine sample and is not influenced by time of col- lection or gender; thus, a random, free-catch urine sample is sufficient. • In healthy dogs and cats, uPr:Cr is less than 0.5. • Glomerular proteinuria typically causes significant loss of albumin from the body; if severe, many ani- mals demonstrate visible edema, particularly limb edema, or abdominal distension caused by free fluid TabLe 1. causes of Proteinuria NONGLOMERULAR CAUSES Urinary tract infection or inflammation serum proteins (inflammatory exudate and erythrocytes) are added to the urine from the urinary or genital tract Hemorrhage (post-renal proteinuria) increased numbers of white and red blood cells are often present upon urine sediment examination Renal tubule damage (chronic kidney disease, acute kidney injury) Renal tubule reabsorption affected, causing mild (trace to 2+) proteinuria because small plasma proteins, which are normally filtered, are not reabsorbed by the damaged renal tubules Hemoglobin Myoglobin Bence-Jones proteins (increased serum protein) When high concentrations of hemoglobin, myoglobin, and bence-Jones proteins (produced by neoplastic plasma cells) are present in the blood, proteinuria results when renal tubular reabsorptive mechanisms are overwhelmed after proteins have been filtered by the glomerulus GLOMERULAR CAUSES Glomerular disease (glomerulonephritis, amyloidosis) severe protein loss from the body, most significantly albumin; glomerular protein- uria is persistent and its magnitude can be quite high (4+ on test strip pad) Physiologic proteinuria stress, temperature extremes (environmental or fever), or strenuous exercise results in transient, increased permeability of glomeruli to plasma proteins; proteinuria is usually mild (trace to 2+) and temporary TVP_2014-0506_TT_Urinalysis-Part2.indd 87 5/25/2014 7:28:46 PM