Chemical Evaluation of Urine

Commercial reagent strips are used to semi-quantitatively measure urine pH, protein, glucose, ketones, bilirubin, and blood. Although test pads are present on many commercial strips for determination of specific gravity, leukocytes, nitrites, and urobilinogen, these are generally not reported in veterinary medicine either because they are not reliable for veterinary use or the information itself is not useful (see below for more details).

pH

In most circumstances, urine pH should not be used as an indicator of acid-base balance in the body. Urine pH generally reflects diet. Protein/meat diets produce acidic urine (pH <7.0), and plant/cereal diets produce alkaline urine (pH >7.0). Paradoxic aciduria (described in Chapter 6: Body Water, Electrolytes, and Acid-Base Balance) is the unique situation where an animal with metabolic alkalosis excretes hydrogen ions into the urine resulting in acidic urine. Urinary tract infection with urease-producing organisms results in alkalinization of the urine.

Precipitation of urine crystals is affected by urine pH. For example, struvite (magnesium ammonium phosphate) crystals can be found in urine of any pH, but they form more readily in neutral to alkaline urine. For this reason and because of increased production of ammonia, urinary tract infection with urease-producing organisms favours the formation of struvite crystals.

Protein

The protein reagent pad is most sensitive to albumin, a small (molecular weight about 66 kilodaltons), negatively-charged protein. The concentration of protein determined by the reagent pad should always be interpreted in relation to the USG. For example, a 2+ protein reaction in urine with a specific gravity of 1.015 is more likely to be significant than the same result in urine with a specific gravity of 1.050. If protein is detected in the urine on the reagent pad, the significance must be determined taking into account physical examination findings, history, and method of urine collection. In order to establish that proteinuria is renal in origin, prerenal and postrenal causes must be ruled out.

Prerenal proteinuria results from an overabundant filtered load of low molecular weight proteins, such as hemoglobin, myoglobin, and Bence-Jones proteins (immunoglobulin light chains sometimes seen with plasma cell neoplasms). However, it is unlikely that these overload proteins will be detected on routine urinalysis since urine reagent strips are particularly sensitive to albumin detection. Prerenal proteinuria does not result in hypoproteinemia/hypoalbuminemia.

The protein result should also be interpreted in relation to sediment findings. Urogenital tract pathology that results in increased vascular permeability such as inflammation or bleeding from whatever cause, will allow leakage of plasma proteins into the urine and is called postrenal proteinuria. Thus, an “active” urine sediment (increased WBCs and/or high numbers of erythrocytes causing a visible red color change of urine) will preclude interpreting proteinuria as being due to glomerular disease. Such cases should be monitored and urine protein reassessed when the sediment findings no longer suggest lower urinary or genital tract disease. As with prerenal proteinuria, postrenal proteinuria does not result in serum decreases in protein/albumin.

Renal proteinuria may be tubular (lesion impairing tubular protein reabsorption) or glomerular (primary lesion/pathology involving the glomerular capillary wall). Increased permeability of the glomerular capillary wall which occurs with glomerulonephritis, renal amyloidosis, and loss of negative charge in the glomerular capillary basement membrane, results in urinary loss of albumin and other proteins of a similar size and charge. The urine protein/creatinine ratio (UPC) is utilized once the proteinuria has been localized to the kidneys. Biochemical analysis of urine protein and urine creatinine, and calculation of the UPC allows for comparisons to reference values by eliminating the variability caused by differences in USG. UPC in the proteinuric range (see protocol manual) indicates that the proteinuria is significant; magnitude may allow the distinction between glomerular or tubular origin. Nonrenal factors such as cardiac disease, extreme exertion, hyperadrenocorticism, and episodes of fever, can also result in increased glomerular permeability. An elevated UPC that is still below the reference limit for well-established glomerular disease, could be due to a preliminary stage of glomerulopathy, elevated levels of plasma protein(s) which are subsequently filtered into the urine, or tubular leakage of protein from congenital tubular diseases or acute tubular injury.

For more information regarding substaging of chronic kidney disease (CKD), please see the website for the International Renal Interest Society (IRIS): http://www.iris-kidney.com/.

Glucose

Glucose is normally completely reabsorbed in the proximal tubules unless the “renal threshold” or transport maximum is exceeded. Glucosuria usually indicates hyperglycemia. Glucosuria together with normoglycemia may indicate congenital or acquired tubular pathology; however, the possibility of prior, undetected hyperglycemia resulting in residual glucosuria should be considered. Cats are particularly prone to epinephrine-induced hyperglycemia that can be triggered by transport to the veterinary clinic, the office visit itself, and physical handling. Time of collection of blood relative to urine, and the amount of mixing of “normal” urine with glucose-containing urine, will affect whether urinalysis and serum biochemical findings are correlated. Particularly in the cat, re-evaluation of serum and urine glucose at a later time and under minimally stressful circumstances will help to determine if a diagnosis of diabetes mellitus or tubular disease should be pursued. History, physical findings, and additional laboratory findings are also particularly useful in this situation (refer to the section on diabetes mellitus in Chapter 10: Endocrine System).

Ketones

Ketonuria is an indication of starvation (negative energy balance), diabetes mellitus, or a high fat diet devoid of carbohydrate. Degradation of fatty acids in the liver results in release of acetoacetic acid into the blood and transport to cells throughout the body as an energy source. A portion of the acetoacetic acid is converted to β-hydroxybutyric acid and, to a lesser extent, acetone. Normally this transport to and diffusion into peripheral tissue occurs so rapidly that the concentration of these compounds remains low in the plasma. The three compounds are collectively called ketone bodies, and the condition where their plasma concentrations are increased is called ketosis (See section in Chapter 10: Endocrine System). Increases in acetoacetic acid and β-hydroxybutyric acid can lead to extreme high anion gap metabolic acidosis. Acetone is very volatile and during states of ketosis, can often be smelled in air being expired from the lungs. The urinalysis reagent strip detects primarily acetoacetic acid and, to a lesser extent, acetone, and does not detect β-hydroxybutyric acid. Ketonuria is not a sign of renal pathology, but indicates that the renal threshold for ketone bodies has been exceeded. Normally, ketones are freely filtered by the glomeruli and completely reabsorbed in the proximal tubules. Ketonuria precedes detectable ketonemia. Ketonuria may be seen concurrently with glucosuria in diabetic patients; ketonuria in the absence of glucosuria suggests lipid metabolism in excess of carbohydrate metabolism.

Bilirubin

Conjugated bilirubin is water soluble, not bound to proteins and excretable by the kidneys. Conjugated bilirubin is freely filtered into the glomerular filtrate of the kidneys and is not reabsorbed. The renal threshold for bilirubin is low in most animals, especially dogs, therefore bilirubinuria may be detected before hyperbilirubinemia. Renal tubular epithelial cells are also capable of bilirubin conjugation in dogs, further contributing to bilirubinuria. Urine bilirubin should always be interpreted in relation to serum bilirubin, CBC findings, additional biochemical test results, and history and physical findings, particularly in the dog. Bilirubinuria in a dog with concentrated urine and no indication of hemolysis or hepatobiliary disease, is unlikely to be significant. Cats, however, have a higher renal threshold for bilirubin than dogs and bilirubinuria is considered significant in this species and if not accompanied by hyperbilirubinemia, is often closely followed by hyperbilirubinemia. Therefore, this finding should be investigated and potential causes of increased bilirubin excretion investigated in the cat. Although the bilirubin found in urine is conjugated, hemolytic disease should be considered as well as cholestatic hepatobiliary disorders, since conjugated bilirubin often predominates in both conditions.

Urobilinogen

Urobilinogen is not reported by many veterinary laboratories because of its lack of usefulness in differentiating causes of hyperbilirubinemia. Increased urobilinogen excretion is expected, but not always found, with hemolytic disease. Although complete biliary obstruction should result in a lack of urobilinogen in the urine, obstruction is rarely complete.

Blood

The blood reagent pad detects hematuria, hemoglobinuria, and myoglobinuria. Intact erythrocytes produce a speckled color change, whereas hemoglobin and myoglobin produce a uniform color change (Chemstrip 10A, Roche Diagnostics). A combination of the two patterns could be due to partial hemolysis of erythrocytes in the sample. If hemoglobinuria is due to hemoglobinemia rather than lysis of erythrocytes within the urinary tract, clear red discolouration of the serum or plasma should be seen. Dark red urine not accompanied by hemoglobinemia or intact red cells on urine sediment examination is consistent with myoglobinuria. Myoglobin is a very small molecule with a low renal threshold and discoloration of the plasma does not accompany excessive muscle breakdown and release of myoglobin. There will be increased release of muscle-origin enzymes (CK and AST) with situations resulting in myoglobinuria. Measurement of serum activities of these enzymes will help determine whether or not the urine discoloration is due to myoglobin. Samples collected by cystocentesis often react positively for intact erythrocytes (hematuria) on the reagent pad, without necessarily appearing red on gross visual inspection. However, intact red cells though often in only low numbers will be seen on the urine sediment examination. This underscores the extreme sensitivity of the test strip for blood.