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Chemical & Microscopic Urinalysis Readings: Tilley- 112-3, 130-1, 234-5, 254-5, 274, 514-5, 572, 640, 874-5, 1016-9
DETERMINING THE CHEMICAL PROPERTIES OF URINE Urine chemicals are most frequently determined using reagent strips or tablets. The strips may consist of a single test or may be a panel (the number of tests varies)--such as the N-Multistix® or Urispec 9-Way®. Tablets, such as the Clinitab®, are specific for a single test. Analysis of urine chemicals is semiquantitive--the results are fairly accurate if close to the reference range and more inaccurate the less "normal" the results. The amount of a substance is related to the degree of concentration. The results should be interpreted in the context of the specific gravity; an increase in proteins can increase the specific gravity- which is the cause? Urine chemicals should also be correlated to the urine sediment. If the chemical test indicates that there are erythrocytes present, but they are not seen in the sediment, the chemical is probably hemoglobin, not RBCs. A 2+ proteinuria is more significant if cells are not present (cells contain protein and can increase the protein level--if there are no cells, then tubular disease or a protein-losing nephropathy may be present). The best guide for using these tests is the package insert-- read it and adhere to it. It is very simple to perform a dipstick test: unfortunately, anything that is easy to use is easy to abuse. Follow directions exactly, using a stopwatch for timing. Dip the stick into the urine and tap the edge to get rid of excess urine. It is important that there is a barrier between the reagent pads, so that urine flowing between the pads does not move the chemicals from the pads. The reagent pads are essentially miniature graduated cylinders. You will be reading color through the urine, so you need a uniform about of urine. URINE CHEMICALS Urine pH is a measurement of the kidney's ability to conserve hydrogen ions. This provides a rough estimate of the body's acid-base status. Many factors, however, can affect urine pH and changes can occur rapidly. A better assessment of body pH is obtained by performing a blood gas evaluation. Diet can have an immediate and short-term effect on urine pH. High protein diets, such as those eaten by carnivores, produces neutral to acidic urine. Herbivores tend to have alkaline urine. Any animal may have alkaline urine immediate after eating due to buffering in response to gastric acids. Alkaline urine is frequently associated with urinary tract infections; the bacteria split urea, forming ammonia. Obstruction and renal tubular disease may also create alkaline urine. If the body (cells and blood) are alkaline due to metabolic alkalosis, urine is usually alkaline as the kidneys conserve hydrogen ions. Acidic urine is commonly seen in animals with diabetes mellitus, especially if the animal is ketoacidotic. Excess or deficient dietary protein may lead to acidosis, as can Fanconi syndrome and metabolic acidosis. Inaccurate assessment of urine pH can occur due to improper preservation, as proliferating bacteria cause an alkaline shift. Excess wetting of the dipstick may result in a decrease in pH, as the acid buffer leaches from the protein pad over the the pH pad. The presence of urine protein is called proteinuria. The reference range is negative to trace in most animals; horses frequently have a higher "normal" level due to the presence of mucus in their urine. Urine is most frequently evaluated using a dipstick, which primarily assesses albumin. The Bumintest® uses sulfosalicylic acid precipitation to measure albumin, globulins and Bence-Jones proteins. Nonpathologic causes of proteinuria include a high protein meal, exercise, and stress. Pathologic causes of proteinuria include renal disease, where glomerular leakage of protein occurs, cardiac insufficiency (filter is not working effectively), and urinary tract infections and hematuria, where protein is associated with the cells present. Many diseases can contribute to proteinuria because the inflammatory response can cause a glomerulonephritis. Numerous errors can cause an error in urine protein assessment. Contaminants, especially if the urine was aspirated from the kennel floor, may increase protein levels. Leaching of the acid buffer from the protein pad may decrease the value, and vitamin C can mask protein, causing a false negative (many dipsticks include an ascorbic acid pad to assist in determining if this has occurred). A high pH may result in a false positive result. The presence of urine glucose is called glucosuria. A healthy animal has little to no glucose in their urine (negative to trace). The kidneys normally reabsorb all the glucose, preserving it to use as an energy source. If the blood level of glucose is too high, it exceeds the ability of the kidney tubules to reabsorb it (exceeds the renal threshold), and the glucose is retained in the urine. Urine glucose may be evaluated using a multi- or single test dipstick, which is specific for glucose. The backup copper reduction test using Benedict's reagent (the Clinitest®) measures several other sugars in addition to glucose. Nonpathologic glucosuria is associated with eating (postprandial), excitement and stress (esp. cats and horses). Pathologic glucosuria is associated with diabetes mellitus, acute renal failure, blockage in cats and milk fever in cattle. Numerous errors can decrease urine glucose values. These include refrigeration, ascorbic acid (vitamin C), salicylates, penicillin and the presence of bacteria. Urine glucose is not a good method to diagnose diabetes mellitus. It can be used for clients to monitor diabetes mellitus in their pet at home, but it is not recommended. False negatives can occur if the animal's level of ascorbic acid is very high and cats can have a glucosuria associated with stress or urinary tract infections. Ketonuria is the presence of ketones in urine. There reference range is negative to trace. Ketones are intermediary metabolites of normal fat metabolism; in a healthy animal, carbohydrates within the cells complete the breakdown. If this does not occur, the ketone level exceeds the renal threshold and ketones are found in the urine. Dipsticks and tablets (Acetest®) are used to evaluate ketone levels. These systems only detect acetoacetic acid and acetone, approximately 1/4 of the ketones present. Assessing these ketones, however, provides are fairly accurate assessment of all of the ketones present. Cold and exercise may cause a ketonuria. Most commonly, the presence of ketones is pathologic. Animals with diabetes mellitus do not have sufficient cellular carbohydrates (glucose) to break down ketones and they can accumulate in the blood, spilling into the urine. Animals in late pregnancy and early postparturition may develop ketosis (also called pregnancy toxemic), a severe and sometimes fatal disorder. Vomiting and diarrhea may also result in ketosis, as can starvation. Like the other urine chemicals, bilirubin should be present in only negative to trace amounts. If larger amounts are present, it is called bilirubinuria. Bilirubinuria occurs when water-soluble (conjugated) bilirubin passes through the glomerulus. Cats have a very high renal threshold, so bilirubinuria is more common in dogs (1+ is normal in concentrated dog urine). Pathologic causes include bile duct obstruction, liver necrosis caused by infectious canine hepatitis, leptospirosis and other infectious diseases, and hemolytic diseases, such as immune-mediated hemolytic anemia. Bilirubin is unstable and is decreased by exposure to light and high levels of vitamin C. Urine blood may be due to intact red blood cells (hematuria) or free hemoglobin from lysed erythrocytes (hemoglobinuria). Although their presence may be determined using a dipstick or tablet, it should be verified by a microscopic exam, which helps differentiate erythrocytes from hemoglobin. Like the other blood chemicals, hemoglobin and erythrocytes should be present only at negative to trace levels. Erythrocytes may be present due to iatrogenic damage, estrus, urinary tract disease or coagulopathies. Hemoglobin is nephrotoxic and may cause renal disease. Other causes of hemoglobin in urine is a hemolytic anemia or the lysis of erythrocytes in dilute urine. Urobilinogen is present in urine due to formation from bilirubin by intestinal bacteria. It is not significant in animals. It is often included on the dipstick and is more useful in human medicine. Dipsticks are used to identify leukocytes in human urine (pyuria). The reference range is negative to trace and most of the white blood cells are neutrophils. Like urobilinogen, however, the results are not reliable. Some dipsticks also include a reagent pad to detect nitrites (nitrituria), formed by urinary bacteria from nitrates. However, this transformation is very slow and doesn't indicate the number or kind of bacteria. It is not considered sensitive or reliable in veterinary medicine.
EVALUATING URINE SEDIMENT The microscopic properties of urine should be interpreted in the context of other data, especially specific gravity. It is important to know the method of collection and to standardized the volume of urine used. Urine should be centrifuged and the supernatent discarded (or saved for use for the physical and chemical urinalysis). The microscope condenser should be lowered so that less light passes through the slide; this will increase contrast and make urine sediment more visible. Some technicians prefer to use stain. However, it is important to filter the stain frequently, because bacteria and yeast grow in it. Stain precipitate may be mistaken for bacteria or crystals.
Erythrocytes indicate bleeding in the urogenital tract, because red blood cells can't pass through the nephron. In fresh urine, RBCs are pale yellow to purple and uniform in size. In concentrated urine, crenation may be seen. Fat droplets, yeasts and crystals may all be mistaken for erythrocytes. If 2% acetic acid is added to the urine, the red blood cells will lyse (make sure that you count them first!!).
Epithelial cells are exfoliated from urinary tract structures. Squamous epithelial cells are produced in the distal urethra, prepuce, vagina, and vulva. They are large, angular and irregular in shape. They stain purple, depending upon the amount of keratinization. The nucleus is dense and small, and the cell may give the overall impression of a fried egg. Usually squamous epithelial cells are contaminants.
Transitional epithelial cells are produced in the proximal urethra, bladder, ureters and renal pelvis. Because these are "stretchy" cells, they vary in size and shape. They are less angular than squamous epithelial cells, with a larger nucleus. They are larger and often more granular than leukocytes. Transitional cells may be normal in an animal, especially if it was catheterized. However, large rafts (clumps) of cells are not normal and should be recorded. They may indicate infection or a transitional cell carcinoma.
Unlike the other epithelial cells, renal tubular cells should be less visible (the reference range is 0 - 1 / hpf). These come from the renal tubules and, when seen intact in urine, indicate renal tubular disease. Renal tubular cells from normal exfoliation have disintegrated by the time urine exits the body. These cells can be difficult to identify. They are slightly larger then leukocytes (20 - 25 um) and have round dark nuclei. |