Introduction to Urinalysis
READINGS: Tilley- 60-61, 86-7,
112-3, 130-1, 254-5, 274, 514-5, 572, 640, 874-5,
INTRODUCTION: Early physicians were called “pisse prophets,” reflecting the importance attributed to urine. These doctors looked at the urine, smelled it and tasted it; patients with diabetes mellitus (“honey water”) could be identified by their sweet-tasting urine. This importance continues today with our more sophisticated testing methods. Urinalysis today is the laboratory evaluation of urine, which allows you to assess the urinary system and which can be used to evaluate other body systems as well. Urination is the expulsion of urine from the bladder. It is also called micturition. Urine is an ultrafiltrate of plasma—it is formed by forcing blood through kidney filter. There are a number of pathologies associated with urination and the urinary system.
URINALYSIS: There are four primary indications for urinalysis:
Urine is unpredictably unstable, so the urinalysis should be performed as soon as possible. After urine has sat on a counter at room temperature for an hour or more, bacteria proliferate, breaking down urea and increasing the urine pH. This increase in alkalinity may cause the disintegration of erythrocytes, crystals and epithelial cells. The bacteria utilize glucose, decreasing the amount present in urine. Bilirubin may be broken down by light and ketones will volatilize. If it is not possible to examine the urine immediately, urine can be refrigerated for up to 12 hours. It is important to allow the urine to return to room temperature before beginning the urinalysis. Freezing can be used to preserve the chemical constituents of urine and one drop of 37% formalin can be added to 30 milliliters of urine to preserve the microscopic components. The constituents present in urine vary based on the time of collection. Preprandial samples are preferred when the focus is identification of the chemical components. Because of the potential for breakdown of microscopic materials, recently formed urine should be used if the focus is on urine sediment.
EVALUATION OF RENAL FUNCTION Review renal anatomy and physiology from your Anatomy and Physiology notes and text. The urinary system consists of the kidneys, the ureters, urinary bladder and urethra. The kidney is the primary homeokinetic (homeostatic) organ of the body. It is responsible for maintaining fluid, electrolyte, and pH balance, elimination of wastes and toxic materials via urine, production of renin, erythropoietin and prostaglandins, and activation of vitamin D. The nephron is the functional unit of the kidney. It consists of the afferent (incoming) and efferent (exiting) arterioles, which supply blood to the glomerulus. The glomerulus is a tuft of coiled capillaries which filters blood. The filtrate is picked up by the Bowman’s capsule, and exits into the renal tubules. The proximal convoluted tubule reabsorbs eighty percent of the filtrate by active and passive transport. The loop of the nephon (loop of Henle) uses a countercurrent multiplier mechanism to concentrate the fluid. Water is reabsorbed in the distal convoluted and collecting tubules in response to ADH (anti-diuretic hormone). Filtration through the glomerulus is regulated by hydrostatic (blood) pressure and albumin (osmosis). Decreased blood pressure in the brain cause the renal juxtaglomerular cells to release renin, which is transformed into angiotensin, resulting in vasoconstriction. The renal filtrate is modified by reabsorption and secretion in the tubules, forming urine. Tubular reabsorption moves materials from the lumen of the tubule into interstitial fluid and finally into the peritubular capillaries, returning the substances to circulation. The renal threshold is the maximum amount of a substance that can be reabsorbed from the filtrate; if the blood level exceeds the renal threshold, the excess material spills into the urine.
PHYSICAL URINALYSIS
A number of
nonpathologic factors can affect the volume of urine produced,
including water intake, environmental temperature and humidity, signalment,
size, food and work. The amount of urine produced by an animal per
day is often estimated. In general, the amount of urine produced by
an animal daily is approximately:
Polyuria can be a physiologic phenomenon: if a healthy animal drinks more water (polydipsia), it will produce more urine. Administration of corticosteroids (in dogs), diuretics, and fluids causes pharmacologic polyuria. There are many pathologic causes of polyuria, including chronic renal failure, diabetes mellitus, diabetes insipidus, hyperadronocorticism in dogs and hyperthyroidism in cats. Pollakiuria is frequently associated with feline lower urinary tract disease and urinary tract infections. Oliguria or anuria may be physiologic, because the animal is panting or sweating or due to decreased intake (water deprivation). If an animal is dehydrated due to a pathologic process, such as vomiting or diarrhea, the volume of urine will decrease. Acute renal failure, obstructive uropathy and ruptured bladed are urinary tract causes of oliguria. Cardiac decompensation, which decreases the blood pressure across the glomerulus, can also decrease urine output. Urine color is influenced by colored metabolic wastes (called urochromes), fluid volume (hydration status), and diet and medications. Normal carnivores typically have light colored urine, while herbivores have amber colored urine due to the presence of food pigments. Urine color is most frequently assessed by measuring out a standardized volume and visually examining it. Descriptions should accurately convey the shade and intensity of the urine color.
Colorless to pale yellow urine may be dilute, due to increased water intake, or associated with diabetes insipidus or end stage renal disease. Dark yellow (amber) urine is concentrated, due to decreased water intake, dehydration or acute renal disease. Yellow-green urine has increased amounts of bilirubin, and may be associated with liver or hemolytic disease. Red urine contains hemoglobin (if it’s clear) or erythrocytes (if it’s cloudy).
Flocculent urine contains grossly visible particles and is generally pathologic, often due to crystals or infection. Urine odor is affected by many things including sex (males generally have a stronger urine odor) and species (such as cats). There are few diagnostically significant odors, but an ammoniacal odor indicates the presence of bacteria and a sweet or fruity odor is associated with ketones (diabetes mellitus). If urine is shaken vigorously, foam will form due to the presence of proteins in urine. Persistent foam can indicate a proteinuria (excess protein in urine). Specific gravity, osmolarity and osmolality are important measurements of the amount of solutes in urine. Most veterinary practices measure specific gravity. Specific gravity is a measurement of the concentration of dissolved solutes in urine, which, in turn, measures the kidney’s ability to concentrate and dilute filtrate. It is not very sensitive, because compensatory hypertrophy of the kidneys may mask significant degradation of renal function. In normal urine, there is an inverse relationship of volume to specific gravity: the greater the volume of urine, the lower the specific gravity. In animals with diabetes mellitus, however, the relationship is direct; diabetes mellitus is associated with high urine volume and high specific gravity.
The reference
range for urine specific gravity is the dog is 1.016 – 1.060.
Decreased urine specific gravity may be physiologic, pharmacologic or pathologic. Polydipsic animals have a physiologically decreased specific gravity. Pharmacologic causes include diuretics, corticosteroids (especially in dogs), and metofane anesthetic. Pathologically low urine specific gravity can be due to chronic renal failure or diabetes insipidus. Increased urine specific gravity can occur due to increased water intake, dehydration, diabetes mellitus and acute renal disease. Isosthenuria occurs when the specific gravity of urine and protein-free plasma are both 1.008 – 1.012. This most frequently occurs in animals with chronic renal failure, which are unable to change the concentration of urine from that of the filtered plasma. A water deprivation test should be performed to evaluate ADH production and action.
REVIEW QUESTIONS:
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