Overview of the Urinary System
Primary functions of the urinary system include: 1) excretion of waste products of metabolism; 2) maintenance of a constant extracellular environment through conservation and excretion of water and electrolytes; 3) production of the hormone erythropoietin, which regulates hematopoiesis, 4) production of the enzyme renin, which regulates blood pressure and sodium reabsorption; and 5) metabolism of vitamin D to its active form (1,25-dihydroxycholecalciferol).
Many abnormalities of the urinary system can be diagnosed from the signalment, history and physical examination findings, serum chemistry profile, urinalysis, and aerobic bacterial urine culture. The history should include information regarding changes in water consumption, frequency of urination, volume of urine produced, appearance of urine, and behavior of the animal. It is also important to obtain information about historical and current drug administration, appetite, diet, changes in body weight, and previous illnesses or injuries.
The physical examination should include palpation of the bladder and examination of external genitalia. In dogs, rectal examination should be performed to evaluate the urethra in both sexes and the prostate in male dogs. Rectal examination in cats may not be feasible because of their small size; however, the kidneys are generally easier to palpate in cats than in dogs. A full neurologic examination should be performed on all animals with micturition disorders. Additional diagnostic tests, such as CBC, blood gas analysis for acid-base status, blood pressure, urine protein:creatinine ratio, iohexol clearance test, survey abdominal radiography, abdominal ultrasonography, contrast studies of the upper and lower urinary tract, cystoscopic examination of the urinary bladder, and renal biopsy may also provide valuable information.
One of the most important diagnostic tests for evaluation of urinary tract disorders is a urinalysis. (Also see Urine Appearance.) Urine may be collected by one of four methods: spontaneous micturition, manual compression of the urinary bladder, catheterization, and cystocentesis. Each method has advantages and disadvantages (see Advantages and Disadvantages of Urine Collection Methods). A urinalysis should include method of collection, urine specific gravity, color, turbidity, pH, glucose, ketones, bilirubin ictotest, occult blood, protein, and leukocytes (urine dipstick leukocyte tests are unreliable in cats). Urine specific gravity should be obtained using a refractometer. Microscopic examination of urine sediment should include RBCs, WBCs, epithelial cells, renal casts, bacteria, yeast, parasitic ova, fat, sperm, and crystals. Delay in analyzing urine samples can result in artifacts (eg, changes in urine pH, formation of crystals, etc), so it is important to note the time when the sample was collected and the time when it was analyzed. If a sample will not be analyzed immediately, it should be refrigerated.
Protein in urine should be evaluated in light of the specific gravity. Protein in a concentrated urine sample may not be significant, whereas the same amount in a dilute sample may be significant. Urine dipsticks provide a semiquantitative assessment of protein and can be influenced by urine pH. Therefore, they should be used only as a screening test for protein, not as a definitive diagnosis of proteinuria. A urine protein:creatinine ratio from a single urine sample or from a 24-hr urine sample is required to quantitate the amount of protein in urine. In dogs, the following International Renal Interest Society (IRIS) guidelines should be used for interpretation of urine protein:creatinine ratios. In dogs, <0.2 = nonproteinuric, 0.2–0.5 = borderline proteinuric, and >0.5 = proteinuric; in cats, <0.2 = nonproteinuric, 0.2–0.4 = borderline proteinuric, and >0.4 = proteinuric. Urine protein:creatinine ratios must be interpreted in the context of other information from the urinalysis. Inflammation and hematuria can falsely increase urine protein:creatinine ratios, although hematuria generally has minimal effects.
Advantages and Disadvantages of Urine Collection Methods
A urinalysis is unreliable to exclude a urinary tract infection (UTI). Not all UTIs are associated with an inflammatory response. In addition, >10,000 bacterial rods/mL and >100,000 bacterial cocci/mL of urine are required to consistently find bacteria in a urine sample using light microscopy. Approximately 25%–30% of all dogs with UTI have urine bacterial counts below these figures at the time of specimen collection, so urine culture is important to exclude a UTI.
Urine samples for bacterial culture may be obtained by the same methods used to obtain samples for urinalysis; however, the preferred method is cystocentesis. Urine obtained by cystocentesis should be sterile. If urine samples are collected by methods other than cystocentesis, a quantitative urine culture should be requested. If the sample is collected by spontaneous micturition or manual compression, significant numbers of bacteria are present if ≥100,000 colony forming units (CFU)/mL of urine in dogs or ≥10,000 CFU/mL of urine in cats are detected. Samples with >10,000–90,000 CFU/mL in dogs and >1,000–10,000 CFU/mL in cats are suspicious for a UTI. If the sample is collected by catheterization, ≥10,000 CFU/mL in dogs and ≥1,000 CFU/mL in cats is significant, whereas samples containing 1,000–10,000 CFU/mL in dogs and 100–1,000 CFU/mL in cats are suspicious for a UTI.
Evaluation of serum chemistries, including BUN, creatinine, calcium, phosphorus, bicarbonate, and serum electrolytes, is useful in many urinary tract disorders and can provide a crude indication of glomerular filtration rate (GFR). Although increases in BUN and creatinine are supportive of renal dysfunction, these tests are influenced by nonrenal factors as well. For example, dehydration can cause increases in BUN and serum creatinine not associated with renal failure. BUN can also be influenced by diet and GI bleeding and is considered inferior to creatinine to evaluate GFR. Serum creatinine levels can be falsely decreased in animals with severe muscle wasting and falsely increased in patients with severe muscle damage. Although BUN and serum creatinine increase as GFR decreases, this relationship is not linear. Large changes in GFR early in renal disease cause only small increases in BUN and serum creatinine, whereas small changes in GFR in advanced renal disease may be associated with large changes in BUN and serum creatinine.
More sensitive methods to detect renal dysfunction include plasma clearance tests (eg, inulin clearance), radionuclide techniques, endogenous creatinine clearance, and exogenous creatinine clearance. However, these tests are impractical to perform routinely in clinical practice. The iohexol clearance test is a recently developed alternative to detect renal dysfunction. It entails recording an accurate body weight, administering a precise amount of iohexol IV, and accurately timing the collection of blood samples as directed after administration. This test does not require timed collection of urine samples or special equipment. Plasma clearance of exogenous creatinine has also recently been validated for use in dogs.
Depending on the cause of the urinary tract disorder, radiographic procedures, sonographic examination, and cystoscopic examination of the bladder may provide additional valuable information. The kidneys have a limited range of responses to disease; therefore, renal biopsies are rarely useful when evaluating renal dysfunction. An exception to this is in animals with significant proteinuria.
Blood gas analysis or serum bicarbonate levels provide useful information on acid-base status, especially in animals with renal dysfunction. Metabolic acidosis is a common problem in chronic renal failure and can result in protein catabolism.