Urinary System, Introduction: Introduction - The Merck Veterinary Manual

Urinary System, Introduction: Introduction

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 hormones erythropoietin and renin, which regulate hematopoiesis, blood pressure, and sodium reabsorption; and 4) metabolism of vitamin D to its active form (1,25-dihydroxycholecalciferol). Many abnormalities of the urinary system can be diagnosed from the signalment of the patient, 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 patient. 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 for evaluation of the prostate in male dogs. Rectal examination in cats may not be feasible due to 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.

Urinalysis:

One of the most important diagnostic tests for evaluation of urinary tract disorders is a urinalysis. (See also Urine Appearance .) Urine may be collected by one of 4 methods: spontaneous micturition, manual compression of the urinary bladder, catheterization, and cystocentesis. Each method has advantages and disadvantages (see Table: 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). Microscopic examination of urine sediment should include RBC, WBC, 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 urine 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 guidelines should be used for interpretation of urine protein:creatinine ratios: 0.0-0.3 = normal; 0.3-1.0 = questionable; and >1.0 = abnormal. In cats, a urine protein:creatinine ratio <0.7 is considered normal. Urine protein:creatinine ratios must be interpreted in the context of other information from the urinalysis. Inflammation and hematuria can falsely elevate urine protein:creatinine ratios.

Bacterial Culture of Urine:

A urinalysis is unreliable for ruling out a urinary tract infection (UTI). Not all UTI 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. About 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 rule out a UTI.

Urine samples for bacterial culture may be obtained by the same methods used for obtaining 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, while samples containing 1,000-10,000 CFU/mL in dogs and 100-1,000 CFU/mL in cats are suspicious for a UTI.

Serum Chemistry Profile:

Evaluation of serum chemistries, including BUN, creatinine, calcium, phosphorus, and serum electrolytes, is useful in many urinary tract disorders and can provide a crude indication of glomerular filtration rate (GFR). Although elevations 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 for evaluating GFR. Serum creatinine levels can be falsely lowered in patients with severe muscle wasting and falsely elevated in patients with severe muscle damage. Although BUN and serum creatinine increase as GFR declines, this relationship is not linear. Large changes in GFR early in renal disease cause only small increases in BUN and serum creatinine, while small changes in GFR in advanced renal disease may be associated with large changes in BUN and serum creatinine.

Additional Diagnostic Tests:

More sensitive methods for detecting 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 for detecting renal dysfunction. It entails recording an accurate body weight, administering a precise amount of iohexol IV, and accurately timing collection of blood samples as directed following administration. This test does not require timed collection of urine samples or special equipment.

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.