Urinalysis 101

Comprehensive Summary of Veterinary Urinalysis

Veterinary urinalysis is a crucial diagnostic tool in evaluating the urinary system and overall health of animals. It consists of three main phases: specimen collection and handling, macroscopic examination (both physical and chemical), and microscopic examination of the sediment.

1. Specimen Collection and Handling

  • The quality and interpretation of a urinalysis depend heavily on the method of sample collection and processing.

  • Ideal Sample: First morning, pre-prandial (before eating) urine sample.

    • Most concentrated, allowing for the best assessment of the kidney's concentrating ability.

    • Most acidic, facilitating accurate pH interpretation.

  • Time Sensitivity: Urinalysis should be performed within 30 minutes of collection to avoid artifacts affecting the accuracy of results.

    • Possible artifacts from delayed analysis include:

    • Bacterial overgrowth from contaminants or pathogens.

    • Dissolution or formation of crystals due to changes in pH and temperature.

    • Deterioration of cells (red and white blood cells).

    • Alterations in pH, color, and odor, along with decreased glucose, ketones, and bilirubin.

  • If immediate analysis is not feasible, refrigerate the sample but ensure it returns to room temperature before examination.

Specimen Collection Methods

  1. Free Catch Sample:

    • Simplest method, poses no risk to the patient.

    • Not sterile due to potential contamination from distal urethra, vulva, prepuce, and fur.

    • Not suitable for bacterial culture; risk of sample drying out and concentrating urine.

  2. Catheterized Sample:

    • Urine collected via a sterile catheter passed into the bladder.

    • Advantages:

      • Sterile sample if aseptic technique is used.

      • Useful for assessing urethral patency.

    • Disadvantages:

      • More challenging in female animals; may require sedation.

      • Risk of iatrogenic trauma or hemorrhage, contaminating the sample.

  3. Cystocentesis Sample:

    • Insertion of a needle through the body wall into the bladder.

    • Gold standard for bacterial culture and sensitivity due to sterility.

    • Disadvantages:

      • Risk of iatrogenic hematuria from needle puncture.

      • Potential injury to surrounding organs or induction of vasovagal events.

2. Urine Macroanalysis (Gross and Chemical Examination)

  • Evaluation of the physical properties of urine and chemical analysis using reagent strips (dipsticks).

Urine Production Terms

  • Polyuria: Excessive urine production; often with polydipsia (excessive drinking), together termed PUPD.

  • Oliguria: Decreased urine output.

  • Anuria: Complete absence of urine production, considered critical.

Physical Properties Evaluated

  • Urine Specific Gravity (USG):

    • Measured accurately with a refractometer; dipstick readings are unreliable.

    • Reflects kidneys' ability to concentrate or dilute urine.

    • Normal ranges:

    • Dogs: > 1.030

    • Cats: > 1.035 (typically better concentrates)

    • Horses/Cows: > 1.025

  • Isosthenuria: USG between 1.008 and 1.012; indicates kidneys are neither concentrating nor diluting.

  • Increased USG: Often due to dehydration, decreased water intake, or fluid loss.

  • Decreased USG: Associated with renal/liver disease, endocrine disorders (e.g., diabetes mellitus), and PUPD.

Urine Color and Transparency

  • Normal Color: Ranges from pale yellow to dark yellow, due to urochromes.

  • Abnormal Colors:

    • Red/Brown: Indicating hematuria (intact red blood cells), hemoglobinuria, or myoglobinuria.

    • Yellow-Brown/Greenish-Yellow: Possible bile pigments.

    • Milky White: Fat, purulent discharge, or crystals; may be normal in horses/rabbits.

  • Transparency:

    • Reported as clear, cloudy, or flocculent; cloudiness indicates cellular material (e.g., red/white blood cells, crystals).

Chemical Analysis Using Reagent Strips

  • Reagent Limitations: USG, urobilinogen, nitrite, and leukocyte pads are unreliable in veterinary settings.

  • pH Testing:

    • Acidic urine (<7) common in carnivores; can result from starvation/high-protein meals, or metabolic/respiratory acidosis.

    • Alkaline urine (>7) common in herbivores; can be due to urinary tract infections or metabolic/respiratory alkalosis.

  • Proteinuria: Should be absent or in trace amounts; major sensitivity for albumin. Factors causing false positives may include:

    • Alkaline urine

    • Prolonged dipstick exposure

    • Active sediment (blood/white blood cells)

  • Persistent proteinuria requires quantitative testing (urine protein to creatinine ratio).

Glucosuria and Ketones

  • Glucose: Typically absent; seen in glucosuria when blood glucose exceeds renal threshold.

  • Common Causes of Glucosuria:

    • Diabetes mellitus, stress (cats), renal tubular dysfunction.

  • Ketones: Should also be absent; presence indicates shift to fat metabolism, primarily seen in diabetes mellitus and late-term pregnant ewes (pregnancy toxemia).

Bilirubin and Blood Tests

  • Bilirubin: Generally absent; small amounts can be normal in dogs.

    • Detected in cats, horses, pigs, sheep indicates abnormalities (liver disease, bile duct obstruction).

    • Bilirubin sensitive to light; exposure may give false-negative results.

  • Blood Testing via Dipstick: Helps differentiate the source of red coloration:

    • Intact red blood cells: spots on the pad.

    • Hemolyzed blood: uniform color change.

  • Centrifugation Differentiation:

    • Hematuria: pellet at the tube's bottom; supernatant clear.

    • Hemoglobinuria: discolored supernatant; blood plasma pink/red.

    • Myoglobinuria: discolored supernatant; normal blood plasma color.

3. Urine Microanalysis (Microscopic Examination of Sediment)

  • Involves centrifuging a urine sample and examining sediment under low and high power.

Low Power Examination

  • Evaluated at 10x magnification for:

    • Casts, mucous threads, and large crystals (e.g., struvite).

High Power Examination

  • Evaluated at 40x magnification for:

    • Erythrocytes (red blood cells), leukocytes (white blood cells), epithelial cells, small crystals (e.g., calcium oxalate), bacteria, and yeast.

Cellular Elements Identified

  • Erythrocytes: More than five per high power field is abnormal, indicating hemorrhage in the urogenital tract;

    • Common cause of iatrogenic hematuria from cystocentesis.

  • Leukocytes: More than five per high power field is significant (pyuria), indicating inflammation;

    • Neutrophils are the most common white blood cells seen.

  • Epithelial Cells Types:

    • Squamous: Largest and most common from the urethra/vagina, no clinical significance.

    • Transitional: Round/pear-shaped, can indicate inflammation or neoplasia if increased.

    • Renal: Very rare; if numerous, can indicate renal tubular degeneration.

  • Casts: Formed from coagulated protein in renal tubules; presence usually indicates pathologic changes.

    • Hyaline Casts: Few are normal; larger numbers indicate tubular damage.

    • Granular Casts: Most common; large numbers suggest renal damage.

    • Waxy and Epithelial Casts: Less common; numerous casts signify significant kidney damage.

Crystals in Urine (Crystalluria)

  • Crystal presence correlates with urine pH:

    • Alkaline Urine Crystals: Magnesium ammonium phosphate (struvite), amorphous phosphates, calcium carbonate.

    • Acidic Urine Crystals: Amorphous urates, uric acid (especially in Dalmatians), calcium oxalate (look like squares with an X).

  • Urate Crystals: Thorny apple appearance; due to genetic mutations or liver shunts.

  • Cystine Crystals: Link to hereditary defects; more common in intact male dogs.

  • Microorganisms:

    • Bacteria shouldn’t be in cystocentesis samples; high bacteria count with white blood cells suggests UTIs.

    • Yeast may be confused with red blood cells but shows budding characteristics.

Other Findings in Urine Samples

  • Possible presence of mucous threads, spermatozoa (in intact males), fat droplets, parasites, and various artifacts.

4. Additional Urine Tests

  • If persistent protein is detected on dipstick analysis and sediment examination shows no active cells, an additional test called the urine protein to creatinine ratio (UPCR) may be warranted.

    • Measures urine protein loss (primarily albumin) against a constant creatinine rate.

    • Performed at diagnostic labs; interpretation based on IRIS guidelines:

    • For Dogs:

      • < 0.2: Non-proteinuric

      • 0.2 - 0.5: Borderline proteinuric

      • > 0.5: Proteinuric

    • For Cats:

      • < 0.2: Non-proteinuric

      • 0.2 - 0.4: Borderline proteinuric

      • > 0.4: Proteinuric

Urinary Tract Infection Testing

  • When UTIs are suspected based on clinical signs (e.g., stranguria, dysuria, pollakiuria, hematuria), a urine culture and sensitivity test is the definitive diagnosis tool.

    • Gold standard sample: Cystocentesis into a sterile container.

    • Culture Portion: Identifies specific bacteria and quantifies them.

    • Sensitivity Portion: Determines the effective antibiotics for treatment.

    • MIC (Minimum Inhibitory Concentration): Lowest antibiotic concentration preventing bacterial growth; time-sensitive test.

  • In-house testing offers quicker results but may lack specificity; external diagnostic labs provide comprehensive antibiotic profiles at increased cost and potential sample degradation risk during shipping.