8.2 Urinalysis Collection and Physical/Chemical Testing

Urinalysis Collection and Physical/Chemical Testing

Urinalysis is a fundamental diagnostic test in veterinary medicine, providing critical information about the urinary system (kidneys, ureters, bladder, and urethra) as well as systemic metabolic diseases like diabetes mellitus and liver disorders. A complete urinalysis consists of physical evaluation, chemical evaluation (reagent strip testing), and microscopic examination of urine sediment. The veterinary assistant is responsible for collecting the sample using various techniques, preserving it correctly, and performing the physical and chemical evaluations.

Urine Collection Methods

There are four primary methods of collecting urine from veterinary patients, each with its own indications, advantages, and risks.

1. Free Catch (Voided Sample):
   • Method: Collecting urine as the animal naturally voids. For dogs, a clean container (such as a ladle or clean plastic dish) is slipped under the urine stream. For cats, non-absorbent litter (e.g., Nosorb) is placed in a clean, disinfected litter box.
   • Clean-Catch Midstream: It is vital to collect a 'midstream' sample. The initial stream of urine should be allowed to pass, as it flushes out normal bacteria, cells, and debris present in the distal urethra and external genitalia. Collecting the midstream portion minimizes contamination.
   • Pros and Cons: Free catch is non-invasive and stress-free. However, because it passes through the distal urethra and vulva/prepuce, it is highly contaminated with external bacteria and cells. It is never suitable for bacterial culture.
2. Manual Expression:
   • Method: Gentle, steady pressure is applied to the urinary bladder through the abdominal wall using the palms of the hands until the urethral sphincter relaxes and urine is expressed.
   • Indications: Most commonly performed on paraplegic or anesthetized patients who cannot urinate on their own.
   • Contraindications and Risks: Never attempt manual expression in animals with suspected urethral obstruction (e.g., male cats with feline lower urinary tract disease) or a fragile bladder wall, as excessive pressure can cause bladder rupture, a life-threatening emergency. It can also cause vesicoureteral reflux (urine backing up into the ureters and kidneys), potentially spreading infection upward.
3. Catheterization:
   • Method: A sterile flexible tube (urinary catheter) is passed through the urethra directly into the bladder.
   • Indications: Used to relieve urethral obstructions, monitor urine output in hospitalized patients, or collect a relatively clean sample.
   • Risks: Catheterization can cause trauma, irritation, and micro-bleeding in the urethra (which can artificially elevate red blood cell counts in the sample). It also carries a high risk of introducing external bacteria into the sterile bladder, leading to an iatrogenic urinary tract infection (UTI).
4. Cystocentesis:
   • Method: Direct puncture of the urinary bladder through the abdominal wall using a sterile needle and syringe.
   • Technique: The animal is placed in lateral, dorsal, or standing recumbency. The bladder is palpated and isolated. A sterile 22-gauge needle (1 to 1.5 inches long) attached to a syringe is inserted through the abdominal wall into the bladder at an oblique angle.
   • Non-Negotiable Safety Rules:
     • Never redirect the needle once it has entered the abdominal cavity. If you miss the bladder, withdraw the needle completely and start over with a new sterile needle. Redirecting can lacerate the bladder, major abdominal blood vessels, or puncture the intestines, leading to peritonitis.
     • Never squeeze the bladder during or after the procedure, as this can cause leakage of urine into the peritoneal cavity.
     • Aseptic Transfer: Discard the first drop of urine in the needle hub (which may contain skin contaminants), then transfer the sample immediately into a sterile container or a red top tube (RTT) for storage.
   • Advantages: Cystocentesis is the gold standard for collecting urine for bacterial culture and sensitivity because it bypasses all lower urinary tract contamination.

Physical Evaluation of Urine

Physical evaluation must be performed on fresh, uncentrifuged urine.

• Color: Normal urine is pale yellow to amber, which is caused by yellow pigments called urochromes.
  • Dark yellow/Amber: Indicates concentrated urine, often due to dehydration.
  • Red or Pink: Indicates hematuria (intact red blood cells) or hemoglobinuria (lysed red blood cells/hemoglobin).
  • Brown or Black: Can indicate myoglobinuria (muscle breakdown products), severe hemoglobinuria, or old blood.
  • Yellow-Brown or Orange: Indicates bilirubinuria, which suggests liver disease or hemolytic anemia.
• Clarity (Turbidity): Normal urine is clear in dogs and cats.
  • Normal Exceptions: Horses and rabbits naturally have cloudy, turbid urine. In horses, this is due to high concentrations of calcium carbonate crystals and mucus. In rabbits, it is due to high calcium excretion.
  • Abnormal Turbidity: In dogs and cats, cloudy urine indicates the presence of suspended particles, such as cells (RBCs, WBCs), bacteria, crystals, casts, mucus, or lipids.
• Odor: Normal urine has a mild, species-specific odor.
  • Ammonia: Indicates bacterial contamination or old urine where bacteria have converted urea to ammonia.
  • Sweet or Fruity: Indicates the presence of ketones, commonly seen in patients with diabetic ketoacidosis.
  • Foul/Putrid: Suggests a severe bacterial infection of the urinary tract.
• Specific Gravity (SG): Urine specific gravity measures the concentration of dissolved solutes in urine, reflecting the kidneys' ability to concentrate or dilute urine. It is measured using a refractometer. Drop a small amount of urine onto the refractometer prism, close the cover, and read the SG scale where the light and dark boundaries meet. Note: specific gravity should never be read from a chemical reagent strip, as they are inaccurate for veterinary patients.
  • Normal Ranges: Dog: 1.015 to 1.045; Cat: 1.020 to 1.060.
  • Isosthenuria (1.008 - 1.012): This occurs when the specific gravity of urine is identical to that of glomerular filtrate (the fluid entering the kidneys). It indicates that the kidneys have lost their ability to concentrate or dilute urine. Persistent isosthenuria is a hallmark sign of chronic renal failure.

Chemical Evaluation (Reagent Strip Testing)

Chemical evaluation is performed using a multi-parameter reagent strip (dipstick). The strip is briefly dipped into a well-mixed urine sample, and excess urine is tapped off to prevent colors from running between pads. The color change on each pad is compared to the chart on the bottle at specific times (typically 30 to 60 seconds).

• pH: Measures hydrogen ion concentration (acidity or alkalinity).
  • Normal canine/feline pH is usually acidic (5.5 to 7.0) due to their carnivore diet. Herbivores (horses, cattle) typically have alkaline urine (pH 7.0 to 8.5).
  • An abnormally alkaline pH in dogs or cats can indicate a UTI caused by urease-producing bacteria (e.g., Proteus or Staphylococcus), which split urea into ammonia, raising the pH.
• Protein: Normal urine should contain little to no protein.
  • Trace protein may be found in highly concentrated urine.
  • Significant protein (proteinuria) suggests renal glomerular disease, inflammation, or hemorrhage within the urinary tract.
• Glucose: Glucose is not normally present in urine.
  • Glucosuria occurs when blood glucose levels exceed the renal threshold (approximately 180 mg/dL in dogs and 280 mg/dL in cats), causing the excess glucose to spill into the urine. This is a primary indicator of diabetes mellitus. It can also occur transiently in highly stressed cats due to epinephrine-induced hyperglycemia.
• Ketones: Ketones are products of rapid, incomplete fat metabolism.
  • Ketonuria is always abnormal and indicates that the body is burning fat for energy instead of glucose. It is commonly associated with diabetic ketoacidosis (DKA), starvation, or prolonged hypoglycemia.
• Bilirubin: Bilirubin is a product of hemoglobin breakdown.
  • Dogs: Trace to 1+ bilirubin can be normal in highly concentrated male dog urine because dogs have a low renal threshold for bilirubin.
  • Cats: Bilirubinuria is always pathological in cats. Even a trace amount of bilirubin in feline urine indicates liver disease, bile duct obstruction, or intravascular hemolysis.
• Blood: The reagent strip pad detects intact red blood cells (hematuria), free hemoglobin (hemoglobinuria), or myoglobin (myoglobinuria).
  • To differentiate hematuria from hemoglobinuria, the sample must be centrifuged.
  • Hematuria: Centrifugation causes intact RBCs to settle at the bottom of the tube as a red pellet, leaving the top liquid (supernatant) clear and straw-colored.
  • Hemoglobinuria: The supernatant remains red or pink after centrifugation because the RBCs have already ruptured, releasing free hemoglobin into the liquid.