Assessing Urine’s Physical Properties

Assessing Urine’s Physical Properties

Ryane E. Englar

12.1 Procedural Definition: What Is This Test About?

Urinalysis is a diagnostic test that involves both macroscopic and microscopic examination of the patient’s sample. Urine samples are routinely obtained via cystocentesis, the preferred method of urine collection for urinalysis and urine culture. Alternate methods of urine collection include urethral catheterization, “free catch” of voided urine, or aspiration of samples off floors and tabletops, the primary disadvantage of the latter being sample contamination. Ideally, six milliliters of urine are obtained by the method of choice to allow for adequate sample size while minimizing trauma to the urinary bladder and urethra during collection.

Urinalysis begins with a macroscopic assessment of the sample. The sample is inspected to characterize its physical properties. This chapter will concentrate on the following properties of urine:

  • volume
  • color
  • clarity
  • odor.

A subsequent chapter (see Chapter 13) will cover the remaining physical property of urine, so‐called urine specific gravity (USG).

12.2 Procedural Purpose: Why Should I Perform This Test?

The evaluation of urine is a critical diagnostic tool that is used in the detection and management of a wide range of medical conditions from local involvement of the urinary tract (e.g. urinary tract infection, UTI) to systemic disease (e.g. diabetes mellitus).

Urinalysis is an essential part of the diagnostic workup for patients that present with aberrant micturition histories, including, but not limited to:

  • dysuria – difficult or painful urination
  • hematuria – bloody urine
  • oliguria – reduced urine production
  • pollakiuria – increased frequency of urination
  • polyuria – excessive urine volume or output
  • stranguria – straining during urination
  • periuria – urinary “accidents” or house soiling
  • unusual or otherwise distinct urine odor.

Urinalysis is also an essential part of monitoring disease progression and/or patient response to case management of urinary tract disease.

Because of its utility as a diagnostic tool, urine is often referred to by clinicians as “liquid gold.”

Urinalysis involves multiple steps:

  1. assessment of urine’s physical features: volume, color, clarity, and odor
  2. measurement of urine specific gravity (USG)
  3. chemical examination of urine constituents
  4. microscopic examination of urine sediment.

As diagnostic machinery advances to the point that laboratory equipment can perform steps three and four on their own, it may be tempting to gloss over urine’s physical features. This is not a new phenomenon. When Laennec invented the stethoscope in 1816, this diagnostic tool in many ways replaced what had previously been used to assess cardiovascular health: the patient’s subjective impressions and physician’s observations. However, clinicians gather unique and essential clues about disease processes through observation.

Gross observations about urine include both visual and olfactory determinations about the sample’s volume, color, clarity, and odor.

12.2.1 Sample Volume

  • Urine production ranges from 20 to 40 ml/kg/day in the average canine or feline patient. This equates to roughly 1–2 ml/kg/h [1]. This amount is contingent upon hydration status and the kidney’s ability to concentrate.
  • Note that oliguria may be pathologic, as from feline lower urinary tract disease (FLUTD) or compensatory, as in a patient’s appropriate response to dehydration.
  • Likewise, polyuria may be physiologic, as from overconsumption of water because of psychogenic polydipsia, or pathologic, as from diabetes insipidus.
  • Sample volume must be interpreted along with sample color and concentration, as measured through USG, to obtain a more complete clinical picture.

12.2.2 Sample Color

  • Urine color is a useful feature because it can be indicative of hydration status or a variety of diseased states. For example, urine that is dark yellow may suggest marked bilirubinuria whereas urine that is discolored red may suggest macroscopic hematuria.
  • A more comprehensive list of urine colors and their associations will be provided under the subheader, interpretation of test results.

12.2.3 Sample Clarity

  • Clarity is sometimes referred to as transparency. The opposite of clear urine is turbid urine. Turbidity or cloudiness may suggest increased number of cells, crystals, casts, or organisms.
  • A more comprehensive list will be provided under the subheader, interpretation of test results.

12.2.4 Sample Odor

  • Urine odor may be linked to intermediate or end‐products of metabolism. For instance, patients with ketoacidosis may develop urine that smells like acetone.
  • A more comprehensive list will be provided under the subheader, interpretation of test results.

In isolation, observations are cheap, readily available, and easy‐to‐identify diagnostic clues. They prompt pattern recognition. For instance, observing hematuria generates a list of the most common differential diagnoses: UTI, urolithiasis, urinary bladder or urethral neoplasia, and thrombocytopenia.

Observations tell us where to look next within our patient and which steps we should take now to rule out differentials.

Observations must therefore be paired with chemical examination of the urine and microscopic examination of the sediment to provide a complete portrait of the patient. This more thorough snapshot of patient health is necessary to diagnose with accuracy.

12.3 Equipment

Examination of urine’s volume, color, clarity, and odor requires the least “supplies.” You need only:

  • exam gloves for handling biofluids
  • a fresh sample of urine within a clean, clear collection container
  • a clear conical centrifuge tube or alternate means of measuring urine volume (see Figure 12.1)
  • your eyes to see
  • your nose to smell
  • a white background against which to assess urine color
  • graph paper or text on a white background against which to assess turbidity.
Photo depicts a 15 ml sterile polypropylene conical centrifuge tube with printed graduations to enable measurement of urine volume.

Figure 12.1 A 15 ml sterile polypropylene conical centrifuge tube with printed graduations to enable measurement of urine volume.

Source: Courtesy of Ryane Englar, DVM, DABVP (Canine and Feline Practice).

12.4 Procedural Steps

  1. Collect urine sample:

    • Cystocentesis

      • Advantages:

        • Ideal for urine culture if handled and transferred sterilely because the sample is not contaminated by the lower urinary tract
        • Minimal risk for iatrogenic UTI
        • Well‐tolerated by most cats and dogs with minimal restraint and without sedation

      • Disadvantages:

        • Client cannot collect sample at home
        • Requires technical skill to palpate and isolate the urinary bladder
        • Challenging to perform blind cystocentesis if urinary bladder is small
        • Ultrasound‐guided cystocentesis is preferred, but requires equipment that not all veterinary practices have access to
        • Common to see iatrogenic microscopic hematuria (<50 red blood cells [RBCs]/ high‐powered field [HPF])
        • Laceration of the urinary bladder is rare, but possible, particularly if the patient flails while the procedure is being performed
        • It is possible to inadvertently aspirate the intestinal tract and/or major abdominal vessels
        • Potential for seeding neoplastic cells throughout the abdomen if there is a mass within the urinary bladder
        • Potential for inducing uncontrolled hemorrhage if the patient has an underlying coagulopathy
        • Potential for urinary bladder rupture if the organ is extremely distended, as from urinary tract obstruction (UTO). Note that in some cases, we intentionally perform therapeutic cystocentesis in cats with UTO to reduce intravesicular pressure.

    • “Free catch” (e.g. voided; tabletop see Figure 12.2)

      • Advantages:

        • Noninvasive
        • Client can collect sample at home
        • Does not require restraint
        • Does not cause iatrogenic hematuria, which can result from cystocentesis or urethral catheterization
        • Easily obtainable from both male and female dogs
        • Can be obtained from cats by providing a box that contains non‐absorbable litter, such as commercially available hydrophobic sand, or even aquarium gravel.

      • Disadvantages:

        • Not ideal in cases that require urine culture due to potential for contamination with leukocytes, epithelial cells, bacteria, or other debris from the distal urinary tract, skin, and/or fur
          Photo depicts cat voiding in litter box.

          Figure 12.2 Cat voiding in litter box. Note that if the cat were tolerant, the cat’s owner could slide a sterile urine collection container under the stream of urine to collect a midstream sample. This technique is typically more common in canine patients than feline patients because cats tend to eliminate in the absence of spectators.

          Source: Courtesy of Ryane Englar, DVM, DABVP (Canine and Feline Practice).

        • Contamination may also be due to the collection container itself if a sterile, single‐use container is not used.

    • Manual urinary bladder expression

      • Advantages:

        • Easily performed in patients who are under general anesthesia (e.g. immediately prior to ovariohysterectomy or orchiectomy)
        • Part of routine care for patients with urine retention, as from decreased detrusor contractility; these patients require manual expression of the urinary bladder.

      • Disadvantages:

        • Too much force can cause iatrogenic hematuria and proteinuria, +/− urinary bladder rupture
        • Not ideal in cases that require urine culture due to potential for contamination with leukocytes, epithelial cells, bacteria or other debris from the distal urinary tract, skin and/or fur
        • Inappropriate in patients who have just undergone cystotomy.

    • Urethral catheterization

      • Advantages:

        • Relatively easy to perform in male dogs

      • Disadvantages:

        • Client cannot collect sample at home
        • Challenging to place urethral catheters in female dogs and female cats
        • Requires sedation and/or anesthesia for female dogs, female cats and male cats
        • Catheter‐induced trauma may contaminate the sample with iatrogenic hematuria and transitional epithelial cells; iatrogenic proteinuria is common
        • Increased risk of iatrogenic UTI secondary to the procedure, particularly in female dogs
        • Not advised to place urethral catheters in male cats unless they are presenting with UTO, in which case urethral catheterization constitutes essential therapy. Placing a urethral catheter in an otherwise healthy male cat for the sole purpose of collecting urine is not advised because there is a risk of iatrogenic UTO by irritating the lining of the anatomically narrow urethra.

  2. Place urine sample in a clean, sterile, leak‐proof container with a closed lid or top, labeled with the following details:

    • animal identification (e.g. name and case number)
    • owner identification: (e.g. client last name)
    • date and time of sample collection
    • how sample was obtained.

  3. Analyze the urine sample within 30 minutes of collection to maximize accuracy of the results [2].

    Prolonged storage may potentiate bacterial overgrowth as well as secondary artifacts such as decreased glucose and changes in pH [2].

    If analysis must be delayed beyond the 30‐minutes mark, then refrigeration of the sample is essential to slow bacterial proliferation and degeneration of both cells and casts [3].

    Refrigerated samples should not be analyzed beyond 24 hours after collection.

    Delayed analysis may result in changes that influence observations.

    For instance, hemoglobin, as from the breakdown of erythrocytes, imparts a red color to the urine. Prolonged exposure of urine to the elements will oxidize hemoglobin. The iron that is contained within hemoglobin (Hb) will transition from the ferrous [Fe+2] to ferric [Fe+3] state, forming methemoglobin (MetHb). MetHb is unable to bind oxygen and will therefore convey a brownish color to the urine instead of reddish (see Figures 12.3a and b). An observer may interpret this brown color as true methemoglobinuria secondary to toxicosis, as from acetaminophen, onion, or garlic toxicosis, when in fact, the color has resulted from delayed analysis.

  4. If the urine sample has been refrigerated, allow it to come to room temperature before analyzing further.
  5. Mix the urine sample well by inverting the sealed container.
    Photos depict (a) an example of feline hemoglobinuria. (b) An example of feline methemoglobinuria.

    Figure 12.3 (a) An example of feline hemoglobinuria.

    Source: Courtesy of Dr. Whitney Rouse;

    (b) an example of feline methemoglobinuria.

    Source: Courtesy of Ryane E. Englar, DVM, DABVP (Canine and Feline Practice).

    Particles that contribute to turbidity may settle out if the sample is left to stand. Turbidity is a measure of sample cloudiness (the opposite of clarity) and is a subjective measurement.

  6. Transfer 0.5–1.0 ml of the sample to a sterile urine collection tube if urine culture is indicated.

    • Select a clean (empty; unused; sterile) red‐top vacutainer tube.
    • Label this vacutainer tube as containing urine to differentiate from serum.
    • Swab the top of the unused (clean; empty) red‐top vacutainer tube with alcohol.
    • If the urine sample was collected via cystocentesis and is still contained within the syringe, remove the used needle from the syringe safely and place a new needle on the syringe.
    • Insert the clean needle (with syringe attached) into the top of the red‐top vacutainer tube.
    • The vacuum in the red‐top vacutainer tube will begin to draw urine into the tube.
    • You need less than 1.0 ml of urine to perform a urine culture. After this volume has been drawn into the tube, you can pull the needle and syringe out of the tube.
    • Remove the used needle from the syringe safely.
    • Uncap a new urine collection tube.
    • Fill this new collection tube with the remainder of your urine sample.

  7. Measure the volume of the remaining sample by pouring the urine into a conical centrifuge tube with printed graduations or any alternate measuring tool, as in a graduated cylinder. Record this volume.

    Note that if the volume measured exceeds 10 ml, then it is acceptable to record this value as “>10 ml.”

  8. Identify and record the color of the urine sample using the appropriate descriptor:

    1. colorless (meaning transparent, like water)
    2. pale yellow
    3. yellow
    4. yellow amber
    5. yellow green
    6. deep yellow
    7. yellow orange
    8. orange
    9. peach
    10. pink
    11. pink‐brown
    12. red
    13. red‐brown or rust
    14. brown
    15. black
    16. blue‐green
    17. milky white
    18. “other.”

    Refer to the subsection below, “Interpreting Test Results,” to consider what each color may indicate.

  9. Evaluate the sample for clarity or turbidity and record the appropriate descriptor:

    1. clear
    2. slightly cloudy
    3. cloudy
    4. turbid
    5. flocculent.

    Refer to the subsection below, “Interpreting Test Results,” to consider what each category of turbidity may indicate.

  10. Smell the urine and record any unusual features.

    Waft air over the open sample toward your nose to take in the aroma without inhaling urine droplets.

    Wearing a mask during this phase of urinalysis will minimize risk of inhalation. Certain infectious diseases are transmitted through the urine, including, but not limited to, leptospirosis.

    Refer to the subsection below, “Interpreting Test Results,” to consider what specific scents may indicate.

12.5 Time Estimate to Perform Test

Five minutes.

12.6 Procedural Tips and Troubleshooting

12.6.1 Urine Collection

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May 3, 2023 | Posted by in SMALL ANIMAL | Comments Off on Assessing Urine’s Physical Properties

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