Urinalysis: What is your interpretation?

Urinalysis is one of our most important clinical diagnostic tools. Unfortunately, most diagnostic reagent strips used to perform routine urinalyses in veterinary laboratories have been designed for human use.

Although these reagent strips provide useful information when used to evaluate urine samples collected from animals, test results obtained with portions of several urine diagnostic reagent strips are unreliable.

Are you familiar with the limitations of the specific brand(s) of diagnostic reagent strip used in your hospital? Here are the limitations of most reagent strips, along with suggestions to minimize errors associated with them:

1. URINE SPECIFIC GRAVITY: values of dogs and cats obtained with reagent strips are usually unreliable. Because the highest value that these reagent pads can detect is 1.025 to 1.030 (Table 1), they are unsatisfactory to detect adequate renal concentrating capacity in dogs or cats.

Osmolality and specific-gravity values for adult dog, cat and human urine

TIP: Use a refractometer to determine urine specific-gravity values.

2. URINE PH test pads included with urinalysis reagent strips have been designed to allow measurement of pH values to within +/- 0.5 units. This is satisfactory for routine evaluation of most patients. However, these test pads will not permit precise determination of urine pH values. Also, care should be used to prevent runover from highly acidic protein test pads to adjacent urine pH test pads to prevent a spurious reduction in pH.

Factors influencing numbers of casts, white cells, red blood cells or crystals in urine sediment of dogs and cats

TIP: Use a (portable) pH meter to confirm results, especially when trying to measure relatively small changes in pH units.

3. GLUCOSE TEST PADS typically contain labile enzymes (glucose oxidase and peroxidase). If these proteinaceous enzymes become inactive, false-negative results will result.

TIP: Become familiar with the expiration date listed on the container by the manufacturer. The viability of enzymes in an unopened container may be prolonged by freezing the package. (Check with the manufacturer for details.) Evaluate test results in conjunction with evaluation of tests for glucose by a different method (such as copper reduction method).

4. BILIRUBIN REAGENT PADS may be unreliable as screening tests in dogs because of a high percentage of false-positive and false-negative results in this species. In contrast, positive bilirubin test results in cats are usually indicative of some underlying disease.

TIP: Consider positive or negative urine bilirubin test results in conjunction with other clinical findings.

5. THE OCCULT BLOOD TEST pad may detect red blood cells, free hemoglobin or myoglobin.

TIP: Evaluate test results in conjunction with evaluation of urine sediment and other clinical findings.

6. PROTEIN TEST PADS are approximately twice as sensitive to albumin compared to globulin, and three times more sensitive to albumin compared to mucoproteins. In addition, false-positive results may be obtained if the urine pH is very alkaline.

TIP: Compare questionable test results to results obtained by another test method, such as the sulfosalicylic acid (turbidometric test). Determination of urine protein-creatinine ratios are a more reliable method to determine the magnitude of pathologic proteinuria.

7. UROBILINOGEN TEST PADS have not been useful in the routine evaluation of canine and feline urine. A computer search of approximately 11,000 canine urinalysis records yielded only 0.015 percent positive urobilinogen tests. A computer search of approximately 2,070 feline urinalysis records yielded only 0.02 percent positive urobilinogen tests, and of these, only 14 were equal to or greater than 1+.

TIP: Urobilinogen test pads are unreliable to screen patients for hemolytic disorders, hepatic disorders or patency of bile ducts. We typically ignore this test result.

8. NITRITE TEST PADS used as an indirect indication of bacteriuria in humans uniformly give false-negative results in dogs and cats. Available information indicates that false-negative results probably are associated with interference caused by ascorbic acid normally present in canine and feline urine.

TIP: Evaluate urine sediment and/or bacterial cultures to rule in or rule-out bacterial urinary tract infections.

9. LEUKOCYTE TEST PADS give false-positive test results in most cats in absence of pyuria.

TIP: Evaluate fresh urine sediment to determine the presence or absence of leukocytes in cats.

10. LEUKOCYTE TEST PADS frequently give false-negative test results in dogs, even when pyuria is present. Although the test is specific for WBC in dogs, it is very insensitive.

TIP: Evaluate fresh urine sediment to determine the presence or absence of leukocytes in dogs.

What is your interpretation of the following urinalysis results?

The following nine case scenarios are designed to facilitate self-assessment of your interpretation of routine urinalysis of several patients admitted to the Veterinary Teaching Hospital, University of Minnesota.

When choosing your answer, remember to distinguish between observations and interpretations, and to consider the best answer in terms of probabilities rather than possibilities.

Please note that hpf means high-power microscope field (450 X), and lpf means low-power microscope field (100 X). See the end of the discussion for my explanations of these answers.

CASE 1

Consider the following results obtained by analysis of a urine sample obtained during normal voiding from a 7-year-old spayed yellow Labrador Retriever.

Color - light yellow

Protein - 4+

Turbidity – clear

RBC - 1 - 2 /hpf

Specific gravity - 1.025

WBC- 0 - 1 /hpf

pH - 6.0

Casts - occasional hyaline/lpf.

Glucose – negative

Epithelial cells – occasional

Acetone – negative

Bacteria – none

Bilirubin – negative

Crystals – none

Occult blood - negative

The best interpretation of the results of this urinalysis is that the patient:

a. Is normal.

b. Has generalized glomerular disease.

c. Has an inflammatory process somewhere along the urinary tract.

d. Has generalized tubular disease.

e. Has findings indicative of congestive heart failure or fever.

CASE 2

Consider the following results obtained by analysis of a voided urine sample from an 8-year-old, male Basset Hound:

Color –yellow

Protein – 1+

Turbidity – clear

RBC - 1 - 2 /hpf

Specific gravity - 1.060

WBC- 0 - 1 /hpf

pH – 7.0

Casts - none

Glucose – negative

Epithelial cells - occasional

Acetone - negative

Bacteria - none

Bilirubin - 1+

Crystals - few amorphous

Occult blood - negative

The best interpretation of the results of this urinalysis is that the patient:

a. Has hepatic disease.

b. Has an inflammatory disorder of the urinary tract.

c. Is normal.

d. Has acute renal failure.

e. Has slight injury affecting the glomerular capillary walls.

CASE 3

Consider the following results obtained by analysis of a voided urine sample obtained from a 5-year-old neutered male Persian cat:

Color – yellow

Protein -trace

Turbidity - clear

RBC - 1 - 3 / hpf

Specific gravity - 1.045

WBC- 0 - 1/ hpf

pH - 7.0

Casts - none

Glucose - negative

Epithelial cells - occasional

Acetone - negative

Bacteria - none

Bilirubin - 1+

Crystals - few amorphous phosphates

Occult blood - negative

The best interpretation of the results of this urinalysis is that this feline patient:

a. Has significant bilirubinuria.

b. Has inflammatory disease syndrome along the urinary tract.

c. Is normal.

d. Has acute renal failure.

e. Has lower urinary tract disease.

CASE 4

Consider the following results obtained by analysis of a fresh urine sample obtained by cystocentesis from a 10-year-old spayed Pomeranian dog.

Color - yellow

Protein - 2+

Turbidity - cloudy

RBC - numerous / hpf

Specific gravity - 1.035

WBC - numerous / hpf

pH - 8.0

Casts - none

Glucose - negative

Epithelial cells - many

Acetone - negative

Bacteria - many cocci

Bilirubin - negative

Crystals - moderate struvite

Occult blood - 4+

The best interpretation of the results of this urinalysis is that the patient:

a. Has an inflammatory process somewhere along the urinary tract caused by bacterial infection.

b. Has an inflammatory process somewhere along the genitourinary tract caused by bacterial infection.

c. Has an inflammatory process somewhere along the urinary tract caused or complicated by bacterial infection.

d. Is normal, and the sample was contaminated during the process of analysis.

e. Has struvite uroliths associated with urinary-tract infection.

CASE 5

Consider the following results obtained by analysis of a voided urine sample obtained from a 10-year-old domestic neutered shorthair cat.

Color - yellow

Protein - negative

Turbidity - clear

RBC - negative

Specific gravity - 1.010

WBC - negative

pH - 6.0

Casts - negative

Glucose - negative

Epithelial cells - negative

Acetone - negative

Bacteria - negative

Occult blood - negative

Crystals - negative

Bilirubin- negative

The best interpretation of the specific-gravity value of this urinalysis is that:

a. The patient's kidneys have lost the ability to concentrate and dilute urine.

b. The patient's kidneys have lost the ability to concentrate urine.

c. The patient's kidneys have lost the ability to dilute urine.

d. No conclusions can be established about the ability of the patient's kidneys to concentrate and dilute urine.

e. The patient has a fixed urine specific gravity, probably as a result of primary renal failure.

CASE 6

Consider the following results obtained by analysis of a voided urine sample from a 9-yr old, male Miniature Poodle:

Color - yellow

Protein - 1+

Turbidity - cloudy

RBC - 5-10/hpf

Specific gravity - 1.037

WBC - 5-10/hpf

pH - 6.0

Casts - many granular/lpf

Glucose - negative

Epithelial cells - moderate/hpf

Acetone - negative

Bacteria - none

Bilirubin - negative

Crystals - few calcium oxalate/hpf

Occult blood - 1+

The most probable interpretation of the results of this urinalysis is that the patient:

a. Has acute primary renal failure.

b. Has chronic primary renal failure.

c. Has acute renal disease.

d. Has chronic renal disease.

e. Is normal.

CASE 7

Consider the following results obtained by analysis of a voided urine sample from a 5-year-old, male Golden Retriever:

Color - yellow

Protein - 1+

Turbidity - cloudy

RBC - 0 - 1/hpf

Specific gravity - 1.045

WBC - 0 - 1/hpf

pH - 7.5

Casts - none

Glucose - negative

Epithelial cells – occasional/hpf

Acetone - negative

Bacteria - none

Bilirubin - negative

Crystals - occasional struvite/lpf

Occult blood - negative

The best interpretation of the significance of the proteinuria is that it is:

a. Clinically insignificant.

b. Indicative of generalized glomerular disease.

c. Indicative of generalized tubular disease.

d. Indicative of damage to the transitional epithelium lining the urinary tract.

e. Indicative of preglomerular proteinuria.

CASE 8

Consider the following results obtained by analysis of a voided urine sample obtained from a 9-year-old spayed female Boxer.

Color – light yellow

Protein - negative

Turbidity - clear

RBC - negative

Specific gravity - 1.003

WBC - negative

pH - 6.5

Casts - none

Glucose - negative

Epithelial cells - none

Acetone - negative

Bacteria - none

Occult blood - negative

The best interpretation of the specific gravity value of this urinalysis is that:

a. There is an insufficient population of viable nephrons to concentrate glomerular filtrate.

b. There is a sufficient population of the viable nephrons to dilute glomerular filtrate.

c. There is a sufficient population of nephrons to concentrate or dilute glomerular filtrate.

d. All of the nephrons of both kidneys are functional.

e. More than two-thirds of the nephrons of both kidneys are nonfunctional.

CASE 9

Consider the following results obtained from analysis of a voided urine sample from an anorectic obese, well-hydrated, 12-year-old male DSH cat:

Color - yellow/orange

Protein - trace

Turbidity - cloudy

RBC - none

Specific gravity - 1.035

WBC = 0-1/lpf

pH - 6.0

Casts - none

Glucose - negative

Epithelial cells - occasional

Acetone - negative

Bacteria - none

Bilirubin = 3+

Crystals - occasional calcium oxalate

Occult blood = negative

The best interpretation of the results of this urinalysis is that the patient:

a. Is normal.

b. Probably has obstructive and/or hepatocellular liver disease.

c. Probably has hemolytic jaundice.

d. Probably has generalized glomerular disease.

e. Probably has calcium oxalate uroliths.

EXPLANATION OF ANSWERS TO INTERPRETATION OF URINALYSIS

The most probable answer to question 1 = b Comments about answers to question 1:

a. Proteinuria of this magnitude is definitely not normal.

b. Marked proteinuria in the absence of significant numbers of red blood cells and white cells is the hallmark of generalized glomerular disease.

c. Absence of significant numbers of white cells eliminates the likelihood of an inflammatory response.

d. Proteinuria of this magnitude would not be expected as a result of primary tubular disease.

e. Proteinuria of this magnitude would not be expected as a result of preglomerular causes.

The most probable answer to question 2 = c Comments about answers to question 2:

a. A mild degree of bilirubinuria in dog urine often is a normal finding. However, bilirubinuria of any magnitude is usually indicative of some underlying disease in cats.

b. Proteinuria, hematuria and pyuria of this magnitude are normal findings, especially in highly concentrated urine.

c. See comment a.

d. Concentration of urine to this degree eliminates a diagnosis of primary renal failure.

e. Proteinuria of this magnitude in highly concentrated urine is not indicative of glomerular disease.

The most probable answer to question 3 = a Comments about answers to question 3:

a. Unlike dogs, bilirubinuria in cats, even when present in small quantities in concentrated urine, is usually indicative of an underlying disorder.

b. Hematuria and pyuria of this magnitude in highly concentrated urine is usually a normal finding.

c. See preceding comment in a.

d. The capacity to concentrate urine to this degree eliminates a diagnosis of primary renal failure.

e. See preceding comment in b.

The most probable answer to question 4 = c Comments about answers to question 4:

a. The results do not permit differentiation between bacterial infection as a cause or complication of urinary tract infection. Most bacterial urinary-tract infections develop secondary to abnormalities in host defense mechanisms.

b. Although this answer is not totally incorrect, to conclude that the genital tract is definitely involved in a sample obtained by cystocentesis is an overstatement.

c. In light of the preceding comments for a and b, this appears to be the best interpretation.

d. The results are definitely abnormal and would not be expected to be caused by contamination.

e. Although magnesium ammonium phosphate uroliths may be present, struvite crystalluria is not by itself a reliable index of their occurrence.

The most probable answer to question 5 = d Comments about answers to question 5:

a. Since a urine specific gravity of 1.010 may represent a normal or abnormal value, further information must be obtained before renal concentrating and diluting capacity can be assessed. However, this specific-gravity value should arouse suspicion that an underlying disorder may be present.

b. See preceding comments for a.

c. See preceding comments for a.

d. See preceding comments for a.

e. See preceding comments for a.

The most logical answer to question 6 = c Comments about answers to question 6.

a. Concentration of urine to this degree indicates that an adequate population of nephrons are functional to prevent primary renal failure. The small quantity of protein and high specific gravity indicate that primary renal failure and primary glomerular disease are improbable.

b. See comments in a. There is no evidence of renal failure: Magnitude of urine concentration indicates that renal failure is improbable. However, observation of many casts indicates that the renal disease is active and could progress to renal failure.

c. Detection of many granular casts is indicative of renal disease affecting at least the renal tubules, even though urine concentration indicates that the disease is not of sufficient magnitude to cause renal failure. The large number of casts suggests (but does not prove) that the disorder is acute. The acute renal disease is associated with an inflammatory response.

d. This answer is possible, although c is more probable. Additional information would be required to prove or disprove either answer.

The findings are definitely abnormal.

The most probable answer to question 7 = a Comments about answers to question 7:

a. Detection of mild proteinuria and occasional struvite crystals in highly concentrated urine is a common finding in dogs.

b. This interpretation is a possibility, but not a probability. Additional follow-up studies would be required to prove glomerular disease.

c. This interpretation is a possibility, but very unlikely in light of other findings.

d. Damage to the mucosa lining the urinary tract would be expected to be associated with an inflammatory response, hematuria and/or many transitional epithelial cells.

e. Further information would be required to verify this conclusion.

The most probable answer to question 8 = b Comments about question 8

a. The conclusion could not be established without knowledge of the patient's hydration status, a water-deprivation test and perhaps a vasopressin-response test. The fact that urine dilution of this magnitude has occurred suggests, but does not prove, that a sufficient population of functional nephrons is present to concentrate urine.

b. Dilution of glomerular filtrate to this degree requires a sufficient population of functional nephrons (at least one-third in dogs) to remove solute in excess of water (e.g. urine dilution).

c. See comment a.

d. It is not possible to evaluate the functional status of all nephrons on the basis of urine specific gravity. Many nephrons may be nonfunctional (up to two-thirds in dogs, and probably more in cats) without loss of the ability to concentrate urine, at least to some degree.

See comment b.

The most probable answer to question 9 = b Comments about answers to question 9

a. Bilirubinuria of this magnitude is not normal.

b. Bilirubinuria of this magnitude is usually associated with hepatocellular or obstructive liver diseases.

c. Hemolytic jaundice may be associated with bilirubinuria, but also is typically associated with a positive occult blood test. Caution: In-vitro studies indicate that positive occult blood tests associated with strong positive tests for bilirubin may be caused by bilirubin, rather than RBC or hemoglobin.

d. Proteinuria of this magnitude is not indicative of significant alteration of glomerular capillary permeability.

e. Calcium oxalate crystalluria is not a reliable index of the occurrence of uroliths, although in patients with signs of urinary tract disease it may warrant further evaluation.

Carl A. Osborne

Dr. Osborne, a diplomate of the American College of Veterinary Internal Medicine, is professor of medicine in the Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota.