| Urine must be at room temperature for accurate measurement of USG and for chemical analysis. These tests are usually done prior to centrifugation; however, if urine is discolored or turbid, it may be beneficial to perform these tests on supernatant (see
urine sediment,
Urine Sediment). |
| The USG is determined using a refractometer designed for veterinary samples, which includes a scale calibrated specifically for cat urine. USG for species other than cats should be determined using the scale for dogs. In healthy animals, USG is highly variable, depending on fluid and electrolyte balance of the body. Interpretation of USG, therefore, depends on the clinical presentation and serum chemistry findings (see also
urinary system introduction,
Urinary System, Introduction: Introduction). An animal that is dehydrated or has other causes of prerenal azotemia will have hypersthenuric urine with a USG >1.025-1.040 (depending on species). Dilute urine in a dehydrated or azotemic animal is abnormal and could be caused by renal failure, hypo- or hyperadrenocorticism, hypercalcemia, diabetes mellitus, hyperthyroidism, diuretic therapy, or diabetes insipidus. Glucosuria increases the
refractive index of urine, resulting in an increased USG despite increased urine volume. |
| Reagent strips such as Multistix® or Chemstrip® can be used to perform several semiquantitative chemical evaluations simultaneously. They are used routinely to determine urine pH, protein, glucose, ketones, bilirubin/urobilinogen, and occult blood. Some reagent strips include test pads for leukocyte esterase (for detection of WBC), nitrite (for detection of bacteria), and USG; these are not valid in animals and should not be
used. Reagent strips are adversely affected by moisture and have a limited shelf life. Bottles should be kept tightly capped, and unused strips should be discarded after their expiration date. |
| Urine pH is typically acidic in dogs and cats and alkaline in horses and ruminants, but varies depending on diet, medications, or presence of disease. Reagent strip colorimetric test pads for pH determination are accurate to within ~0.5 pH units. For example, a reading of 6.5 means the actual pH is likely to be between 6.0 and 7.0. A bacterial urinary tract infection with a urease-producing microbe will result in alkaluria. Urine pH will affect crystalluria because some
crystals, such as struvite, form in alkaline urine, while other crystals, such as cystine, form in acidic urine. |
| The protein test pad detects primarily albumin in urine. Proteinuria can be seen with inflammation, hemorrhage, or glomerular disease. A positive reaction must be interpreted in light of USG, pH, and urine sediment examination. For example, a trace amount of protein in concentrated urine is less significant than a trace amount of protein in dilute urine. Alkaluria will give a false positive reaction. Likewise, presence of other proteins, such as Bence-Jones proteins, will give
false negative results. Proteinuria can be measured using sulfosalicylic acid precipitation, which detects albumin and globulins. If proteinuria is present with an inactive urine sediment, its significance can be verified and quantitated by dividing the urine protein concentration by the urine creatinine concentration (urine protein to urine creatinine ratio; UP:UC). Interpretation of a UP:UC is as follows: <0.5:1 is normal, 0.5-1.0:1.0 is questionable, and >1.0:1.0 is
abnormal. It is important to ensure that hematuria, pyuria, and infection are not present before determining a UP:UC because inflammation and hemorrhage result in significant proteinuria. |
| Glucosuria is not present normally because the renal threshold for glucose is >180 mg/dL in most species and >240 mg/dL in cats. With euglycemia, the amount of filtered glucose is less than the renal threshold and all of the filtered glucose is reabsorbed in the proximal renal tubules. Glucosuria can result from hyperglycemia (due to diabetes mellitus, excessive endogenous or exogenous glucocorticoids, or stress) or from a proximal renal tubular defect (such as primary
renal glucosuria or Fanconi syndrome). If glucosuria is present, blood glucose concentration should be determined. |
| The ketone test pad detects acetate and acetoacetate, but not β-hydroxybutyrate. Ketonuria is associated with primary ketosis (ruminants), ketosis secondary to diabetes mellitus (small animals), and occasionally with prolonged fasting or starvation. A false positive reaction can occur with presence of reducing substances in urine. |
| Presence of conjugated bilirubin in urine will result in a positive reaction. A tablet test, Ictotest®, can be used to detect bilirubin as well. Bilirubinuria occurs when conjugated bilirubin exceeds the renal threshold as with liver disease or hemolysis. In dogs with concentrated urine, a small amount of bilirubin can be normal. Pigmenturia may result in a false positive reaction. Urobilinogen, formed from bilirubin by intestinal microflora, is
absorbed into the portal circulation and is excreted renally. A small amount of urinary urobilinogen is normal. Increased urinary urobilinogen occurs with hyperbilirubinemia; a negative test may be observed with biliary obstruction. However, the test is not specific enough to be clinically useful. |
| The occult blood test pad uses a “pseudoperoxidase” method to detect intact RBC, hemoglobin, and myoglobin. A positive reaction can be due to hemorrhage (hematuria), intravascular hemolysis (hemoglobinuria), or myoglobinuria. The latter 2 processes can be distinguished by examination of plasma—plasma will appear pink to red after intravascular hemolysis, while myoglobin is rapidly cleared from plasma, resulting in clear plasma. As with other colorimetric test pads,
discolored urine may yield false positive results. A positive result should be interpreted with microscopic examination of urine sediment. |
