Contributors: Melissa Wallace, Roger Powell
Species: Feline | Classification: Lab Tests
- Changes in the chemical and physical characteristics of urine occur in many diseases, eg urogenital disease, hepatic disease, hematological disorders, endocrinopathies, neoplasia and musculoskeletal disease.
- Changes in these characteristics can aid in the diagnosis of many diseases in ill animals and act as a cost-effective screening test in asymptomatic animals, or for monitoring specific disease changes and therapeutic response.
- Upper (pyelonephritis) and lower urinary tract infection (Cystitis: bacterial).
- Urolithiasis (Urolithiasis), ureter (ureterolithiasis) Ureter: obstruction, idiopathic cystitis Idiopathic cystitis.
- Diabetes: mellitus (Diabetes mellitus) and insipidus Diabetes insipidus.
- Acute renal failure Kidney: acute renal failure, less use if anuric though.
- Portocaval shunt Congenital portosystemic shunt (CPSS), ammonium biurate or urate crystals.
- Transitional cell carcinoma Bladder: neoplasia. Diagnostic wash or catheter suction aspirates, the latter more accurate but often requires targeted imaging. Direct percutaneous aspiration controversial due to field theory and perceived or unclear to minimal risk of metastatic seeding.
WASH: Empty bladder, infuse 10-20 ml phosphate-buffered saline, massage bladder transabdominally, suction saline off into an EDTA and plain tube, examine sample.
- PUPD: polyuria /polydipsia.
- With serum-plasma urea Blood biochemistry: urea and creatinine test Blood biochemistry: creatinine and albumin too Blood biochemistry: albumin - to assess renal function Renal function assessment.
- Alongside liver enzymes and serum (pre + 2 hr post-prandial) bile acids with possibly ammonia test (Blood biochemistry: ammonia NB very labile, separated frozen-chilled plasma in 20 mins, smoking and disinfectant artifact) - to assess hepatic encephalopathy Hepatic encephalopathy due to portocaval shunt Congenital portosystemic shunt (CPSS).
- With serum glucose test Blood biochemistry: serial blood glucose and fructosamine Blood biochemistry: frucotsamine - to confirm diabetes mellitus and prolonged hyperglycemia respectively.
- With ultrasonography Ultrasonography: bladder and urinary tract Ultrasonography: kidney - to assess hematuria and urogenital structures such as the prostate.
- With hematology Hematology: complete blood count (CBC) - in severe (intravascular) hemolytic disease Anemia: immune-mediated hemolytic.
- With hematology and biochemistry such as cholesterol - to assess endocrinopathy.
Source of Test Material
- 1-5 ml.
Sample Collection Technique
- Collect at least 1ml to allow most routine analysis and a standard volume for sediment examination.
- Trauma during collection (catheterization, compression or cystocentesis) may contaminate the sample (RBCs especially, catheterization epithelial cells also). Note how the sample is taken to allow interpretation and comparison.
- The longer the delay between collection and analysis, the less reliable the results.
- If not analysing in 1-2 hours, the sample should be refrigerated (2-8°C or 35-45°F).
- Return urine to room temperature before analysis (for dipstick and refractometer USG).
- Protect sample from exposure to light (UV light degrades urobilinogen and bilirubin).
- Use of non absorbent cat litter for sampling at home relies on a clean and rinsed litter tray to prevent detergent or other cleaning products contaminating the urine collected.
Timing of test
- The composition of urine varies throughout the day.
- Sample for screening test at any time of day but a first morning sample provides the greatest amount of information initially.
- Repeat or serial testing for specific parameters, such as fresh for cells, may be required.
Early morning sample
- Advantage: most representative and likely to be concentrated with higher yield of cells, casts and bacteria.
- Disadvantage: as the urine is stored in vivo, cells and casts degrade naturally, especially if dilute and alkaline.
Recently formed sample
- Advantage: cytological and cast detail best.
- Disadvantage: may be affected by recent feeding and water intake, with fewer cells or casts exfoliating.
- Sample can be refrigerated at 2-8°C (35-45°F) for a few hours (up to 6-12 h for many urine parameters).
- If sample is to be stored >12 h and is of sufficient volume, it can be split to give a plain sample and a preserved sample at room temperature. If using a preservative, the correct volume must be used to maintain the proper ratio and not distort the urinary material. The preservative must be chosen specifically as it will typically prevent some of the routine analysis otherwise performed on the plain sample, especially chemical based (eg dipsticks).
- Boric acid (0.8%): for bacterial numbers up to 72 h, fill to the line and invert to gently mix and dissolve. It will also stabilize cells to a degree.
- (Diluted) Formaldehyde (1 drop 40% per 30 ml): for cells and casts especially but requires special staining and special handling.
- Refrigerate the separate plain sample/aliquot.
- The longer the delay between collection and analysis, the less reliable the results.
- Temperature >8°C (>45°F) for 1-2 hr causes artifacts, even preserved samples unsuitable if exposed to temperature extremes (heat causes degradation, freezing lyses cells).
- Package according to Post Office or shipping regulations Transportation of diagnostic specimen.
- Color: to check for possible interference and abnormal (non yellow) colors (eg red-blood).
- Turbidity: macroscopic parameter which results from noted sediment findings (eg lipid droplets in iatrogenic samples).
- Warm refrigerated sample to room temperature as the USG and enzymatic tests are all temperature dependent.
- Centrifuge a set volume (minimum 0.5 ml) at slow speed for 10 mins (eg 500g, see conversion formula for rpm).
- Decant supernatant for chemical analysis and USG, leaving sediment for microscopy (eg 1 drop following 0.5 ml centrifugation).
- Determine specific gravity (USG) Urinalysis: specific gravity by refractometer on the supernatant.
- Dipstick specific gravity pads do not correlate well with refractometry and should not be used. Veterinary specific refractometers are ideal which have a specific cat reference scale too. Otherwise the standard (human) scale can be converted using the formula:
- Feline SG = (0.846 x reading) + 0.154
- Perform dipstick analysis Urinalysis: dipstick on the supernatant, comparing the color change of dipstick to the reference range at the correct time of reading (often 1 min / 60 s). The color change is typically enzymatic so time and temperature dependent too.
- Rapidly and completely immerse the dipstick and then tip sideways to blot off excess and prevent cross-reaction between the pads. Alternatively if a small volume, use a pasteur pipette to streak or pipette urine drops onto each pad, before tipping off the excess.
- Leave 0.25 ml urine in the Eppendorf or centrifuge tube.
- Resuspend the sediment by repeatedly flicking or ‘racking’ the centrifuge tube. Repeated (5 times) gently withdrawing and expressing via the pipette can also be used.
- Using a fine tip pasteur pipette, once evenly resuspended, transfer one drop of sediment to a microscope slide and place a coverslip (18 x 18mm typically) over it.
- Lower the substage condenser on the microscope to maximise the diffraction.
- Systematically examine the entire specimen under the lower power objective (x10-20) initially to check and semi-quantify (occ., +, ++, +++) for large epithelial cells, casts and crystals.
- Examine the sediment under the high power objective (hpf, x40) to confirm the morphology of certain elements (eg small calcium oxalate crystals) and to detect bacteria. Average the number of red and white cells per hpf by counting at least 5 fields of view.
- 1 drop of Sedistain TM or a supravital stain such as Sternheimer-Malbin Staining techniques: Sternheimer-Malbin can be added to the standard sediment volume to highlight some cell features, notably nuclei for WBC identification. It does though obscure crystal colours.
- However, stain deposits or precipitates and bacterial contaminant growth in the reused bottle and its pipettor, as well as varying the number of drops used, all lead to misleading results and mis-diagnosis, such as confusing precipitate as bacteria to mis-diagnose a UTI.
- Can be performed in-house by veterinary clinics with an SOP and appropriate training.
- Access to variable speed centrifugation quickens the analysis but is not essential.
- All external laboratories but potentially greater in vitro change.
Technician (Extrinsic) Limitations
- A standard operating procedure (SOP) should always be used on a designated plain and standardised volume of urine to provide accurate, reproducible and consistent results. This allows disease changes and therapy to be monitored over time accurately, no matter who performs the testing. Experience adds to the familiarity and speed over time.
Normal (Reference) Values
- Amber yellow to straw.
- Very dilute urine tends to be almost colourless, while concentrated urine is dark yellow.
- Transparent and clear.
Specific gravity (USG)
- 1.001->1.060 (depending on hydration and water intake) Urinalysis: specific gravity.
- Hyposthenuria Hyposthenuria: <1.008.
- Isosthenuria: 1.008-1.012.
- Hypersthenuria: >1.012.
- Acidic as carnivorous: 6-7 typically Urinalysis: dipstick analysis.
Some normal cats may be outside this, in vitro storage also potentially affecting pH.
- Must interpret in conjunction with USG and sediment Urinalysis: protein. Myelomatous paraproteins will not cause a positive reaction on standard dipsticks.
- Concentrated urine (USG>1.030) may have trace protein, dilute urine possibly too little for detection though.
- Protein /urine ratio (UPCR), creatinine <0.2, Urinalysis: creatinine.
- Part of the IRIS staging scheme for renal failure.
- Negative Urinalysis: glucose.
- Negative Urinalysis: ketone.
- Negative Urinalysis: bilirubin.
- Trace (due to intact enterohepatic recirculation) Urinalysis: urobilinogen.
- Negative Urinalysis: hemoglobin.
- Cystocentesis and catheterization: typically a few red cells in low numbers (single figures to low doubles per hpf) depending upon technique and degree of trauma during needle advancement, or trauma and difficulty when catheterized) Hematuria.
- Free flow / voided sample: <3 red cells / hpf unless during estrus in females.
- Negative Urinalysis: nitrate.
White blood cells (WBCs)
- Convention states <3 / hpf but typically less than 1 in health Urinalysis: white blood cells. NB again estrous cycle factors in females for voided or catheterized samples can contain low numbers (<10 / hpf typically)
- Dipstick pads are based on esterase reactions and are human derived. In cats they are unreliable and should not be used, most commonly giving a false positive with no pyuria or infection indicated.
- Occasional in most samples Urinalysis: epithelial cells.
- Increased numbers in voided sampling from females during estrus.
- Increased numbers or even small sheets of cells stuck together if catheterized.
- Negative in cystocentesis sample, few when catheterized to low numbers if voided, but free flow samples when mid-stream and collected well in a sterile container are also usually negative Urinalysis: culture and sensitivity
- <1 hyaline casts/low power field (x 10-20 lens) Urinalysis: centrifuged sediment.
- May have struvite if more alkaline and stored in vitro especially Urinalysis: centrifuged sediment.
- Otherwise can be pathological and associated with urolithiasis or risk of deposition, the crystal habit though and appearance varying due to time, temperature, saturation and pH.
- Hematuria, hemoglobinuria and bilirubinuria are the most common causes. If macroscopically red due to blood, can naturally increase protein ratios and cellular components.
- Milky-floccular: pyuria.
- Yellow-orange: bilirubin
- Red: RBCs, hemoglobin, myoglobin.
- Red-brown: methemoglobin, myoglobin, RBCs, hemoglobin.
- Brown-black: methemoglobin (NB aged degenerating samples, in vivo or in vitro, >24 hrs)
- Commonly lipid droplets when collected, especially cystocentesis samples from mesenteric fat, or if catheterized, from lubricant gels.
- Pyuria also a common cause.
- Mucous, especially during oestrous if an intact female and voided.
- Contaminants from the collection (tray or vessel).
- Contamination with other fluids (eg seminal and prostatic or vaginal).
- <1.008 or >1.012 supports renal tubular function.
- Hyposthenuria generally suggests that renal failure is not present. If persistent, especially morning samples, it suggests a lack of ADH (physiologically due to increased/excessive water intake or (central) diabetes insipidus) OR resistance to ADH (nephrogenic diabetes insipidus)
- Resistance to ADH may be due to hyperadrenocorticism Hyperadrenocorticism, hypercalcemia Hypercalcemia: overview, hypokalemia Hypokalemia, myeloma Multiple myeloma, amyloidosis Amyloidosis, pyelonephritis Pyelonephritis, drug therapy, or may be congenital.
- A specific gravity >1.007 and <1.040 -1.045 in a patient that is clinically dehydrated or is azotemic Azotemia supports renal disease and failure Kidney: chronic kidney disease.
- Incorrect temperature (not at room temperature).
- Non calibrated refractometer (use at least deionized water to confirm baseline USG at 1.000).
- Exogenous drugs or fluid such as dextran Dextrans and radiographic contrast agents.
- If <7:
- Respiratory or metabolic acidosis Acid base imbalance.
- Diabetic ketoacidosis Diabetic ketoacidosis. Glucose and anion gap
- Primary renal failure Kidney: acute renal failure.
- Severe vomiting Vomiting. Check electrolytes Blood biochemistry: overview.
- Severe diarrhea.
- Pyrexia Pyrexia: overview.
- (Starvation due to protein-muscle catabolism).
- If >7:
- Transiently post-prandial.
- Urinary tract infection Cystitis: bacterial caused by urease-producing bacteria (eg Staphylococcus spp, Klebsiella spp, Proteus spp).
- Respiratory or metabolic alkalosis.
- Vomiting especially gastric, or upper gastrointestinal obstruction.
- Urine protein:urine creatinine ratio Urinalysis: protein >0.2 = protein loss. If >3-5, consider glomerular involvement Glomerulonephritis or amyloidosis Amyloidosis.
Hypertension, congestive heart failure, urinary tract inflammation or hematuria may produce proteinuria Proteinuria, < 3 commonly.
- Nephrotoxic drugs, eg sulfonamide, aminoglycoside, cephaloridine.
- Benign sediment and normal globulins or polyclonal gammopathy is usually due to glomerulonephropathies including: systemic lupus erythematosus Systemic lupus erythematosus, sarcoptic mange Skin: sarcoptic mange, various neoplasias, ehrlichiosis Ehrlichiosis, pyometra Pyometra, chronic inflammatory disease, glomerulonephritis.
- Diabetes mellitus is the most common cause. Assay also fructosamine if greater than 1-2 weeks duration (time for glycosylation).
- Proximal renal tubular dysfunction due to early aminoglycoside toxicity, early amphotericin-B toxicity Amphotericin B, acute renal failure , Fanconi syndrome, primary renal glucosuria.
- Can be seen mildly and transiently with lower urogenital blockages and obstruction (‘FIC’). Unclear why but possibly stress and adrenaline related. Renal threshold about 10-12 mmol/L so ‘stress’ hyperglycemia in cats will often increase above this, resulting in temporary glucosuria.
- Certain exogenous drugs such as cephalosporins and salicylates (increase).
- Exogenous medication or dietary ascorbic acid in quantity (decrease).
- Other drugs have very varying effects depending on the enzyme or reaction (eg Formaladehyde increases or decreases).
- Assay blood glucose as secondary to hyperglycemia most commonly, and also electrolytes for acid-base or anion gaps. Urine dipsticks detect commonly only acetoacetate, the serum/plasma often also more β hydroxybutyrate.
- Prolonged fasting.
- Diabetic ketoacidosis Diabetic ketoacidosis.
- Certain drugs such as phthaleins and sulphydryls (color change immediately cf. developing).
- Always abnormal in cats and often precedes serum/plasma hyperbilirubinemia. Assay serum bilirubin Blood biochemistry: bilirubin, ALT, ALP (with GGT) and measure hematocrit (HCT / PCV).
- Hepatic disease and cholangitis.
- Post-hepatic bile duct obstruction.
- As labile and degraded in UV light, often not particularly useful but its lack supports obstructive cholestatic pathology due to blocked entero-fecal recirculation.
- Hematuria with hemolysis is the most common cause.
- Check for hemoglobinaemia (intravascular hemolytic disease – red plasma). Check HCT, blood smear and bilirubin.
- Lastly, consider myoglobinuria Myoglobinuria (acute muscle trauma, hyperthermia and myositis). Measure serum creatine kinase (CK) Blood biochemistry: creatinine phosphokinase and aspartate aminotransferase (AST) Blood biochemistry: asparate aminotransferase.
- Urinary tract infection is a common cause so urine culture test is indicated, even if no evidence of pyuria or bacteriuria is found.
- Most commonly non-septic inflammation (‘FIC’/FLUTD Feline lower urinary tract disease (FLUTD)).
- Renal infarct Kidney: thromboembolism.
- Urinary parasites.
- Violent or very prolonged exercise.
- Toxicity of cyclophosphamide → cystitis.
- Idiopathic benign renal hematuria.
- Contamination with certain detergents, such as hydrogen peroxide and bleaches.
- Culture urine test.
- Specific but not sensitive so will miss bacterial infection if used alone.
- Urinary tract infection with nitrate reducing bacteria.
- Urinary tract infection is the most common cause. Culture urine using plain refrigerated sample if cystocentesis or boric acidified sample if voided and especially if posting/shipping.
- Urinary tract infection.
- Sterile cystitis.
- Calculi: urethral.
- Prolonged or violent exercise.
- Glomerulonephritis (rare).
- Urinary tract infections.
- Contamination from external genitalia (free flow).
- If voided and asymptomatic, bacteria can be seen without causing clinical signs and infection that needs treatment.
- Hyaline casts: associated with proteinuria.
- Epithelial, fatty, granular and waxy casts: associated with conditions causing degeneration and necrosis of tubular epithelial cells, eg dehydration, nephrotoxins, tubular hypoxia, acute oliguric renal failure.
- Red blood cell casts: hemorrhage into renal tubules or severe glomerular inflammation.
- White blood cell casts: renal inflammation, upper urinary tract infection, eg pyelonephritis.
- May be normal or abnormal. Use caution in interpretation but supports saturation risk and their lack does not exclude urolithiasis.
The following may be abnormal
- Struvite (magnesium ammonium phosphate – rectangular bevelled, ‘coffin lid’).
- Calcium oxalate: dihydrate (maltese cross/pyramids) and monohydrate (hemp seed).
- Calcium phosphate (amorphous clear).
- Ammonium urate (brown irregular to ‘thorny apple’).
- Bilirubin (yellow filigree or star-like).
- Cystine (clear hexagonal disks).
Errors and Artifacts
- Not using a fresh urine sample.
- Incorrect storage of sample (eg temperature).
- Incorrect handling of sample (eg preservative).
- Contamination from collection method or vessel (eg kidney bowl or tray).
- Unknown collection technique.
- Inconsistent use of SOP or method for analysis (eg volume centrifuged).
- EXOGENOUS DRUGS.
(R is the radius of the centrifuge arm in cm. RPM is the revolutions per minute or speed required when spinning samples).
- Recent references from VetMedResource and PubMed.
- Ling G V, Norris C R, Franti C E, Eisele P H, Johnson D L, Ruby A L & Jang S S (2001) Interrelations of Organism Prevalence, Specimen Collection Method, and Host Age, Sex and Breed among 8,354 Canine Urinary Tract Infections (1969-1995). JVIM 15(4), 341-347 PubMed.
Other sources of information
- Osbourne C A et al (1999) Urinalysis: A Clinical Guide to Compassionate Patient Care. Bayer.