Gribbles Veterinary Pathology 109

URINALYSIS

Analysis of urine is an important diagnostic aid for many disease processes.

Ideally all procedures should be carried out on fresh urine (< 4 hours old).

It is advantageous to use an early morning urine, as this is likely to be concentrated, which facilitates evaluation of tubular capacity to concentrate urine. This urine is also more likely to contain a higher yield of cells, casts or bacteria, be more acidic (which prevents dissolution of proteinaceous structures), and concentrated urine is less likely to lyse cells (urine with an SG < 1.008 can result in cell lysis).

Whatever technique is used for collection, the urine should be placed into a clean container (sterile if bacteriology is required). Aim for at least 5 ml and send the sample as quickly as possible, as cells, casts and crystals disintegrate with time. Refrigerate the sample if there is to be a delay, in which case dipstick analyses and specifi c gravity using a refractometer should be done at the clinic on the fresh sample after collection.

Samples at room temperature can undergo variable increases in pH, which in turn promotes lysis of cells and may alter the type of crystals present.

Collection Collecting urine by voiding or cystocentesis are the preferred methods, because of simplicity and atraumatic effects on the bladder.

Mid stream free fl ow / Voided urine May be contaminated with cells, bacteria and other debris from the genital tract or on the skin and hair, but is generally acceptable for urinalysis and sediment exam.

The risk of contamination can be reduced by discarding the fi rst portion of the sample (mid stream free fl ow - using two containers in the collection).

CystocentesisThis is the collection method of choice, but can induce microscopic haematuria (1+ blood on dipstick) - especially in cats. It is the recommended technique for culture, and where the aim is to localise the source of haematuria, pyuria and bacteriuria.

Ballottment prior to cystocentesis helps to mix bladder contents, and may increase fi ndings in sediments /crystals etc.

Catheterisation These samples can contain 1+ blood on dipstick

Urine TestsUrinalysis includes a visual description of colour, turbidity and dipstick examination (pH, protein, glucose, ketones, urobilinogen and blood), a Specifi c Gravity (SG) and sediment examination.

Important considerations are:

Urine specifi c gravity (SG) - This should be taken [and measured] before fl uid administration.

- It helps to verify that an azotaemia is due to renal failure.

- Dilute urine (SG < 1.008) tends to cause osmotic induced lysis of cells.

- Dipsticks are unreliable and should not be used to measure urine SG.

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Feline urine has higher relative specifi c refractivity then human or canine urine. Use of human based urine SG scale leads to falsely high results for feline urine, especially for concentrated urine samples. Some veterinary refractometers include a separate scale for feline urine SG. Alternatively, a conversion factor can be used to obtain feline SG from medical refractometers. Using the human SG scale will also give falsely lowered results for guinea pig and rabbit urine.

Conversion of SG results from a medical refractometer to feline urine SG values:

Medical Refractometer Results Feline SG

1.005 1.004

1.010 1.008

1.015 1.013

1.020 1.017

1.025 1.021

1.030 1.025

1.035 1.030

1.040 1.034

Conversion calculation:

Feline specifi c gravity = (0.846 x medical refractometer SG)+0.154

Note: Detection of reproducibly lowered SG in the geriatric animal, in the absence of other non-renal causes, may suggest early renal disease (before the manifestation of azotaemia).

pH (Urine dipstick)Bromophenol blue indicator is used on the test strip. Therapeutic doses of Frusemide can cause urine to become acidic.

Urine Protein (Dipstick protein – albumin)This should be interpreted in the light of the urine SG and pH.

A trace protein reading equates to 0.3g/L and is clinically insignifi cant (unless very dilute urine)

In dogs with a normal urine concentration (SG > 1.030), a 1+ proteinuria equates to 0.5g/L and is regarded as normal.

Any protein is potentially abnormal with an SG < 1.030 (dogs), and < 1.035 (cats). The dipstick will not detect Bence Jones proteinuria (protein found in multiple myeloma) or protein in very dilute or acid urine. A pH > 7 will result in at least a 1+ false proteinuria on dipstick. This can also be seen with some antibiotics.

If in doubt, a quantitative assessment of urine protein excretion can be done by measuring the protein:creatinine ratio (this should only be performed if the sediment is free of red blood cells, infl ammatory cells and bacteria), and there is no pyrexia evident (see urinary protein:creatinine ratio later in the chapter).

Proteinuria in the absence of infl ammatory sediment or blood is strong evidence for glomerular disease. Gribbles Veterinary Pathology offers the E.R.D. – Healthscreen urine tests. The E.R.D. tests detect small amounts of albumin in the urine (microalbuminuria). Microalbuminuria can be detected in normal animals and can also be associated with pre-renal, renal or post-renal causes. It should always be correlated with history, clinical exam and other test results. Post-renal disease is common, and you are advised to check for the possibility of an active urine sediment. Some apparently healthy animals have albumin in their urine. A signifi cant correlation has been found between increasing age and positive test results in apparently healthy animals.

Detection of microalbuminuria during routine health examination suggests the presence of either an underlying disease process causing early renal damage or lower urinary tract disease.

Gribbles Veterinary Pathology 111

Urine GlucoseGlucosuria of any degree should be investigated, particularly in dogs. The renal threshold for glucose in dogs = 10mmol/l.

Cats can have marked hyperglycaemia due to stress/excitement, which can exceed the renal threshold (> 16mmol/l), but there is an indication that in diabetic cats the threshold may be lower (11mmol/l). Thus, hyperglycaemia in feline samples must be interpreted in view of clinical signs and other laboratory data. Also, ruminants have a lower threshold for the formation of urinary glucose, so its presence is less signifi cant.

Use of certain drugs (e.g. Penicillins, Cephalosperins, Xylazine) can induce a glucosuria, and false positives may occur in cats with FLUTD with urethral obstruction –this may be attributed to unknown oxidising substances in urine reacting with the indicator in the test strip.

The presence of glucose in urine results in an increase in USG:

3+ Glucose (> 1g / dl) adds 0.004 – 0.005 to USG

4+ Glucose (> 2g / dl) adds 0.008 – 0.010 to USG

Urine KetonesAre invariably present in cases of ketoacidotic diabetes mellitus.

Urine BilirubinAt least 20% of normal dogs will produce a 1+ bilirubinuria. Dogs can convert haem (if haemoglobinuria) to bilirubin in renal tubule cells. The bilirubin found in urine is always conjugated.

In cats the renal threshold is at least 9x higher than in dogs, and thus any bilirubin is signifi cant – but can be seen in 10-20% of Diabetes Mellitus (DM) , and 20% of hyperthyroid cases.

Urine BloodCystocentesis can induce microscopic haematuria - especially in cats, but cystocentesis sampling should not induce >trace to 1+ blood on dipstick. Catheterisation samples can contain up to 1+ blood on dipstick.

A positive test for blood with reagent test strips, but a failure to fi nd intact RBC in the sediment can occur with haemoglobinuria, myoglobinuria, or minute haematuria i.e. very small numbers of RBC, or lysis of red blood cells.

RBC > 1+on dipstick is abnormal, but dilute urine causes lysis of RBC, and concentrated urine crenates RBC.

Urine Leukocytes, bacteria (nitrite) and SGSome dipsticks purport to detect the above, but these tests are unreliable when used on animal urine and are of no diagnostic value.

Sediment examination This is a microscopic examination of the sediment after a measured amount of sample is spun. This is examined at 40x for the numbers of WBC, RBC, epithelial cells, crystals, casts and presence of bacteria. Some unusual sediments may be stained for further cytological examination.

CrystalsDetection of crystals is not synonymous with uroliths, and clinical signs associated with them. The clinical signifi cance of the presence of crystalluria should always be associated with the clinical signs, as these may be incidental fi ndings in normal animals, and thus their presence should not be overinterpreted. It is an important consideration that crystals can dissolve or may even form in vitro, and the presence of these in vitro formed crystals does not justify therapy. Conversely, some crystals (e.g cystine or ammonium urate), or large aggregates (e.g calcium oxalate or magnesium ammonium phosphate [struvite]) in apparently normal animals may be of diagnostic signifi cance. In patients with confi rmed urolithiasis, the microscopic evaluation of the crystal composition should not be used as the sole criterion of the mineral composition of bladder stones or urethral plugs.

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Crystals found in alkaline urine- Ammonium biurate: seen in healthy mammals, Dalmatians, suggest liver dysfunction and Portosystemic

Shunt (PSS).

- Calcium oxalate dihydrate: (can occur at all pH’s) and can be seen in healthy mammals, but uncommonly (or less commonly than the monohydrate) in apparently normal cats. Indicate a predisposition to formation of uroliths. Can also be seen in hyperCa, or hyperoxaluria (incl ethylene glycol toxicosis).

- Calcium oxalate monohydrate: (can occur at all pH’s) not seen commonly in healthy mammals, but large aggregates should prompt consideration of hyperCa, or hyperoxaluria (incl ethylene glycol toxicosis).

- Calcium phosphate: seen in healthy mammals

- Struvite (magnesium ammonium phosphate): commonly seen in apparently normal dogs and cats with alkalinuria, cats that have sterile or infection induced struvite uroliths, non struvite uroliths, have uroliths of mixed composition, and in UTI without uroliths.

Crystals found in acid urine- Bilirubin: in bilirubinuria

- Cholesterol: uncommon, seen in healthy dogs, suggest hypercholesterolaremia

- Cystine: rare, associated with liver disease

- Drug crystals: (sulfa, contrast media, primidone)

- Hippuric acid: very rare, can be confused with calcium oxalate monohydrates

- Leucine: rare, suggest liver disease

- Tyrosine: rare, suggest liver disease

- Urate, sodium: or ammonium (pH variable) seen in healthy mammals, Dalmatians, suggest liver dysfunction and PSS

- Uric acid: seen in healthy mammals, (not commonly in cats) Dalmatians, may suggest liver dysfunction

- Xanthine: rare, may refl ect treatment with allopurinol

Quantitative UrinalysisUrine Protein:Creatinine Ratio

The urine protein:creatinine ratio is used to assess the amount of protein being lost in the urine and therefore its signifi cance. It is more accurate than the dipstick reading and as it is a ratio removes the diffi culties of interpreting protein levels in urine of different SG. It correlates well with the 24 hour urine protein determination.

A full urinalysis must be done on the same sample to look for haemorrhage or infl ammation. If there is any blood measured by the dipstick or the presence of an infl ammatory sediment, then the protein:creatinine ratio can not be used as an indicator of glomerular disease.

Urine Cortisol:Creatinine RatioSee Endocrinology chapter – diagnosis of hyperadrenocorticism.This is a useful screening test for canine hyperadrenocorticism as a low (normal) result makes the disease unlikely. It is therefore useful in those cases where hyperadrenocorticism is unlikely but needs to be defi nitely excluded. The test has low specifi city, (there are a lot of false positives) so that further tests are needed to confi rm that a high result is due to hyperadrenocorticism.

It is thought that the test has similar sensitivity and specifi city in cats but there are few published reports.

Urine Fractional ExcretionThe concentration of a specifi c substance in urine can be measured and when compared with the concentration of that substance in blood and the ratio of creatinine in blood and urine, a fractional excretion ratio or percentage can be calculated. The most common substances measured are the electrolytes - Na, K, Cl, Ca, PO4. Calculating fractional excretions can help determine if there is excessive loss of the particular substance in urine e.g. potassium-losing nephropathy. Reference ranges are a guide only and the values obtained should always be interpreted with the view – is this appropriate given the level in the blood?

A fresh urine sample is required as well as a serum sample (red top tube) taken at the same time.