Urine Analysis - Pathology
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Introduction
Urine Analysis can be a quick and inexpensive method to determine abnormalities in a patient.
Techniques
Urine samples should be processed by noting their gross appearance, testing with a urinary dipstick, measuring the specific gravity with a refractometer and by microscopic examination.
Many of the measured parameters will be adversely affected by time, so prompt assessment of urine samples is essential for accuracy.
Gross Appearance
Colour
In the majority of the domestic species the urine should be yellow in colour. The colour of the urine is very dependant on the specific gravity so it is important to account for this. If the urine is more concentrated it will be darker in colour and visa versa as a general rule . Equine urine can become brown if left standing. Discoloured urine should be taken into consideration when carrying out tests which involve a colour change such as dipsticks. The abnormal urine colour could affect the result.
Turbidity
If the urine is not clear then this could indicate a variety of pathological states. In the horse it is normal for it to be turbid due the mucous secreted in the renal pelvis and proximal ureters.
Odour
It is normal for urine to have a slight odour from the ammonia. The odour increases with concentration and in some species such as the cat a pungent urine is normal. However a strong smelling urine could indicate a pathological state such as an active bacterial infection.
Specific Gravity
Urine specific gravity (USG) is measured using a refractometer calibrated for veterinary use. It must additionally be calibrated on each use using water which must match the water line on the refractometer read-out prior to measuring USG, as refractometer measurement is sensitive to room temperature. Adjustments are made using the screw on the top of the refractometer - take care not to get this screw wet when cleaning the refractometer.
Specific gravity relates to the urine concentration. It is basically a measure of the density of particles in the urine.
Below are the normal urine specific gravities for the common domestic species.
Urinary Dipsticks
pH
In dogs and cats an acidc pH is normal. In the ruminants and horses the opposite is true and the urine is alkali. The pH does however vary with diet and medications as well as in pathological states so this needs to be accounted for. The normal range is 5-9
Protein
It is not common to find protein in the urine of normal animals. However it can be normal for small amounts to be found in the urine of dogs with a high specific gravity. Therefore this needs to be taken into account.
Glucose
The presence of glucose in the urine is termed glucosuria and occurs when the level of glucose reaches the renal threshold for glucose reabsorption. It is not normal to find glucose in the urine.
Ketones
It is not normal for ketones to be present in the urine and they are indicative of pathological states
Bilirubin and Urobilinogen
Small amounts of bilirubin can be found in dogs with concentrated urine therefore this needs to be interpreted in light of the urine specific gravity. However as a rule it is not normal to find bilirubin in urine.
Urobilinogen is formed from bilirubin in the intestine and small quantities are normally found in the urine.
Blood
The presence of blood in the urine is not normal and is usually related to a pathological state.
Microscopic Examination
Cellular Deposits
- Epithlial cells
- Renal
- Squamouns
- Transitional
- Leucocytes
- Red blood cells
- Intact
- Ghost
- Crenated
- Bacteria
- Other
- Fat droplets
- Yeast bodies
- Sperm
- Artefacts
Casts (Cylinduria)
- Hyaline
- Waxy
- Granular
- Cellular
- Fatty
- Haemoglobin
- Myoglobin
- Artefacts
- mucus threads
Crystalline and Amorphous Deposits
- Calcium carbonate
- Struvite
Calcium Oxalate
- Dihydrate
- Monohydrate
- Amorphous phosphate
- Leucine
- Tyrosine
- Cystine
- Amorphous urates
- Uric acid
- Sodium urate
- Dicalcium Phosphate
- Ammonium Biurate
Culture and Sensitivity
Free catch samples which are collected from naturally voided urine will inevitable be contaminated by the bacteria of the external genitalia and skin. Where culture is essential for diagnositc and treatment purposes, a sample obtained by cystocentesis or catheterisation will be required. Catheterisation carries the risk of introducing infection iatrogenically.
Culture on blood agar and CLED (with Andrades indicator - for E coli) plates will help with a speedy diagnosis of causal organisms.