Jean-Pierre Cotard - 2001
The method used to obtain a sample of urine has a significant influence on subsequent analysis. In domestic carnivores, three sampling methods are usually used:
- Spontaneous Sampling
- Urethral Catheterization
The advantages and drawbacks of each method are discussed below.
1. Spontaneous Sampling
This is certainly the easiest method. Unfortunately, the passage of urine through the uro-genital track leads to contamination and, thus, limits the number of tests which can be reliably performed. Assessment of proteins or cytobacteriological examinations, for example, can be more difficult with this method.
Although this method often has to be rejected by the veterinarian, it can still be used to follow up certain diseases. For example, spontaneous sampling is useful for monitoring glucosuria, ketonuria or proteinuria, once formally identified by other sampling techniques. In this case, it is preferable to use urine sampled in the middle of urination (midstream clean catch).
2. Urethral Catheterization
This technique is simple, but certain precautions are necessary to facilitate interpretation of results. The catheterization must be carried out abiding by the basic rules of asepsis: sterile catheter and local disinfection of the preputial region in males and the perineal and vulval region in females. These precautions are essential to avoid bacterial contamination of the bladder and to allow correct interpretation of cytobacteriological findings. With catheterization, it is recommended to discard the initial urine withdrawn (to avoid contamination of the sample with bacteria and cells from lower urinary tract) and to use the following sample withdrawn.
Urinary catheterization has to be carried out without trauma to the animal. Urinary blood and protein tests are extremely sensitive and calorimetric methods can easily provide misleading results. This method is not suitable for very nervous or very small animals for which cystocentesis is preferable.
This method is not used frequently in Europe, although it is certainly the best way to sample urine for laboratory analysis. Before puncturing the bladder through the abdominal wall, it is important to check its size. A half-empty bladder may not facilitate sampling. If the animal is obese, it can also be difficult to palpate the bladder. The animal is positioned on its back or side and the zone over the bladder should be shaved and disinfected.
The operator palpates and immobilizes the bladder with one hand. With the other hand, a fine needle (23 gauge, 5/8" long for cats) is introduced at an angle of 45° from the front to the back towards the neck of the bladder.
The advantage of this method is that it avoids any risk of urine contamination by blood or bacteria, making it the method of choice if sediment examination, estimation of proteinuria or bacterial culture of the urine is required.
In addition, this method can be used to relieve urethral obstruction when catheterization is not suitable. The only risk is if an unskillful operator punctures the intestines or other organs, confusing them with the bladder. If ultrasonography is available, cystocentesis can be easily performed.
If the animal is not cooperative, light sedation may be necessary.
Handling of Samples
Biochemical, bacteriological or cytological examinations have to be carried out as soon as possible. In practice, if the examination is done immediately, no preparation is necessary. However, certain precautions have to be taken if the examination has to be postponed for a few hours.
Urine can be preserved at 4° C for some hours but it is important to let the sample reach room temperature before carrying out the analysis. This procedure can, however, alter the cells present in the urine and lead to crystal formation or dissolution. Deep-frozen urine does not allow cytological examinations, but is suitable for biochemical tests, with the exception of certain enzymatic activities such as gamma glutamyltranspeptidase.
The use of a urine preservative agent is possible, but may interfere with subsequent analysis and the risk of handling certain toxic products (e.g., toluene) has to be considered. Toluene, formalin and boric acid can be used as preservatives.
Chemical analyses are possible with toluene. Formalin is mainly used for the examination of urinary sediments. Boric acid is the medium of choice to perform a bacteriological examination for up to four days. In addition, it preserves cells and crystals and allows biochemical analysis and pH determination in the majority of the cases.
To analyze urine, the minimal equipment required is:
- a refractometer to measure specific gravity
- urine dipsticks
- a microscope
- a centrifuge