A Hüttig, Reutlingen, Germany
Diagnosis requires collection of urine and the goal of a urine collection procedure is to obtain the highest quality urine sample with lowest risk of complications to the patient. The method of collection, collection container used, interval between collection and analysis, and method of handling of the sample in this interval are all important aspects of the process, as these will affect both the results and their interpretation. Furthermore, the volume of urine collected is another important consideration. The usual goal is to collect between 2 and 5 mL to enable determination of urine specific gravity, dipstick chemistries, sediment examination and storage of a portion of the urine in case the initial results of the urinalysis lead to further diagnostics tests, such as measurement of the urine protein-to-creatinine ratio or bacterial urine culture.
A urine sample can be collected at any time, particularly for screening analysis. However, urine characteristics may vary throughout the day, so to facilitate interpretation of results, it is helpful to record both the timing of urine collection and whether the animal was fasted or was recently fed. It seems that early morning is the best sample to use when evaluating renal function - it follows the longest period of fasting and likely low water intake, so the urine specific gravity may better reflect the patient's urine concentrating ability.
Any delay between collection and analysis should be minimized to minimize in vitro changes. Urine samples should be analyzed within 60 minutes of collection at room temperature to minimize changes in pH, specific gravity, and crystal formation in vitro secondary to temperature and time of storage. Due to this, the presence of crystals observed in stored samples should be validated by reevaluation of fresh urine.
There are at least three ways to obtain urine from the bladder: mid-stream free catch (by natural voiding or manual compression of the bladder), transurethral catheterization of the bladder, or cystocentesis.
This method of collection can be conducted at any time of day or night, and it is a noninvasive and easy alternative but could be criticized as increasing the possibilities that the sample being not usable or having false-positive results.
There are useful methods of collection of voided samples (e.g., use a clean container to collect urine from an inert kitty litter or directly during the micturition) and potentially unreliable methods (e.g., collection of urine from a dirty litter box or scooping urine from the floor). Furthermore, the time interval between collection and urinalysis, and method to conserve the sample in the interim (e.g., refrigerated or held at room temperature) are also key factors in judging the usability of the sample and the interpretation of the results.
This method could be useful if the patient is small, generally <12 pounds (<5.5 kg), and calm and compliant. However, the urinary bladder can be traumatized if excessive digital pressure is applied, it could be painful to some patients (especially those with lower urinary tract disease) and may lead to misinterpretation if hematuria results from the procedure. Furthermore, urine (potentially contaminated with bacteria) may be forced into the prostate gland, ureters, or renal pelvis and parenchyma if bladder compression is not accompanied by simultaneous relaxation of the urethral sphincter. Because of technical challenges inherent to this method and the attendant risk to the patient, this method could be substituted by natural voiding for most situations where a urine sample is desired.
All free catch urine samples are assumed to be contaminated with bacteria, cells, and debris from the genitourinary tract or skin and hair. However, they may be satisfactory for screening purposes, such as determination of urine specific gravity, dipstick analysis, and determination of the urine protein-to-creatinine ratio particularly if the sample was collected, handled and stored properly, and the urine sediment examination is normal. If a potential abnormality is found on dipstick analysis or urine specific gravity measurement, it is often preferable to confirm and follow-up this finding by first obtaining a urine sample by another means to avoid the risk of false positive findings.
In almost all cases, a free catch urine sample is considered unsatisfactory if bacterial culture is being considered. However, although both urine cultures and urinalyses are more accurate in catheterized or by cystocentesis specimens, the magnitude of difference in the interpretation of results usually is small but should be factored into clinical decision making. For example, taking ≥100,000 CFU/mL as a veterinary cut-off value for voided urine could be appropriate for determining significant bacteriuria in most dogs and cats with suspected UTI if specimens are refrigerated and cultured on the day of collection. Despite all this, the use of voided urine specimens for bacteriological culture in dogs and cats is discouraged because contamination from external genitalia could lead to misinterpretation of results.
In the authors' opinion, catheterization of the urethra solely to collect a urine sample is best avoided. There is a risk of iatrogenic bacterial infection, even when the procedure is performed in a seemingly aseptic and atraumatic fashion, as passage of the catheter invariably transports bacteria from the distal urethra into other areas of the urinary tract (e.g., prostate gland or urinary bladder). Urine samples obtained by this method are often contaminated with bacteria and cells from the distal urethra and adjacent tissues, complicating interpretation of bacterial culture and sediment examination.
Needle puncture of the urinary bladder to collect the required amount of urine has advantages to the patient and aids interpretation of test results. In general, cystocentesis is better tolerated than catheterization, and there is less risk of introducing an iatrogenic infection than with catheterization. It also avoids or reduces contamination of urine samples with bacteria, cells, and debris from the lower urogenital tract, thereby facilitating interpretation of results and assisting localization the sources of hematuria, pyuria, and bacteriuria. For bacterial culture of urine, this is by far the best method of sample collection as a properly collected cystocentesis sample should be expected to be sterile.
Passage of the needle through the bladder wall can cause microhematuria, especially if cystitis is present. Typically, this results in <20 red blood cells per high power field in the sediment examination but no other artifactual findings in the urinalysis. To avoid escape of urine into the abdomen, some recommend that the needle be directed at a 45-degree angle through the bladder wall, rather than at a right angle to it. This approach will create an oblique pathway that should close immediately as the needle is withdrawn.
Many veterinary practitioners prefer to perform cystocentesis with the animal restrained exactly upright in dorsal recumbency, others prefer to restrain the animal in lateral recumbency, and still others prefer to perform this procedure in a standing animal. The size and compliance of the patient could play a role in the selection of approach.
Regardless of animal position, antiseptic treatment of the skin is performed first, then the needle is inserted through the skin and the ventral bladder wall, where there is less risk of touching large vessels or the ureters.
In female dogs, and both sexes in cats, the needle is inserted through the linea alba or the lateral abdominal wall. For male dogs a paramedian site is chosen, close to the penis, after first evaluating the size of the prostate to avoid contacting an enlarged gland. Use of ultrasound guidance is recommended if a prostatic cyst is suspected.
If the bladder cannot be palpated for some reason (e.g., low urine volume, adiposity, oedema, abdominal splinting), the use of ultrasound guidance should be considered, if available. If there is any doubt, or for inexperienced clinicians, ultrasound guidance will provide more confidence with the process. For the ultrasound-guided method of cystocentesis, a syringe may be used, but some prefer to use a Becton, Dickinson's Vacutainer™ device. Singlehanded sample collection is then possible, the other hand being used to manipulate the ultrasound probe. This avoids environmental contamination, and the vacuum tube remains closed until the sample is analyzed. Use of boric acid tubes to collect urine where bacterial culture is required can lead to false negative cultures.
In summary, the method of collection and handling of a urine sample is critical to proper interpretation of the results of urinalysis and urine culture. Ideally, this means an early morning urine sample in a fasted patient that is collected by cystocentesis and analyzed shortly (<1 hour) after collection by determination of the urine specific gravity with refractometer, dipstick chemistries, and the most important, microscopic urine sediment examination. A portion of the urine should be aseptically stored (20°C for 1-3 days) in case subsequent analyses, such as urine culture or determination of the urine protein-to-creatinine ratio, are deemed appropriate.
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