Urine Collection Methods in Veterinary Medicine

Dr. Rosemary Stolzer

Urine Collection Methods in Veterinary Medicine

Adapted from the IRIS education article "Urine collection in dogs and cats" (2022), originally written by Alexander Hüttig (Reutlingen, Germany) and updated by Xavier Roura.

A urinalysis starts with collecting urine, and the goal of any collection procedure is to obtain the highest-quality sample with the lowest risk of complications for the patient. The collection method, the container used, the time between collection and analysis, and how the sample is handled in that interval all matter, because each affects both the results and their interpretation. The volume collected is another important consideration. The usual goal is 2 to 5 mL, enough to measure urine specific gravity, run dipstick chemistries, examine the sediment, and store a portion in case the initial results call for further tests, such as a urine protein-to-creatinine ratio or a bacterial urine culture.

Timing and Handling

A urine sample can be collected at any time, particularly for screening. However, urine characteristics vary through the day, so to help with interpretation, record both the time of collection and whether the animal was fasted or recently fed. An early morning sample appears to be best for evaluating kidney function: it follows the longest period of fasting and probably of low water intake, so its specific gravity may better reflect the patient's ability to concentrate urine.

Any delay between collection and analysis should be kept to a minimum to limit changes in the sample. Urine should be analyzed within 60 minutes of collection when kept at room temperature, to minimize changes in pH, specific gravity, and crystal formation caused by temperature and storage time. For the same reason, crystals seen in a stored sample should be confirmed by examining fresh urine.

There are at least three ways to obtain urine from the bladder: a mid-stream free catch (by natural voiding or by manual compression of the bladder), transurethral catheterization, and cystocentesis.

Mid-Stream Free Catch

Natural Voiding

This method can be used at any time of day or night, and it is noninvasive and easy. Its drawback is a higher chance that the sample will be unusable or give false-positive results.

Some ways of collecting voided samples are useful, such as using a clean container to collect urine as the animal urinates, or collecting it from a clean litter box filled with non-absorbent litter. Others are unreliable, such as collecting urine from a dirty litter box or scooping it from the floor. The time between collection and urinalysis, and how the sample is stored in the meantime (refrigerated or at room temperature), are also key to judging whether the sample is usable and to interpreting the results.

Manual Compression of the Bladder

This method can be useful if the patient is small, generally under 12 pounds (5.5 kg), and calm and cooperative. However, excessive pressure can injure the bladder, the procedure can be painful for some patients (especially those with lower urinary tract disease), and blood in the urine caused by the procedure can be misinterpreted. In addition, urine that may be contaminated with bacteria can be forced into the prostate, ureters, or kidneys if the urethral sphincter does not relax as the bladder is compressed. Because of these technical challenges and risks, natural voiding is preferable in most situations where a free-catch sample is wanted.

Interpreting Free-Catch Samples

All free-catch urine samples should be assumed to be contaminated with bacteria, cells, and debris from the genital tract, skin, and hair. However, they may be satisfactory for screening, such as measuring urine specific gravity, dipstick analysis, and the urine protein-to-creatinine ratio, particularly if the sample was collected, handled, and stored properly and the sediment examination is normal. If a possible abnormality is found on the dipstick or specific gravity, it is often best to confirm it on a sample collected by another method, to avoid acting on a false-positive result.

A free-catch sample is considered unsatisfactory in almost all cases where bacterial culture is being considered. Urine cultures and urinalyses are more accurate on samples obtained by catheterization or cystocentesis, although the difference in interpretation is usually small and should be weighed in clinical decisions. For example, a cut-off of 100,000 colony-forming units (CFU) per mL or more in voided urine may be appropriate for identifying significant bacteriuria in most dogs and cats with a suspected urinary tract infection, if samples are refrigerated and cultured on the day of collection. Even so, voided urine is discouraged for bacterial culture in dogs and cats, because contamination from the external genitalia can lead to misinterpretation.

Transurethral Catheterization

In the authors' opinion, catheterizing the urethra solely to collect a urine sample is best avoided. There is a risk of introducing a bacterial infection even when the procedure appears aseptic and atraumatic, because passing the catheter inevitably carries bacteria from the distal urethra into other parts of the urinary tract, such as the prostate or bladder. Samples obtained this way are often contaminated with bacteria and cells from the distal urethra and nearby tissues, which complicates the interpretation of bacterial culture and sediment examination.

Cystocentesis

Collecting urine by needle puncture of the bladder benefits the patient and makes test results easier to interpret. Cystocentesis is generally better tolerated than catheterization and carries less risk of introducing infection. It also avoids or reduces contamination with bacteria, cells, and debris from the lower urogenital tract, which makes results easier to interpret and helps to locate the source of blood, white blood cells, or bacteria in the urine. For bacterial culture, it is by far the best collection method, because a properly collected cystocentesis sample should be sterile.

Passing the needle through the bladder wall can cause a small amount of blood in the urine, especially if cystitis is present. This typically results in fewer than 20 red blood cells per high-power field on sediment examination, with no other artifacts in the urinalysis. To reduce leakage of urine into the abdomen, some clinicians recommend directing the needle through the bladder wall at a 45-degree angle rather than at a right angle. This creates an oblique track that should seal as the needle is withdrawn.

Some veterinarians prefer to perform cystocentesis with the animal restrained on its back, others with the animal lying on its side, and still others with the animal standing. The size and cooperation of the patient can influence the choice.

Whatever the position, the skin is first cleaned with an antiseptic. The needle is then passed through the skin and the ventral bladder wall, where there is less risk of hitting large blood vessels or the ureters.

In female dogs, and in cats of both sexes, the needle is inserted through the midline (linea alba) or the lateral abdominal wall. In male dogs, a site to one side of the midline, close to the penis, is chosen, after first assessing the size of the prostate to avoid an enlarged gland. Ultrasound guidance is recommended if a prostatic cyst is suspected.

If the bladder cannot be felt, for example because of low urine volume, obesity, edema, or abdominal tensing, ultrasound guidance should be considered if available. If there is any doubt, or for less experienced clinicians, ultrasound guidance adds confidence. For ultrasound-guided cystocentesis, a syringe can be used, but some clinicians prefer an evacuated tube collection system. This allows single-handed sample collection while the other hand holds the ultrasound probe, avoids contamination from the environment, and keeps the tube closed until the sample is analyzed. Collecting urine for bacterial culture into boric acid tubes can lead to false-negative culture results.

Summary

The method used to collect and handle a urine sample is critical to interpreting urinalysis and urine culture correctly. Ideally, this means an early morning sample from a fasted patient, collected by cystocentesis and analyzed within an hour of collection: urine specific gravity by refractometer, dipstick chemistries, and, most important, microscopic examination of the sediment. A portion of the urine should be stored aseptically in the refrigerator in case further tests, such as urine culture or the urine protein-to-creatinine ratio, turn out to be needed. Refrigerated samples remain suitable for quantitative bacterial culture for up to 24 hours.

Urine Collection Methods in Veterinary Medicine

Urine Collection Methods in Veterinary Medicine

Timing and Handling