Samples for urinalysis should be collected in a sterile manner and in adequate volume.
Time of day
The time of day at which urine is collected can be important in some regards:
- AM (or fasting) samples tend to have:
- higher USG (decreased or no water intake)
- lower pH
- more formed elements (if longer period of formation)
- PM (or non-fasting) samples tend to have:
- lower USG
- more alkaline pH
- possibly fewer formed elements (if shorter period of formation).
As much urine as possible should be collected. Ideally, the amount of urine evaluated in a urinalysis should be standardized (we try to standardize to 5 ml in our laboratory, but frequently obtain samples with lower urine volumes than this).
Method of collection
This should always be provided when submitting samples. Results will be affected by method of collection, as indicated below.
- Off the floor: This is the least optimal way of obtaining urine, but sometimes is the only method available to clients (e.g. obtaining from kitty litter). This will have the highest number of contaminants, including the possibility of fecal elements.
- Free catch: A mid-stream catch is ideal for minimizing contaminating bacteria, but these still may be seen. May see contaminating bacteria, squamous cells from the distal urinary tract, genital tract or skin.
- Catheterized: May slough off transitional epithelial cells so they are seen in higher quantities than normal. If traumatic or the urethra is irritated, hemorrhage may occur. This is done mostly in male dogs who are easier to catheterize than female dogs or other species. This sample should be sterile.
- Cystocentesis: This is the cleanest (most sterile) method of obtaining urine and is ideal. However it is traumatic, and can result in a microscopic hematuria (urine is still of normal color and is not red). More than 100 RBC/HPF may be seen in samples, mimicking a true hematuria. Increased protein is not usually seen with a microscopic hematuria (but may not accompany a true hematuria either).
A sterile container is preferred for urine collection. Culture can be performed on such samples as long as it is maintained and handled in a sterile manner. Glass non-anticoagulant (red top) tubes can be used for storing urine, however plastic (red top) tubes are not recommended, because they introduce crystalline artifacts, which mimic true crystals, into the sample.
The time of collection relative to the time of analysis is important because delay in analysis can result in:
- altered urine pH (increases)
- microbial proliferation (contaminants or pathogens)
- degradation of formed elements (cells and casts)
- degradation of chemical analytes (bilirubin, ketones)
If analysis cannot be performed promptly (<30 min), the urine should be stored refrigerated to maximize cell preservation and minimize bacterial growth, but analysis should still be performed within 12 hours. After this time, cells will begin to lyse (usually by 24 hours, unless the urine contains a substantial amount of protein, which helps preserve cells), because urine is a "toxic" environment, and bacteria will proliferate. We do not recommend adding any preservatives, e.g. formaldehyde, toluene, to the sample because they all introduce artifacts in results (which vary, depending on the preservative). Before analysis, samples should be brought to room temperature. Note that artifacts are still seen with storage. Calcium oxalate and magnesium ammonium phosphate crystals all may develop over time with storage (i.e. were not present in a freshly examined sample).