Module 8: Introduction to the Routine Urinalysis
Module 8.2: Sample Collection
Collection Methods
By far the most important step of the urinalysis is sample collection. As the method in which you collect (and store) the sample can affect the interpretation.
-
Off the floor:
This is the least optimal way of obtaining urine, but sometimes it 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 collected as sterile as possible.
-
Cystocentesis:
This is the cleanest (most sterile) and the ideal method of obtaining urine especially when culture is likely needed. It is important to remember it is a traumatic method of collection and can result in microscopic hematuria (urine is still of normal color and is not red). It is not uncommon to see more than 100 RBC/HPF mimicking true hematuria. Increased protein is not usually seen with iatrogenic hematuria (but may not accompany true hematuria either).
Sample storage
The time of collection relative to the time of analysis is important.
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 the analysis cannot be performed promptly (<30 min), the urine should be stored refrigerated to maximize cell preservation and minimize bacterial growth, but the analysis should still be performed within 12 hours. After 12 hours, 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 “caustic” environment, and bacteria will proliferate. The addition of preservatives, e.g. formaldehyde, toluene, is not recommended as 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). If the sample has been refrigerated, it is important to allow the urine to return to at least room temperature (ideally body temperature) prior to evaluation.
