Tips for crystal-clear crystalluria interpretation
Here are some important considerations when looking for crystalluria:
1. Remember that crystalluria can only be confirmed by evaluating properly collected and processed urine sediment.
2. Examine fresh, nonrefrigerated urine samples.
3. Use refrigeration with caution when evaluating for crystalluria since refrigeration of urine samples may cause the formation
of various types of crystals in vitro (Figure 8).
Figure 8: Variable sizes of calcium oxalate dihydrate crystals in the urine sediment of a dog. This sample was preserved by
refrigeration (160X original magnification; unstained).
4. Keep in mind urine handling and collection factors likely to cause in vitro crystal formation or dissolution, which include
bacterial contamination, temperature changes, length of storage and changes in urine pH. For example, if a urine sample with
calcium oxalate crystals becomes contaminated with urease-producing microbes after collection, urine pH may become alkaline,
resulting in struvite crystalluria as well, confusing diagnosis.
5. Evaluate the pH of the urine since many crystals tend to form and persist in certain pH ranges. A pH meter will provide
more accurate measurements than most diagnostic pH reagent strips.
6. When more than one hour is expected to elapse between the time of collection and the time of analysis, measure urine pH
at the time of collection and again at the time of urinalysis. Differences between the values suggest that in vitro changes
have occurred and should be considered when interpreting the significance of crystalluria. This point is especially applicable
when sending urine samples to diagnostic laboratories.
7. Be on the lookout for larger crystals (Figure 8), which indicate that the composition of urine is conducive to crystal
8. Determine whether crystals are aggregating (Figures 4, 9 & 10) because crystals that aggregate represent a greater risk
for urolith formation.
Figure 9: Aggregate of calcium oxalate dihydrate crystals in the urine sediment of a dog.
9. Verify the composition of urine crystals (Figure 11). Contact diagnostic laboratories for details about how to prepare
and submit specimens.
Figure 10: Scanning electron micrograph of an aggregate of calcium oxalate dihydrate crystals in the urine sediment of a dog.
10. Consider whether other risk factors for urolithiasis (e.g., breed, sex, age, diet, concomitant illnesses) are present.
Figure 11: Sodium urate crystals in the urine sediment of a dog. These needlelike crystals are often mistaken for tyrosine
crystals (128X original magnification; unstained).
11. Interpret the quantity of crystals in context of urine specific gravity values. A concentrated urine sample is usually
more conducive to crystal formation.
12. Consider when the patient was last fed. Diet-associated crystalluria could be expected to increase in the postprandial
13. Don't forget that crystalluria may be influenced by diet, including water intake. Urine crystal formation that occurs
while patients are consuming hospital diets may be dissimilar to urine crystal formation that occurs when patients are consuming
diets fed at home.
14. Remember: Detection of crystalluria is not synonymous with the presence of uroliths. Crystalluria often is present in
absence of uroliths. Conversely, uroliths can be present without concomitant microscopic crystalluria.
15. Repeat the urinalysis when significant crystalluria is detected in animals with otherwise normal urinary tract function.
Persistent crystalluria usually represents a greater risk for urolith formation.
Dr. Osborne, a diplomate of the American College of Veterinary Internal Medicine, is professor of medicine in the Department of Small
Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota.
For a complete list of articles by Dr. Osborne, visit