Optical crystallography is based on use of a polarizing microscope to identify crystalline components of uroliths (Figure
5). Representative sections of the stone are identified with the aid of a dissecting microscope.
These samples (or crystal grains, as they are sometimes called) are then viewed with a polarizing scope. Optical properties,
such as refractive index and birefringence then are recorded and compared with known standards to determine the mineral type(s).
Infrared spectroscopy is based on the observation that, when infrared waves encounter a sample, some are absorbed by the sample
(absorbance) and some pass through the sample (transmission). The resulting spectrum is a molecular fingerprint of the sample
Figure 6: Infrared spectroscopy.
Because no two unique molecular structures produce the same infrared spectrum, these spectra can be compared to known reference
spectra for identification.
Table 1 Dos and don'ts of urolith submission for quantitative urolith analysis
This procedure is useful in identifying unknown materials, determining the quality and consistency of samples and quantifying
the amounts of different calculogenic substances within the sample.
Unfortunately, some laboratories persist in performing qualitative analysis. In our view, qualitative analysis does not meet
a current standard of practice in the United States.
All laboratories do not report the results obtained by quantitative analysis in the same fashion. Regardless of the method
of analysis, the location(s) of minerals within the urolith should be further specified (Figure 7).
Figure 7: Structure of a urolith.
Layers that are grossly visible by examination of cross-sections of stones may or may not contain different types of minerals
(Figures 3 and 4).
Formation of so-called metabolic uroliths (e.g., calcium oxalate) followed by formation of infection-induced magnesium ammonium
phosphate (e.g., infection-induced struvite) may result in distinct laminations detected by survey radiography or by examination
of the cut surface of the stone.
Table 2 Minnesota Urolith Center quantitative urolith analysis form
If significant hemorrhage occurs intermittently, it may affect the appearance of layers on the cut surface without a corresponding
change in the mineral composition of the urolith.
We acknowledge the financial support of Hill's Pet Nutrition in providing an educational grant to the Minnesota Urolith Center
and extend thanks to our veterinary colleagues for their financial support and confidence in the center.
Carl A. Osborne, DVM, PhD, Dipl. ACVIM
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.
Jody P. Lulich, DVM, PhD, Dipl. ACVIM
Dr. Lulich, a diplomate of the American College of Veterinary Internal Medicine, is a professor in the Department of Small
Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota.