Ca nine Uroliths :
Fre q uently A ske d
Questions a nd
Their Answers
Lori A. Koehler, CVT a,*, Carl A. Osborne, DVM, PhDa,
Michelle T. Buettnera, Jody P. Lulich, DVM, PhDa,
Rosalie Behnke, MS, DVMb
KEYWORDS
Urolith Calculi Question Crystalluria Urine pH
This article is devoted to answering frequently asked questions from veterinarians,
veterinary technicians, and pet owners about urolithiasis. The authors do not stop
with determining the mineral composition of uroliths; but also respond to questions re-
lated to the detection, treatment, and prevention of various types of uroliths from the
urinary tract.
Originally it was suggested that this article be divided into veterinary and owner-
related questions, but with the wealth of information available on the Internet, many
owners have become quite curious and knowledgeable about uroliths affecting their
pets, especially as it relates to nutrition. They frequently have many of the same ques-
tions asked by veterinarians and veterinary technicians.
The information has been divided into the following topics: urolith analysis, urolith
types, diagnosis, treatment and prevention, urolith recurrence, urinalysis, diet, water,
and miscellaneous.
UROLITH ANALYSIS
1. What Are Uroliths?
Answer: Uroliths are aggregates of crystalline and occasionally noncrystalline solid
substances that form in one or more locations within the urinary tract. The urinary tract
is designed to eliminate wastes in liquid form. When urine becomes oversaturated with
lithogenic substances, uroliths may form and can interfere with the complete and
frequent voiding of urine.
Supported in part by an educational gift from Hill’s Pet Nutrition, Topeka, KS.
a
Veterinary Clinical Sciences Department, Minnesota Urolith Center, College of Veterinary
Medicine, University of Minnesota, 1352 Boyd Avenue, St. Paul, MN 55108, USA
b
Hill’s Pet Nutrition, 400 SW 8th Avenue, Topeka, KS 66603, USA
* Corresponding author.
E-mail address: koehl002@umn.edu (L.A. Koehler).
Vet Clin Small Anim 39 (2008) 161–181
doi:10.1016/j.cvsm.2008.09.007 vetsmall.theclinics.com
0195-5616/08/$ – see front matter ª 2008 Elsevier Inc. All rights reserved.
,162 Koehler et al
Uroliths typically are composed of one or more mineral types (Table 1).1 These
minerals may be pure, deposited in layers, or they may be mixed throughout the uro-
lith. In addition some drugs may precipitate as crystals within the urinary tract and be
incorporated into the urolith.
If foreign substances, such as suture material, hair, or plant material are present
within the lumen of the urinary tract, they can become the nidus for urolith formation.
2. Why Should Uroliths Be Analyzed?
Answer: While guessing the mineral composition of uroliths by their appearance is
sometimes possible, this method is subject to considerable error. Erroneous guesses
of the mineral composition in turn often leads to formulation of erroneous therapy.
Detection of the composition of the interior core of uroliths and/or microscopic surface
crystals may also escape detection. To develop an effective treatment plan, knowl-
edge of the composition and structure of the entire urolith is essential.
3. What Methods of Analysis Are Recommended?
Answer: Two general methods of urolith analysis are available: qualitative analysis
and quantitative analysis.
Qualitative analysis is a colorimetric test designed to identify the chemical compo-
nents of a substance or mixture. Drops of test reagents are added to an aliquot of pul-
verized urolith; the appearance of different colors indicates various anions or cations
Table 1
Crystalline substances that may be detected in uroliths
Chemical Name Crystal Name Formula
Oxalates
Calcium oxalate monohydrate Whewellite CaC2O4 C H2O
Calcium oxalate dihydrate Weddellite CaC2O4 C 2H2O
Phosphates
b-tricalcium phosphate (calcium Whitlockite b-Ca3(PO4)2
orthophosphate)
Carbonate apatite Carbonate apatite Ca10 (PO4 C CO3 C OH)6 (OH)2
Calcium hydrogen phosphate Brushite CaHPO4 C 2H2O
dihydrate
Calcium phosphate Hydroxyapatite Ca10 (PO4)6 (OH)2
Magnesium ammonium Struvite MgNH4PO4 C 6 H2O
phosphate hexahydrate
Magnesium hydrogen phosphate Newberyite MgHPO4 C 3H2O
trihydrate
Urice acid and urates
Anhydrous uric acid Same C5H4N4O3
Uric acid dihydrate Same C5H4N4O3 C 2H2O
Ammonium acid urate Same C5H3N4O3 C NH4
Sodium acid urate monohydrate Same C5H3N4O3Na C H2O
Cystine Same (SCH2CHNH2COOH)2
Amorphous silica Same SiO2
Xanthine Same C5H4N4O2
, Canine Uroliths: FAQs 163
that are present. Because this method requires pulverizing the sample into a sand of
powdery consistency, the layers of different minerals frequently identified by quantita-
tive methods of analysis typically cannot be identified by qualitative methods. Also,
this method is not designed to determine the approximate percentages of different
minerals that are present. Likewise, these tests are not designed to identify some bio-
genic components of uroliths such as silica or xanthine. In addition, crystalline drug
metabolites are missed. One study comparing a qualitative chemical test to quantita-
tive physical tests revealed false positive and false negative results. Test results were
in agreement in only 92 of 223 cases.2
Quantitative methods of analysis are designed to determine the composition of
a urolith, and the amounts or proportions of the components of a urolith. Several phys-
ical methods of quantitative urolith analysis may be used to determine and quantify the
mineral composition of the sample. At the Minnesota Urolith Center, we most fre-
quently use optical crystallography (polarized light microscopy) and infrared spectros-
copy. On occasion, we use energy dispersive spectroscopy and x-ray diffraction
techniques. Some laboratories include high-performance liquid chromatography to
identify different forms of purines.3
4. What Is Optical Crystallography?
Answer: Optical crystallography encompasses the use of a polarizing light micro-
scope to identify crystalline and/or noncrystalline components of uroliths by matching
them to known refractive index oils. Representative sections of the urolith selected for
microscopic examination are identified with the aid of a dissecting light
stereomicroscope.
5. What Are Basic Principles of Infrared Spectroscopy?
Answer: Infrared spectroscopy is based on unique wave patterns generated when in-
frared waves encounter a sample. Some waves are absorbed by the sample (absor-
bance) and some waves pass through the sample (transmittance). The resulting
spectrum is a molecular fingerprint of the sample. Because no two unique molecular
structures produce the same infrared spectra, results can be compared with known
reference spectra for identification. This procedure is useful in characterizing urolith
components that cannot be identified with the polarizing light microscope, in deter-
mining the quality and consistency of samples, and for quantifying the amounts of dif-
ferent substances within the sample.
6. How Should the Results of Urolith Analysis Be Interpreted?
Answer: At the Minnesota Urolith Center, the following anatomic classification is used
to describe different portions of the uroliths (Fig. 1).
Nidus—central area of obvious initiation of urolith growth, which is not necessarily
the geometric center of the sample.
Stone—the major body of the urolith.
Shell—a complete, outer, concentric lamination of the urolith.
Surface crystals—an incomplete, outer lamination of the urolith.
Although all portions of the sample are analyzed, if all areas are composed of the
same mineral composition, it is listed only under the ‘‘stone’’ area. Occasionally layers
are encountered within the urolith or between the main layers that are listed as bands
or incomplete bands on the report. Tiny focal deposits within the urolith may also be
