Page 1196 - Clinical Small Animal Internal Medicine

1134 Section 10 Renal and Genitourinary Disease

test for urolith analyses, where the entire urolith is linked marker test to identify dogs at risk for early stone
VetBooks.ir crushed and then analyzed. An example of why this is formation is being evaluated.
important would be with a primary metabolic stones (i.e.,
urate) which may have a large (or several) outer covering
(s) of struvite. If only the outer layer was assessed or if the Imaging of the Urinary Tract
urolith was crushed and then analyzed, then ongoing Radiography
diagnostics and therapy would be misdirected, at the Most uroliths are radiopaque on survey abdominal radi­
struvite rather than the urate portion. ographs. Radiopaque uroliths include calcium phosphate,
calcium oxalate, struvite and silicate (Figures 123.11 and
Stone Characterization 123.12). Survey radiography is suitable for identification
Uroliths are defined based on their mineral composi­ of these radiopaque uroliths, provided they are greater
tion as noted earlier. The following terms are used to than approximately 2–3 mm in diameter. However, urate,
identify type. cystine, and xanthine uroliths are generally character­
ized as radiolucent or only slightly radiopaque on survey
Pure urolith: when 70% or more of the urolith is com­
● radiographs. The failure rate for detection of uroliths by
posed of one type of crystalloid, it is named for that survey radiography was 2–27% dependent on urolith
crystal type. In dogs and cats, most stones have one size and composition.
major crystal component. Clinicians may try to guess the chemical composition
Mixed urolith: when less than 70% of the urolith is
● of a urolith based on the density, shape, and number of
composed of one mineral, it is called a mixed urolith. uroliths on radiographs. One aid for this type of “guessti­
This means that the urolith does not contain one pre­ mate” is the pneumonic POCUS, which is used by clini­
dominant mineral type but is a combination of two or cians to remember the order of the stones that are most
more types. radiodense to radiolucent: calcium phosphate, calcium
Compound urolith: this is a urolith in which each layer
● oxalate, struvite, silicate, cystine, and urate calculi.
is composed of a different mineral. Uroliths that have Contrast‐enhanced radiographic procedures or ultra­
formed around foreign material in the urinary tract are sound are needed to identify uroliths composed of com­
often classified as compound. Suture material, urinary pounds similar to the radiographic density of soft tissue.
catheter, metal pellets, plant material, hair strands, Double‐contrast cystography has been associated with
and other foreign objects (ex. porcupine quill) have all high sensitivity and low false‐negative detection rates
been reported to be a part of uroliths of various min­ (Figure 123.13). Small uroliths are detected more reliably by
eral types. double‐contrast cystography or ultrasonography. However,
ultrasound is superior to double‐contrast cystogram in
experienced hands and for very small stones and sand.
Specific Tests to Detect Cystinuric Dogs
On average, dogs with type I cystinuria have a several‐
fold higher urinary COLA excretion than non‐type I
cystinuric male dogs. A simple urinary screening test to
determine if a dog is cystinuric is available through the
Metabolic Screening Laboratory, Section of Medical
Genetics, at the University of Pennsylvania (http://
research.vet.upenn.edu/penngen). This test can detect
any type I cystinuric animal, but not necessarily all dogs
with non‐type I cystinuria. High‐performance liquid
chromatography can be further used to determine the
amount of cystine and other amino acids in urine.
However, this form of quantitative amino acid analysis is
currently restricted to a few laboratories (PennGen and
LaboKlin) and is relatively expensive. Based on studies,
dogs with either cystine levels of >200 μmol/g creatinine
or COLA values of >700 μmol/g creatinine are consid­
ered cystinuric. Moreover, for Newfoundlands and
Labrador retrievers, a breed‐specific mutation test is Figure 123.11 Abdominal radiographs (lateral view) showing
available that detects not only cystinuric dogs but also bladder stones in a dog. Source: Courtesy of Stephanie Nykamp,
asymptomatic carriers. Finally, in mastiffs, a preliminary Radiologist, Ontario Veterinary College.