Page 899 - Small Animal Clinical Nutrition 5th Edition

Feline Lower Urinary Tract Diseases 931

VetBooks.ir Box 46-1. Managing Cats with Crystalluria.

Detection of crystalluria on microscopic examination of urine sedi-
ment does not mean a cat subsequently will develop urolithiasis; retrieved urolith can provide that information. However, crystalluria
can be used along with other factors (e.g., history, age, breed, uri-
however, it does indicate increased risk for urolith development. nary pH, radiographic appearance, other urinalysis findings and
Crystalluria in cats with normal anatomy and physiology of the uri- biochemistry profile results) to predict mineral type. Uroliths may be
nary tract is usually of no clinical significance; these crystals are present in cats without crystalluria. In this case, factors that influ-
voided before they grow to sufficient size to interfere with urinary enced formation and growth of crystals may be absent transiently.
tract function and health. Factors typically responsible for this phenomenon include food
Crystals that form in vitro after a urine specimen is collected are changes, anorexia, increased water intake, different urinary pH val-
often of no clinical importance. Temperature, evaporation and pH ues and in vitro changes in urine specimens that are not fresh. The
are in vitro variables that may cause a urine specimen to become crystal type may be different from the urolith type in some cases.
oversaturated, leading to crystal formation. Importantly, in vitro This dichotomy exists when cats are assumed to have one urolith
conditions may cause crystals to dissolve or grow. It is possible that type, which is not confirmed by quantitative analysis (e.g., assumed
urine kept at room temperature after collection may lose carbon to have struvite uroliths and fed according to struvite dissolution or
dioxide into the atmosphere, which could affect pH, and subse- preventive protocols, when in reality the cat has calcium oxalate
quently, presence of crystalluria. However, a recent study found uroliths and calcium oxalate crystalluria). Finally, cats may have
that urinary pH was not affected by storage time or temperature. more than one crystal type concurrently (e.g., struvite and calcium
Analysis of stored urine from cats consuming a mixture of moist oxalate crystals may be identified in the same urine sample).
and dry food was associated with significant increases in struvite Struvite crystals may occur in: 1) normal cats, 2) cats with infec-
crystalluria compared with evaluation of a fresh urine sample. In tion-induced struvite uroliths, 3) cats with sterile struvite uroliths, 4)
another study that included 31 dogs and eight cats, calcium cats with non-struvite uroliths, 5) cats with uroliths of mixed min-
oxalate and struvite crystals formed in vitro in urine that was stored eral type and 6) cats with urinary tract disease other than uroliths
at either room or refrigeration temperature. However, this phenom- (e.g., feline idiopathic cystitis). Calcium oxalate dihydrate crystals
enon occurred more commonly at refrigeration temperature. In occur uncommonly in normal cats. Large quantities of these crys-
addition, length of crystals that formed in vitro was significantly tals alone or in combination with calcium oxalate monohydrate
increased in urine stored at refrigeration temperature compared crystals in fresh urine specimens probably indicate a hypercalciuric
with samples stored at room temperature. or hyperoxaluric disorder (e.g., ethylene glycol toxicity, calcium
Bacterial contamination of urine specimens also may affect crys- oxalate urolithiasis).
talluria. Urease-producing bacteria (e.g., Staphylococcus spp., Should all cats with crystalluria be treated? In the absence of
Proteus spp.) alkalinize urine, possibly altering crystal composition urolithiasis or a history of urolithiasis, cats with struvite or calcium
and disrupting cellular components in urine (e.g., red and white oxalate crystalluria should be monitored serially. Cystine, ammoni-
blood cells). Other bacteria produce acid metabolites with similar um urate or xanthine crystalluria should be investigated and the
consequences. To prevent bacterial overgrowth, urine samples cause treated. Frequent detection of large crystals and aggregates
should be refrigerated if they cannot be evaluated promptly. of crystals in a fresh urine sample may be clinically important,
However, this is not ideal for evaluation of crystalluria. especially if the cat has a history of urolith formation. In this case,
The nutritional history, including water intake, must be consid- preventive nutritional management should be implemented, and
ered when evaluating significance of crystalluria. Cats consuming the cat should be encouraged to increase water intake. Finally,
moist food are less likely to have crystalluria than those eating dry urine sediment examination should be performed periodically to
food exclusively. In addition, results of urine sediment examination monitor effectiveness of preventive therapy in patients with a his-
may not accurately reflect what occurs in the home environment if tory of uroliths.
cats are fed different foods in the hospital before urine collection.
Crystalluria should not be used as the sole criterion to predict The Bibliography for Box 46-1 can be found at
mineral type of confirmed uroliths; only quantitative analysis of a www.markmorris.org.

Urinary pH consumed. Consequently, it is difficult to interpret a single uri-
Urinary pH influences formation of several crystal types. Al- nary pH value, especially if the type of food and time of eating
though exceptions occur, crystal types tend to form and persist are unknown. Furthermore, it has been reported that simply
at certain urinary pH ranges (Table 46-7) (Osborne et al, 1995). putting a cat in a carrier and traveling to a veterinary hospital
In general, struvite uroliths are associated with more alkaline can increase urinary pH (Buffington and Chew, 1996).
urinary pH values (>6.4) and calcium oxalate uroliths are asso- In laboratories, pH meters with glass and reference elec-
ciated with lower urinary pH values. Overlapping values can be trodes are used to make pH measurements.The electrode must
detected; however, in cats with either urolith type. The method be calibrated periodically against buffers of known pH. For
of measuring urinary pH should be considered because the urine clinical purposes, urinary pH can be measured with pH meters
dipstick method is not as accurate as a pH meter. In addition, or indicator paper. Most multi-test reagent strips and test tapes
urinary pH varies throughout the day due to the influence of use indicator paper impregnated with two indicator dyes:
food, time of eating, method of feeding and amount of food methyl red and bromthymol blue. The typical pH range is