Page 909 - Small Animal Clinical Nutrition 5th Edition

Feline Lower Urinary Tract Diseases 941

VetBooks.ir Box 46-2. Determining Risk for Urolith Formation or Recurrence.

Analytical data indicate that urine often is supersaturated with degree of supersaturation because we know less about changing
respect to most common urolith components. Thus, the question is urinary inhibitors. One way to express urinary saturation is to deter-
not why a specific patient formed a urolith, but rather why doesn’t mine the relative supersaturation (RSS) of a calculogenic substance
every patient form uroliths? Inhibitors in urine probably explain the (e.g., struvite or calcium oxalate). RSS is determined by measuring
less than predicted prevalence of urolith formation. The current the concentration of a number of urinary analytes, including sodi-
therapeutic strategy is to reduce risk factors by decreasing the um, calcium, oxalate, magnesium and potassium. The concentra-
tions of these analytes are entered into a computer program that
calculates the saturation of the urolith elements compared with a
standard human urine sample. RSS has limitations because it is
highly dependent on urine volume and involves comparison with
standard values for human urine.
Another technique for predicting likelihood of crystal formation is
the activity product ratio (APR) (Figure 1). The APR for calcium
oxalate is the mathematical product of the activity of calcium and
the activity of oxalate.Activity is different than simple concentration
of the substance of interest in an aqueous solution. Activity refers
to the ionic activity, which is influenced by the concentration of the
substance of interest, other substances in urine and factors such
as pH and temperature. Like RSS, APR involves measurement of a
number of analytes (e.g., sodium, potassium, calcium, oxalate,
magnesium) in urine that are entered into a computer program.
Unlike RSS, however, the APR technique requires incubation of
seed crystals (e.g., calcium oxalate) in an aliquot of urine. After
incubation, urinary analytes are measured again and the post-incu-
bation activity product is determined. Dividing the pre-incubation
activity product by the post-incubation activity product yields the
APR. An APR less than one indicates that crystals dissolved during
incubation. The APR provides a better indication of risk of crystal
formation than RSS because APR considers the influence of
unmeasured inhibitors and promoters and is not unduly influenced
by urine volume. APRs can be used to evaluate quantitatively the
influence of nutrients, complete foods and drugs on the risk of
crystal formation.
Several studies have reported effects of various foods on urinary
saturation values for calcium oxalate and struvite (APR and/or RSS)
Figure 1. Schematic depicting how APRs are calculated. Urine
from a cat is analyzed for pertinent minerals (Tube A) and then is in healthy cats; however, only one has reported these values in cats
incubated with a seed crystal. After incubation, the urine is ana- with a history of forming uroliths. Although decreasing values for
lyzed for the same mineral constituents (Tube B). Dividing the pre- urine saturation indicate decreased risk for urolith formation, these
incubation activity product by the post-incubation activity product values serve as surrogate markers. There are no reported studies
yields the APR. The risk of urolith formation increases with an APR correlating urine saturation values (APR or RSS) in urolith-forming
greater than 1. This means the seed crystal has grown and the cats with rate of urolith recurrence.
urine is supersaturated and/or contains inadequate concentrations
of crystal inhibitors. The risk of urolith formation decreases with an The Bibliography for Box 46-2 can be found at
APR less than 1. This means the seed crystal became smaller dur-
ing the incubation process and the urine is undersaturated or con- www.markmorris.org.
tains adequate concentrations of crystal inhibitors.

CALCIUM OXALATE UROLITHS associated with a significant decrease in urine calcium excretion
Based on a small study of cats with calcium oxalate uroliths and urine calcium oxalate saturation compared with feeding
and information extrapolated from canine and human patients, their regular food. Hypercalcemia promotes urinary excretion
it seems reasonable that factors promoting hypercalciuria of calcium and therefore may increase the risk of calcium ox-
and/or hyperoxaluria are involved in the pathogenesis of calci- alate urolithiasis. Mild hypercalcemia (11.1 to 13.5 mg/dl) has
um oxalate urolithiasis in cats (Lulich et al, 1991, 2004; Ste- been identified in approximately 35% of cats with calcium
venson et al, 2004; Seiner et al, 2003, 2005). Hypercalciuria was oxalate uroliths (Osborne et al, 1996a). Radiopaque uroliths
identified in 10 cats with calcium oxalate uroliths; feeding a were diagnosed in seven of 20 cats (35%) with idiopathic
c
therapeutic food formulated to prevent urolith recurrence was hypercalcemia in one study; uroliths were removed from two