Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

V. Renal Function
849
from the fact that BUN only refers to the nitrogen part of
the UR molecule, in the United States it is often expressed
in mg/dl, whereas in Europe UR is usually expressed in SI
units (mmol/L). It is sad to see how reference values that
were presented in the previous edition of this book were
erroneously converted to American units in the Journal of
Veterinary Clinical Pathology (Harr, 2002) .
To convert BUN in mg/dl to urea in mmol/L, the following
steps must be performed:
1. Convert BUN (mg/dl) to urea (mg/dl) by multiplying
BUN with the quotient of the molecular mass of
urea and the molecular mass of nitrogen in the urea
molecule (12 4 1 2 14 16)/(2 14).
2. Convert urea (mg/dl) to urea (mmol/L) by dividing urea
(mg/dl) by the molecular mass of urea (60).
3. Convert mg/dl to mg/L by multiplying the result by 10.
Or more simply: to convert BUN (mg/dl) to urea
(mmol/L), divide BUN with total mass of nitrogen in urea
molecule and multiply by 10 (10/28) 0.357). The conversion
from UR (mmol/L) to BUN (mg/dl) is 1/0.357 2.8.
F . Postprandial Effects
In raptors, a significant postprandial increase in plasma
UA and UR concentrations occurs ( Lumeij and Remple,
1991, 1992 ). Postprandial UA was similar to that in birds
with hyperuricemia and gout and was well above the
theoretical limit of solubility of urate in plasma. It is not
clear why under physiological conditions, no urate deposits
occur in raptors, which have hyperuricemia for at least
12 h after ingesting a natural meal ( Fig. 28-12 ). A similar
Uric acid/creatinine (mmol/l)
2000
1600
1200
800
400
0
4
0
0 4 8 12 16 20 24 28
time (hours)
FIGURE 28-12 Fasting and postprandial nonprotein nitrogen substances
in plasma of peregrine falcons, Falco peregrinus (mean SD).
Symbols: , [uric acid] ( μ mol/L); Δ , [urea] (mmol/L); O, [creatinine]
( μ mol/L); ↓ , feeding quail. Reprinted with permission from Lumeij and
Remple (1991) .
6
5
4
3
2
1
Urea (mmol/l)
effect was noted in the piscivorous blackfooted penguin
( Spenicus demersus ) ( Kolmstetter and Ramsay, 2000 ). To
avoid physiological food-induced elevations in UA and
UR that can complicate interpretation of plasma chemistry
results in raptorial or piscivorous birds, blood samples
should ideally be collected after a 24-h fasting period. In
small passerines, this is obviously not an option!
G . Other Changes Associated with Renal
Failure
Hyperkalemia is a particular problem in acute renal failure
that may lead to severe electrocardiographic changes and
eventually to cardiac arrest. Hypocalcemia and hyperphosphatemia
are usual in humans with renal failure. The former
may lead to hypocalcemic tetany, especially with rapid
correction of acidosis. In birds, special attention should
be paid to these variables for further documentation of
changes in renal disease because these changes may have
therapeutic implications. Anemia has been documented in
birds with chronic renal failure.
H . Murexide Test
Macroscopically, the aspirated urates from articular gout
look like toothpaste. The presence of urate can be confirmed
by performing the murexide test or by microscopic
examination of aspirates of tophi or joint accumulations.
The murexide test is performed by mixing a drop of nitric
acid with a small amount of the suspected material on a
slide. The material is evaporated to dryness in a Bunsen
flame and allowed to cool. Then one drop of concentrated
ammonia is added. If urates are present, a mauve color will
develop.
I . Birefringent Crystals
Microscopically sharp needle-shaped crystals about the
size of a leukocyte can be seen in smears of joint fluid
from patients with articular gout. A polarizing microscope
is helpful in identifying the typical birefringent crystals.
Birefringent literally means splitting a ray of light in two.
Crystals bend light and become visible in joint fluid when
viewed through a microscope with crossed polarizing
filters. When a compensator plate is used on the microscope,
monosodium urate crystals parallel to the axis of
the compensator appear yellow (negatively birefringent).
In humans, pseudo-gout is diagnosed by finding positively
birefringent calcium pyrophosphate dihydrate crystals that
appear blue when parallel to the axis of the compensator.
Calcium pyrophosphate dihydrate crystals are smaller and
are rhomboid shaped.