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

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Chapter | 3 Carbohydrate Metabolism and Its Diseases
The binding of glucose to proteins occurs firmly
and constantly over the life span of a particular protein.
Therefore, these glycated proteins reflect the average blood
glucose concentration over the half-life of the protein.
Thus, they offer a means and are used to evaluate longterm
average blood glucose levels in diabetics.
1 . Hemoglobin A1c
In the case of canine HbA1c where the canine erythrocyte
has a life span of 100 days and a half-life of about 60 days,
HbA1c reflects the average blood glucose over the previous
2 months before sampling. In the cat, with an erythrocyte
life span of 70 days and a half-life of about 40 days, HbA1c
could be used as a measure of the average blood glucose
over the previous 6 weeks. This means that bimonthly
samplings for the dog and 6 weekly intervals in the cat
for HbA1c could be used to monitor long-term blood
glucose control. The use of HbA1c is a well-established
means for monitoring long-term average blood glucose in
human diabetics. The techniques for measuring HbA1c,
however, have not been applicable to animals. The methods
are complex as well as labor and equipment intensive.
The methods all rely on the structural properties of HbA1c
for their separation so that methods for human HbA1c are
not directly applicable to animal HbA1c. Methods used for
human HbA1c are affinity and ion-exchange high performance
liquid chromatography (HPLC), electrophoresis,
immunoassay, and colorimetrically to measure 5-hydroxymethylfurfural-thiobarbituric
acid (HMF-TBA). Of these,
the HPLC method is most widely used in humans. A report
evaluating a number of methods for canine HbA1c indicated
that the HMF-TBA method is the most promising
(Hooghuis et al ., 1994 ) .
Because HbA1c is directly related to the amount of red
cells, anemias or polycythemias must be ruled out. Also,
bimonthly samplings may not detect the long-term changes
in glucose in a timely manner.
2 . Fructosamine
The total serum proteins or albumin have half-lives of 2
to 3 weeks and 7 to 9 days, respectively. This means that
FrAm or Galb could be used as indicators of the average
blood glucose over the previous 2 weeks. This biweekly
time interval has the advantage of detecting changes in
glucose control more quickly than HbA1c and allows for
timely clinical intervention. Furthermore, the FrAm assay
is a colorimetric assay based on its reducing properties, so
it is an assay readily performed in any clinical laboratory.
An improved version of the original kit is now available
from the manufacturer (Roche Diagnostic Systems, Inc.,
Rahway, New Jersey). Using this improved version, Jensen
and Aaes (1992) reported a reference range for FrAm of
259 to 344 μ mol/l (301 21.3 μ mol/l). This result is lower
than that originally reported by Kawamoto et al . (1992) for
dogs by a factor of about 10. By extrapolation, the reference
range for cats as reported by Kaneko et al . (1992)
would be 219 to 347 μ mol/l (283 32 μ mol/l).
In an extensive study of 253 diabetic dogs either treated
or nontreated with insulins, Davison et al . (2005) found
that FrAm and HbA1c concentrations compared similarly
regardless of type of insulin, insulin injection regime, duration,
or treatment or dose.
C . Tolerance Tests
1 . Glucose Tolerance Tests
Glucose tolerance (GT) in its original definition referred
to the amount of glucose that could be ingested by an animal
without producing a glucosuria, hence, tolerance for
glucose. Because, in the normal animal, the absence of a
glucosuria indicates only a limited rise in blood glucose
where the renal threshold is not exceeded, GT now refers
to the blood glucose curve following glucose administration.
Accordingly, an animal with an increased glucose tolerance
is one that has a limited rise and rapid fall in blood
glucose (i.e., can tolerate extra glucose). The animal with a
decreased tolerance has an excessive rise and a slow return
to its baseline level (i.e., cannot tolerate extra glucose).
This is the typical diabetic type of GT curve.
It is important to ascertain the nature of the animal’s
diet, especially in the omnivores and carnivores, before
performing this test. A carbohydrate-only diet favors a
minimum rise in the tolerance curve, whereas a carbohydrate-free
diet (meat only) favors a high or diabetic type of
glucose tolerance curve. Therefore, for optimum results,
the diet must be standardized by placing the dog on a standard
diet of 100 to 200gm carbohydrate plus fat and protein
per day for 3 days before performance of the test. The GT
curve is also affected by the status of the intestinal absorptive
process (i.e., inflammation, increased motility, thyroxine).
Furthermore, the variations that result from absorption,
the excitement attending intubation, or tranquilization can
be avoided by use of the intravenous test.
a . Oral Glucose Tolerance Test
Section VII.D described the blood glucose curve following
the oral administration of a test dose of glucose. The
oral glucose tolerance test (OGTT) is ineffective in the
ruminant because the ingested carbohydrate is almost
totally fermented by the rumen microflora. The OGTT
has been used in dogs by feeding of a test meal consisting
of 4 gm glucose/kg b.w. mixed with a few grams of
horse meat. A fasting blood sample is taken, the test meal
is given, and blood samples are taken at 30-min intervals
for 3 h. The OGTT curves in dogs receiving a standard
daily diet of either glucose or galactose with meat had normal
curves as described in Section VII.D. The maximum