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

References
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b . Performance
In the morning, 0.01 mg dexamethasone per kg body
weight is administered intravenously. Blood for cortisol
measurements is collected immediately before and at 3 and
8 h after dexamethasone administration.
c . Interpretation
A plasma cortisol concentration exceeding 40nmol/liter at
8 h after dexamethasone administration can be regarded
as diagnostic for hyperadrenocorticism with a diagnostic
accuracy of 0.83 (95% confidence limits 0.76 to 0.88)
( Rijnberk et al ., 1988 ). The measurements at 0 and 3 h
are not needed for the diagnosis of hyperadrenocorticism
but may be informative in the differential diagnosis. Quite
commonly the high value at 8 h is preceded by a lower
value at 3 h. Thus, at 8 h the pituitary-adrenocortical system
escapes from the suppression by dexamethasone. If the
value at 3 or 8 h is at least 50% lower than the basal value,
the disease may be regarded as pituitary dependent.
3 . High-Dose Dexamethasone Suppression Test
a . Indication
Differentiation between pituitary-dependent hyperadrenocorticism
and hypercorticism arising from adrenocortical
tumors is possible with the high-dose dexamethasone suppression
test.
b . Performance
Blood for cortisol determination is collected immediately
before and 3 to 4 h after administration of 0.1 mg dexamethasone
per kg body weight.
c . Interpretation
If the plasma cortisol declines by more than 50%, the diagnosis
of pituitary-dependent hyperadrenocorticism is justified.
A decrease of less than 50% can be due either to an
adrenocortical tumor or to dexamethasone-resistant pituitarydependent
hyperadrenocorticism. Differentiation between
these two forms requires additional tests (see Section IV.D).
4 . Urinary Corticoid/Creatinine Ratios
(with High-Dose Dexamethasone Test)
a . Indication
Determination of corticoid/creatinine ratios can be performed
when hyperadrenocorticism is suspected.
b . Performance
The owner is asked to collect morning urine samples at
set times (e.g., 7 A.M.) on 3 consecutive days. On the preceding
evenings, the animal should have its last walk at
identical times (e.g., 11 P.M.). After collection of the second
urine sample, dexamethasone is administered orally.
At 8-h intervals, the owner administers 0.1 mg dexamethasone
per kg body weight.
c . Interpretation
Corticoid/creatinine ratios exceeding 10 10 6 can be
regarded as compatible with hyperadrenocorticism with a
diagnostic accuracy of 0.91 (95% confidence limits 0.85 to
0.95) ( Rijnberk et al ., 1988 ). When the ratio of the third
urine sample is 50% lower than the mean of the first two
ratios, the diagnosis of a pituitary-dependent hyperadrenocorticism
is justified. A lesser decrease may be due to
either adrenocortical tumors or dexamethasone-resistant
pituitary-dependent hyperadrenocorticism (for differentiation,
see Section IV.D).
d . Comment
This test is now generally regarded as a sensitive screening
test for the diagnosis of hyperadrenocorticism ( Feldman,
1995 ). There has been some debate on its specificity
( Feldman, 1992 ; Rijnberk and Teske, 1992 ), and recently
it was stated that the test lacks specificity ( Feldman, 1995 ).
Indeed as with other screening tests the specificity is not
100% ( Kaplan et al ., 1995 ; Smiley and Peterson, 1993 ),
and in conditions that are associated with increased adrenocortical
function, such as hepatic encephalopathy
( Rothuizen et al ., 1995 ), elevated ratios can be found.
In this discussion, two points remain somewhat underexposed.
First, a high sensitivity and not a high specificity
is essential for a screening test. The predictive value of
a positive test result depends on the sensitivity of the test
and also on the prevalence of the disease in the population
studied. When the testing is limited to dogs suspected of
the disease, the population under study will have a high
prevalence of disease, and as a result the predictive value
of a positive test result will be high. Second, as always, the
diagnostic accuracy of a test depends also on the quality of
the test procedure. In this respect, it should be mentioned
that easily elevated C/C ratios can be obtained when the
urines are collected under stressful conditions, such as in
the hospital. This will increase the number of false-positive
test results (i.e., lower the specificity).
When C/C ratios are measured in a population suspected
of hyperadrenocorticism, with urine collections at home,
not only is the sensitivity of the test high, but also the specificity
(0.77) is comparable to that of the low-dose dexamethasone
suppression test (0.73) ( Rijnberk et al ., 1988 ).
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