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

V. Overview and Conclusions
403
FIGURE 13-13 Indocyanine green used for quantitative assessment of
hepatic excretory function.
chronic forms of liver disease. It often is impossible to differentiate
between different hepatic diseases on the basis
of multiple laboratory tests alone, and hepatic imaging or
liver biopsy is required. Hepatic tests, however, continue
to have an important place in evaluating and monitoring
clinical patients with liver disease and in understanding the
underlying pathophysiological mechanisms essential for
successful treatment.
will be negative (true negatives). A laboratory test is said to
be sensitive to the extent that it detects individuals with
the disease (true positives divided by the total number
with the disease [i.e., true positives plus false negatives]).
The test is said to be specific to the extent that a negative
result detects patients that are free of the disease (true negatives
divided by the number of subjects without the disease
[i.e., true negatives plus false positives]).
In the clinical setting, the sensitivity and the specificity
of hepatic tests are less important than the positive predictive
value, defined as the probability that a positive test
result indicates the presence of the disease (true positives
divided by true positives plus the false positives), and the
negative predictive value, the probability that a negative
test result indicates the absence of the disease (true negatives
divided by the true negatives plus false negatives).
The predictive value of a test, unfortunately, depends on the
population being studied and the proportion of individuals
in the population with the disease (prevalence). Even when
test sensitivity and specificity are high and the number
of false-positive tests is low, if there are few subjects with
the disease in the population, a positive test result will have
relatively low positive predictive value, whereas a negative
test result will have proportionately high negative predictive
value. If the prevalence of a disease in a population
is high, however, a positive result for a test with even low
sensitivity and specificity will have high positive predictive
value, whereas a negative result would have proportionately
low predictive value for the absence of the disease.
Although the sensitivity of a test is often discussed, the
prevalence of the disease and the positive and negative predictive
values of tests are often ignored in discussions of
interpretation of laboratory results. Importantly, laboratory
test results from a selected group of individuals with a high
prevalence of liver disease cannot be compared to a population
in which the prevalence is low or to that in which the
predictive value of a test in one population is similar to that
of the other population.
As indicated previously, it is unusual that a single test
for hepatic injury or function is performed, but rather a
“ profile ” of hepatic test results ordinarily is obtained. The
combined results of a panel of tests often provide increased
sensitivity and specificity and improved predictive value in
assessing severity or in differentiating between acute and
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