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Newsletter
Highlight :
By Prof. Joseph Lau, Chinese University of Hong
Kong.
The
HKEA Newsletter 2002, 6(2), p.5.
HOW
GOOD IS IT? EVALUATION OF SCREENING PROGRAMMES
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Sensitivity
and specificity are often used to measure accuracy of screening tests.
When a screening test has a range of values, different cut-off points
can be used and they result in different sets of sensitivity and specificity.
There is always a trade-off between these sets of sensitivity and
specificity values and a choice should be made based on the application
of the screening test. A highly sensitive test can be used to "rule-out"
a diagnosis in face of a negative screening result and a highly specific
test can be used to "rule-in" a diagnosis in the presence
of a positive result. Like other statistics, sensitivities and specificities
are subject to chance and can be described by confidence intervals;
cautions should be given to cases where these statistics are derived
from small samples. |
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When
the sensitivity values are plotted against the (1-specificity) values,
a receiver-operating characteristics curve is obtained. The Area
Under the Curve (AUC) measures the performance of a screening test.
The maximum area of 1.0 says that the screening test is as good
as the gold standard; an area of 0.5 says that the test is totally
useless. One can use special statistics to compare the areas of
two different ROC curves generated by applying two screening test
to same individuals, so as to determine whether the performance
of one test is statistically different (better or worse) from another
one.
A
more updated method to assess the performance of a screening test
is the likelihood ratio, defined as sensitivity/(1-specificity).
It is used to generate post-test odds and probabilities which can
be compared with pre-test odds and pretest probabilities (prevalence
of disease). A large change between the pre-test and post-test values
would suggest that screening is worthy as it would change a clinical
practice. Positive and negative predictive values are defined as
the probabilities that someone with a positive or negative screening
result is actually a case or a non-case. They are intuitively important.
However, as they are dependent on prevalence of disease; they cannot
be used to assess accuracy of screening test.
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