![]() ![]()
This table reports the sensitivity and 1-specificity
for every possible cutoff for positive classification.
The sensitivity is the
proportion of HIV-positive samples with assay
results greater than the cutoff.
1-specificity is
the proportion of HIV-negative samples with assay
results greater than the cutoff.
Cutoff 0 is equivalent to assuming that everyone is HIV-positive.
Cutoff 9 is equivalent to assuming
that everyone is HIV-negative. Both extremes
are unsatisfactory, and the challenge
is to select a cutoff that properly balances the
needs of sensitivity and specificity.
For example, consider cutoff 5.5. Using this
criterion, assay results of 6, 7, or 8 are
classified as positive, which leads to a sensitivity
of 0.989 and 1-specificity of 0.012. Thus,
approximately 98.9% of all HIV-positive samples
would be correctly identified as such, and 1.2% of
all HIV-negative samples would be incorrectly
identified as positive.
If 2.5 is used as the cutoff,
99.7% of all HIV-positive samples
would be correctly identified as such, and 3.3% of
all HIV-negative samples would be incorrectly
identified as positive.
Your choice of cutoff will be mandated by the need
to closely match the sensitivity and
specificity of traditional tests.
Please note
that the values in this table are at best guidelines
for which cutoffs you should consider. This table
does not contain error estimates, so there is no
guarantee of the accuracy of the sensitivity or
specificity for a given cutoff in the table.
|
|
Coordinates of the Curve |