A quick examination of a test's sensitivity (S), specificity (E) and Youden index can provide useful information as to its usefulness in diagnosis.
Youden index =
= (sensitivity) + (specificity) - 1
Youden
S
E
Interpretation
<= 0.10
NA
NA
test of little value
1.00
1.0
1.0
perfect test, good at any disease prevalence
>= 0.90
NA
NA
good test; additional testing may be needed if the prevalence of disease is low
0.11 to 0.89
>= 0.98
low
test good for ruling out a diagnosis if negative (high negative predictive value); need additional testing to exclude false positives
0.11 to 0.89
low
>= 0.98
test good for making a diagnosis if positive (high predictive value); need additional testing to find false negatives
0.11 to 0.89
other
other
test performance determined by disease prevalence (the lower the prevalence the worse the performance); multiple tests will be needed to include or exclude a diagnosis
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