sensitivity & specificity

Negative predictive value (NPV) answers the question:

'How likely is it that this patient does not have the disease given that the test result is negative?'

Positive predictive value (PPV) answers the question:

'How likely is it that this patient has the disease given that the test result is positive?'

a test with a high sensitivity but low specificity results in

many patients who are disease free being told of the possibility that they have the disease and are then subject to further investigation

the ability to make a diagnosis or screen for a condition depends

both on the discriminatory value of the test and on the prevalence of the disease in the population of interest

PPV and NPV are

dependent on the population being tested and are influenced by the prevalence of the disease

Likelihood ratio

how much more likely is it that a patient who tests positive has the disease compared with one who tests negative.

Likelihood ratio =

sensitivity / 1 - specificity

The sensitivity of a clinical test refers to

the ability of the test to correctly identify those patients with the disease.

The specificity of a clinical test refers to

the ability of the test to correctly identify those patients without the disease

True negative

the patient does not have the disease and the test is negative

False positive

the patient does not have the disease but the test is positive.

True positive

the patient has the disease and the test is positive.

False negative

the patient has the disease but the test is negative.

NPV =

true negatives / (true negatives + false negatives)

specificity =

true negatives / (true negatives + false positives)

sensitivity =

true positives / (true positives + false negatives)

PPV =

true positives / (true positives + false positives)