PHTH 550 - Sensitivity & Specificity

Negative predictive value (formula)

True negatives / True negatives + False negatives

Sensitivity (definition)

Ability of the test to correctly identify those patients with the disease

Specificity (definition)

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

The ability to make a diagnosis or screen for a condition depends on..

Discriminatory value of the test Prevalence of the disease in the population of interest

What happens when the cut-off point is lowered?

Fewer false negatives More false positives = Highly sensitive but low specificity

What happens when the cut-off point is raised?

Fewer false positives More false negatives = Highly specific but low sensitivity

Sensitivity (interpretation)

High Sensitivity & Negative test helps to rule a condition Out

Specificity (interpretation)

High Specificity and a Positive test helps to rules the condition In

Negative predictive value (definition)

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

Positive predictive value (definition)

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

Likelihood ratio (definition)

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

Perfect test

Never positive in a patient who is disease free Never negative in a patient who is diseased

What is dependent on the population being tested and is influenced by the prevalence of the disease?

Positive predictive value Negative predictive value

What is the area under the receiver operator curves?

Represents the overall accuracy of a test Approaching 1.0 = high sensitivity and specificity

What is independent of the population being tested and isn't influenced by the prevalence of the disease?

Sensitivity Specificity

Likelihood ratio (formula)

Sensitivity / 1 - Specificity

What is the best alternative and why?

Subject patients who are positive to a test with high sensitivity/low specificity to a second test with low sensitivity/high specificity Nearly all of the false positives may be correctly identified as disease negative

High sensitivity important when..

Test is used to identify a serious but treatable disease

Specificity (formula)

True negatives / True negatives + False positives

Sensitivity (formula)

True positives / True positive + False negatives

Positive predictive value (formula)

True positives / True positives + False positives