Sensitivity and Specificity
of diagnostic tests

The probability of the test finding disease among those who have the disease or the proportion of people with disease who have a positive test result.

Sensitivity = true positives / (true positives + false negatives)

The probability of the test finding NO disease among those who do NOT have the disease or the proportion of people free of a disease who have a negative test.

Specificity = true negatives / (true negatives + false positives)

The percentage of people with a positive test result who actually have the disease.

Positive predictive value = true positives / (true positives + false positives)

The percentage of people with a negative test who do NOT have the disease.

Negative predictive value = true negatives / (true negatives + false negatives)

The odds that a positive test result would be found in a patient with, versus without, a disease.

Likelihood Ratio Positive (LR+) = Sensitivity / (1 - Specificity).

The odds that a negative test result would be found in a patient without, versus with, a disease.

Likelihood Ratio Negative (LR-) = (1- Sensitivity) / Specificity.