Sensitivity and specificity

In medical terms, sensitivity is the percentage of people who test positive for a disease that have that disease. A highly sensitive test will essentially rule out those who do not have disease. Highly sensitive tests are often used as “screening tests.”

Specificity is the percentage of people without the disease who test negative for that disease. A highly specific test can help rule in those who have the disease.

These values are measured against a “gold standard” test. A popular example is to compare a surgical biopsy versus a fine needle aspiration for detecting breast cancer.See attached table.

‘a’ represents the true positives, or the number of people who test positive that actually have the disease

‘c’ is the number of people who have the disease but test negative, or the false negatives

‘b’ is the number of people who test positive but do not have the disease, or the false positives

‘d’ is the number of people who do not have the disease and test negative, or the true negatives

The sensitivity of a test is the number of people who test positive (a) divided by the total number of people with the disease (a+c).

The specificity of a test is the number of people who test negative (d) divided by the total number of people without the disease (b+d).