How is specificity calculated in medical testing?

Specificity in medical testing is a measure of a test's ability to correctly identify individuals who do not have the disease. It is calculated using the formula:

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

This formula assesses the proportion of actual negative cases (those without the disease, represented by true negatives) relative to all individuals who tested as negative (which includes both true negatives and false positives). A high specificity indicates that the test is effective at ruling out disease in individuals who are healthy, thereby minimizing the number of false positives.

For example, if a hypothetical test correctly identifies 90 out of 100 healthy individuals as negative (true negatives) but mistakenly labels 10 healthy individuals as positive (false positives), the specificity would be calculated as 90 / (90 + 10) = 0.90 or 90%. This measure is crucial in screening processes to ensure that healthy individuals are not incorrectly diagnosed, leading to unnecessary anxiety and further testing.