Specificity of a Test

Specificity is the test’s ability to correctly designate a subject without the disease as negative.

Specificity of a test is the percentage of true negatives out of all subjects who do not have a disease or condition.

Simply, specificity is the proportion of people without disease correctly diagnosed as negative by a test. In other words, it is the ability of the test or instrument to obtain normal range or negative results for a person who does not have a disease. 

The formula to determine specificity is the following:

Figure: Deriving the Formula for Specificity

• A highly specific test means that there are few false positive results.
• High specificity tests perform well for diagnosis because of low false positive errors. In the specific context of tuberculosis (TB), tests with low specificity have the disadvantage that (among other things) many subjects without TB disease will screen positive and potentially receive unnecessary chemotherapy for 6 months or more (and possibly invasive, risky or expensive) follow-up diagnostic or therapeutic procedures.
• A specific test is used for ruling in a disease, as it rarely misclassifies those WITHOUT a disease as being sick. A perfectly specific test, therefore, means no healthy individuals are identified as diseased.

A good TB test is one that has both high sensitivity and specificity.

Resources

• Screening Tests: A Review with Example, L. Daniel Maxim,Ron Niebo & Mark J. Utell.
• Sensitivity and Specificity Explained: A Cochrane UK Trainees Blog. 

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