Specificity Calculator

Created by Anna Szczepanek, PhD
Reviewed by Steven Wooding
Last updated: Jan 18, 2024


Welcome to the specificity calculator! We've built it to help you quickly determine how well a diagnostic test can identify true negative cases. In the brief article below, we will answer the following questions:

  • What is the specificity of a test?
  • What is the equation for specificity?
  • How do you calculate specificity?
  • How do you use this calculator?

Enjoy!

What is the equation for specificity?

The specificity equation reads as:

Specificity = TN / (FP + TN),

where:

  • TN – Number of true negative cases; and
  • FP – Number of false positive cases.

That is, specificity describes the proportion of healthy people that tested negative to the number of healthy people regardless of their test result, i.e., among true negative cases and false positive cases. Therefore, specificity corresponds to the probability of having a negative test result, conditioned on being healthy.

How do I interpret test specificity?

If a test has high specificity, then there is a high probability that a healthy person will get a negative test outcome. In other words, a healthy person will rarely obtain a positive result.

Hence, if a test has high specificity and you get a positive result, you will very likely have the disease. However, if in a test with high specificity, you get a negative outcome, there is no reason to celebrate: specificity does not take into account false negatives!

⚠️ Extreme example: Imagine a test that always gives a negative result. Its specificity is 100% (all healthy people will get a negative result). However, all sick people will get a negative result as well! This test is completely useless despite having 100% specificity.

How to use this specificity calculator?

Calculating specificity is very easy in theory, but actual computations may be time-consuming. Omni's specificity calculator will help! You only need to:

  1. Input data: the number of true negatives TN and false positives FP.

  2. The specificity of your test appears immediately!

  3. You can also compute sensitivity, provided that you have the necessary data.

  4. Even more coefficients are available when you choose the advanced mode.

Omni features several other tools related to test accuracy and confusion matrix:

FAQ

How do I calculate specificity?

To determine specificity, you need to:

  1. Know the number of true negative cases TN.

  2. Know the number of false positive cases FP.

  3. Determine the number of healthy people: TN + FP.

  4. Execute the specificity equation:

    Specificity = TN / (FP + TN).

Anna Szczepanek, PhD
table for sensitivity and specificity
Specificity
True negative
False positive
Specificity
%
Sensitivity
True positive
False negative
Sensitivity
%
Extra
For more results, click the advanced mode below.
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