Specificity Calculator

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!

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.

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!

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:

• False positive calculator;
• Sensitivity calculator; and
• Most advanced sensitivity and specificity calculator.

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).