True Negative Calculation

What is a True Negative?

A true negative occurs when a diagnostic test correctly identifies a person as not having a particular condition. In a 2×2 contingency table used in medical testing, true negatives are found in the bottom-left cell.

True negatives are important because they indicate the test's ability to correctly rule out a condition. A high number of true negatives suggests the test is reliable for identifying negative cases.

True Negative Formula

The formula for calculating true negatives is:

Where:

• Total Negative Cases = Number of people who actually do not have the condition
• False Positives = Number of people who tested positive but do not actually have the condition

How to Calculate True Negatives

Step-by-Step Calculation

1. Determine the total number of negative cases in your population.
2. Identify the number of false positives (people who tested positive but don't have the condition).
3. Subtract the false positives from the total negative cases to get the true negatives.

Interpreting True Negatives

True negatives are particularly valuable in:

• Screening programs where correctly identifying negative cases is important
• Public health monitoring to track disease prevalence
• Quality control in diagnostic testing

However, true negatives alone don't tell the complete picture. They should be considered alongside true positives, false positives, and false negatives.

Worked Example

Suppose you have a test for a certain disease with the following results:

To find the true negatives:

1. Total negative cases = 20 (false negatives) + ? (true negatives) = 100
2. False positives = 15
3. True negatives = Total negative cases - False positives = 100 - 15 = 85

The complete table would show 85 true negatives in the bottom-left cell.