Sensitivity
Definition:The proportion of individuals with a specific disease that are detected by a surveillance system or laboratory test. Sensitivity addresses the question, "of those who truly have disease, what proportion did the test or system detect?"
Low surveillance sensitivity
- May not detect many cases or miss outbreaks
- Usually less resource intensive
High surveillance sensitivity
- Detects most cases
- May also capture many non-cases or misidentify outbreaks
- May be very resource intensive
Calculating Sensitivity
Sensitivity can be calculated using a 2x2 table and the following formula: Sensitivity = A /(A+C)
| Disease Yes | Disease No | Total |
|
| Detected Yes | True + (A) | False + (B) | A+B |
| Detected No | False – (C) | True – (D) | C+D |
| A+C | B+D | A+B+C+D |
Example: In the diagrams below, red people truly have the disease; the outlined boxes those people whom the test or surveillance system detected, that is, identified, as having the disease. The diagrams below show a population of 50 people.
Diagram 1
Since all five people who have the disease (people colored red) are within the outlined box (those detected by the test or surveillance system), the sensitivity is 100%.
| Disease Yes | Disease No | Total | |
| Detected Yes | 5 | 0 | 5 |
| Detected No | 0 | 45 | 45 |
| 5 | 45 | 50 |
Sensitivity calculation = A / (A+C) = 5 / (5+0) = 100%.
Diagram 2
In the diagram below, five people truly have the disease (people colored red). The test or system detected, that is, identified, ten people with the disease (10 people in outlined box).
Of the people colored red (had disease), all were detected, so the sensitivity of the test or sytem is 100%.
Note that five people were detected as having the disease, when they truly didn't have the disease. These cases are called false positives.
| Disease Yes | Disease No | Total | |
| Detected Yes | 5 | 5 | 10 |
| Detected No | 0 | 40 | 40 |
| 5 | 45 | 50 |
Sensitivity calculation = A / (A+C) = 5 / (5+0) = 5 / 5 = 100%.
Diagram 3
In the diagram below, six people truly had the disease (people colored red); some were detected or identified as having the disease (people within the outlined box) and some were not. Of the six people who truly had the disease, four were detected. So the sensitivity is 67%.
Note that two people were not detected as having the disease, even though they truly did. These cases are called false negatives.
| Disease Yes | Disease No | Total | |
| Detected Yes | 4 | 4 | 8 |
| Detected No | 2 | 40 | 42 |
| 6 | 44 | 50 |
Sensitivity calculation = A / (A+C) = 4 / (4+2) = 4 / 6 = 67%.
