Sensitivity and Specificity
Sensitivity is how well a test catches disease; specificity is how well it rules it out.
What it means
Two measures of how good a test is, pulling in opposite directions.
Sensitivity — how good it is at picking up people who do have the condition. A highly sensitive test is good for ruling out: if it is negative, you probably do not have it.
Specificity — how good it is at correctly clearing people who do not have it. A highly specific test is good for ruling in: if it is positive, you probably do have it.
Almost no test is excellent at both, and pushing one up usually pushes the other down. That trade-off explains a great deal about why medicine works the way it does — sensitive tests are used first to screen, specific tests second to confirm.
The D-dimer is the classic example: very sensitive, not specific. A negative one effectively excludes a clot; a positive one means very little on its own.
In practice
In discussions about why one test was chosen over another, and in explanations of why a positive result needs confirming.
Why it matters
Ask what a negative result actually rules out. For some tests it is nearly everything; for others it is very little.
Do not treat a positive screening result as a diagnosis. Screening tests are deliberately built to be sensitive rather than specific, which is exactly why most positives turn out to be nothing.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026