I have searched prior posts and I haven’t seen a good explanation that seems to help my seemingly tiny brain grasp this concept.
Highly sensitive tests rule OUT a disease. To me, this means if the test is negative, it’s likely to be a true negative because you are pretty certain that it’s an accurate negative result.
However, I just did a review question that told me sensitivity detects true positives and now I’m confused.
For example, D-dimer is a highly sensitive test. If it’s not elevated, we feel fairly certain the disease we are ruling out is ruled out. Wouldn’t this be a true negative? (Obviously not according to the world, but can someone please ELI5??)
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In medical testing terminology, sensitivity refers to a test’s ability to correctly identify those with the disease (true positive rate). A highly sensitive test will catch most people who have the disease — if you have the disease and you take this test, it’s unlikely to miss it.
On the other hand, specificity refers to a test’s ability to correctly identify those without the disease (true negative rate). A highly specific test will correctly rule out most people who don’t have the disease.
Using your D-dimer example: If a D-dimer test is negative (which rules out blood clots), it’s considered a true negative because of its high sensitivity — if there was a clot, such a sensitive test likely wouldn’t have missed it. The confusion comes from thinking about sensitivity from the perspective of negatives rather than positives: while high sensitivity does make true negatives more likely when the result is negative, its primary definition revolves around accurately identifying positives.
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