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??)
In: 0
Short Answer – Sensitivity only measures True Positives. There are 3 other possible outcomes, False Positives, True Negatives, and False negatives. Just because a test is really good getting true positives you can’t claim it’s correctly identifying true negatives though.
For example, you have 100 sick people, let’s agree they are all sick. You run your test and you ID 1 sick person and 99 “healthy people”.
Since your 1 sick person was correctly sick, your sensitivity is 100%, but we know the 99 people you claimed were healthy are in fact sick, so all false negatives.
The other concept your missing is ‘specificity’, the rate of true negatives, which in this case is 0%.
In general it’s a trade off, you get great sensitivity or specificity but not both. In the case of medical diagnoses you’d err on the side of specificity, meaning you’re minimizing false negatives at the risk of getting reducing your false positive rate. The idea being it’s better to suspect someone has cancer and give them more tests than to suspect someone is healthy and have them die of cancer.
Latest Answers