Please can someone explain what these mean exactly? For context I am looking at the study of Ottawa ankle rules. This study looked to develop a set of rules that could be used in order to detect significant ankle fractures
In: 3
Broadly speaking:
– Efficacy: does a treatment do the main thing we want it to?
– Sensitivity: does a test/assessment pick up on the thing we want it to measure?
– Specificity: when that test/assessment registers positive, how often is it a false positive?
Without looking up that particular study, I imagine sensitivity/specificity applies to their method of detecting fractures. Efficacy as a term is more used to describe treatments, and doesn’t usually refer to diagnostic tests.
Efficacy means does it work, sensitivity means does it detect what it’s supposed to, specificity means does it detect things it’s not supposed to.
Sensitivity: the odds that someone who has the disorder has a positive test.
Positive predictive value: the odds that someone with a positive test has the disorder.
Specificity: the odds that someone without the disorder has a negative test.
Negative predictive value: the odds that someone with a negative test does not have the disorder.
Sensitivity and specificity tend to be inversely related. The better a test is at detecting something, the more likely there is to be false positives.
These concepts are related to the positive predictive value (The chance that a positive test means there is a real problem) and the negative predictive value (The chance that a negative test means there is no problem).
The Ottawa ankle test has about a 20% positive predictive value and a 95% negative predictive value. This means that there will be a lot of false positives but very few false negatives making it a good screening test. It will misidentify a lot of problems that aren’t there, but is very unlikely to miss a problem if it is there. The test is meant to be used to determine if further testing is needed. So a false positive means that someone gets an x-ray or other imaging study that wasn’t necessary, but this ensures that no one with a true injury will be missed.
This is why tests are often layered. You start with a screening test to ensure you don’t miss anything, before moving on to a more invasive confirmatory test to confirm your results. The screening test helps reduce the number of people getting the more invasive test while still ensuring that you don’t miss anyone.
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