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
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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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