Pretest Probability is defined as the probability of the target disorder before a diagnostic test result is known.
The Likelihood Ratio (LR) is the likelihood that a given test result would be expected in a patient with the target disorder compared to the likelihood that that same result would be expected in a patient without the target disorder.
The Number Needed to Treat (NNT) is the number of patients you need to treat to prevent one additional bad outcome (death, stroke, etc.).
When a sign, test or symptom has an extremely high specificity (say, over 95%), a positive result tends to rule in the diagnosis.
When a sign, test or symptom has a high sensitivity, a negative result rules out the diagnosis.
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