Diagnostic and screening tests - specificity

Definition

Specificity is the proportion of people who truly do not have the disease who test negative - a highly specific test has few false positives, making it useful for ruling a diagnosis in when positive (mnemonic: SpPin - a highly Specific test, when Positive, rules in)
A fixed property of a given test (does not change with disease prevalence in the tested population), making it directly comparable across different clinical settings, unlike predictive values

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