Measures of diagnostic accuracy

Diagnostic accuracy measures the ability of a test to detect a condition when it is present and detect the absence of a condition when it is absent.

A perfect diagnostic test can discriminate all subjects with and without the condition and results in no false positive or false negatives. However, this is rarely achievable, as misdiagnosis of some subjects is inevitable. Measures of diagnostic accuracy quantify the discriminative ability of a test.

• Sensitivity / Specificity
  Sensitivity and specificity are the probability of a correct test result in subjects with and without a condition, respectively.
• Likelihood ratios
  Likelihood ratios are the ratio of the probability of a specific test result for subjects with the condition against the probability of the same test result for subjects without the condition.
• Predictive values
  Predictive values are the probability of correctly identifying a subject's condition given the test result.
• Youden J
  Youden's J is the likelihood of a positive test result in subjects with the condition versus those without the condition. It is also the probability of an informed decision (as opposed to a random guess).