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.

Comparison of the result of a diagnostic test to the true known condition of each subject classifies each outcome as:
True positive (TP) Test result correctly identifies the presence of the condition.
False positive (FP) Test result incorrectly identifies the presence of the condition when it was absent.
True negative (TN) Test result correctly identifies the absence of the condition.
False negative (FN) Test result incorrectly identifies the absence of the condition when it was present.

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.