Conditions impacting test performance:
(1) A mild or inactive disease may test negative if the test result reflects disease activity.
(2) The presence of other conditions that can cause a false positive result.
(3) The distribution of disease severities (mild, moderate and severe).
This should be suspected:
(1) when real world performance does not match that reported in the literature
(2) when the study group section describes a skewed selection of subjects
(3) exclusion of patients with conditions that may cause a false positive test
(4) exclusion of patients with mild disease
This may be done accidentally or intentionally. A diagnostic test manufacturer may compare performance of the test in patients with severe disease vs people who are healthy, thereby enhancing the sensitivity and specificity for marketing purposes.
