Sometimes a test may show a different sensitivity and/or specificity from that reported in the literature. The type of change may point to possible reasons for this change.
Example: A rapid diagnostic test may report very high sensitivity and specificity, but significantly lower values are observed in practice. This can be achieved by carefully selecting the study population so that people with disease have severe disease and the people without disease are totally normal (no comorbid condition that can cause a false positive.
Observation
Possible Explanation Based on Population
Possible Explanation Based on Testing
lower sensitivity
original study had more patients with more severe disease resulting in fewer false negatives
error in test performance in the current study resulting in more false negatives
lower specificity
original study had fewer patients with comorbid conditions resulting in fewer false positives
error in test performance in the current study resulting in more false positives
higher sensitivity
current study has more patients with more severe disease
improved test performance in the current study with fewer false negatives
higher specificity
current study has fewer patients with comorbid conditions causing false positives
improved test performance in the current study with fewer false positives
Things to look for when investigating the phenomenon:
(1) inclusion and exclusion criteria for the original study
(2) differences between the population used in the original and current study
(3) any differences in how the test is being performed
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