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To assess whether a new test (e.g. a diagnostic tests or medical device testing for disease or non-disease status) is equivalent to an existing test, the following measures can be reported. They can be of importance for the submission of premarket notification (510(k)) or premarket approval (PMA) applications for diagnostic devices (tests) to the American Food and Drug Administration (FDA).
A new test is usually compared to an existing and established test or a general trusted reference. If the existing test (or reference) is not perfect, the FDA recommends to report the positive and negative percent agreement (PPA/NPA). This is calculated using false positives, true positives, false negative and true negatives and calculated like this (1):
Existing test New test R+ R- T+ TP FP TP+FP T- FN TN FN+TN TP+FN FP+TN N PPA = TP * 100 / (TP + FN) NPA = TN * 100 / (TN + FP)
Measures of accuracy
"FDA recommends you report measures of diagnostic accuracy (sensitivity and specificity pairs, positive and negative likelihood ratio pairs) or measures of agreement (percent positive agreement and percent negative agreement) and their two-sided 95 percent confidence intervals. We recommend reporting these measures both as fractions (e.g., 490/500) and as percentages (e.g., 98.0%)." (2)
Sensitivity and specificity are explained here.
In general th FDA recommends to report (2)
- the 2x2 table of results comparing the new test with the non-reference standard
- a description of the non-reference standard
- measures of agreement and corresponding confidence intervals.
1 - Workshop notes: Assessing agreement for diagnostic devices
2 - FDA recommendation "Statistical Guidance on Reporting Results from Studies Evaluating Diagnostic Tests"
3 - STAndards for the Reporting of Diagnostic accuracy studies (STARD)
4 - Wikipedia page