Screening for dementia: Q* index as a global measure of test accuracy revisited

Receiver operating characteristic (ROC) curves intersect the downward diagonal through ROC space at a point, the Q* index, where by definition sensitivity and specificity are equal. Aside from its use in meta-analysis, Q* index has also been suggested as a possible global parameter summarising test accuracy of cognitive screening instruments and as a definition for optimal test cut-off. Area under the ROC curve (AUC ROC) is a recognised measure of test accuracy. This study compared different methods for determining Q* index (both graphical and calculation from diagnostic odds ratio) and AUC ROC (integration and calculation from diagnostic odds ratio) using the dataset of a prospective screening test accuracy study of the Mini-Addenbrooke’s Cognitive Examination. The different methods did not agree. DOR-based calculations gave a very sensitive cut-off but with poorer global metrics than the graphical method. DOR-based calculations are not recommended for defining optimal test cut-off or test accuracy.