Studies were then graded according to the Quality Assessment of Diagnostic Accuracy Studies (QUADAS) tool [24], which was modified for the purpose of the review. This tool considers 14 criteria relevant to diagnostic studies accounting for bias (items 3 to 7, 10 to 12), variability (items 1 and 2) and reporting (items 8 and 9). Items 4 (regarding the time period between index and reference test) and 7 (regarding the independency of the reference test) were omitted with regard to the selection criteria and the previously applied CEBM criteria. Additionally, item 3 (regarding the ability of the reference test to correctly classify the target condition) was applied to CT findings, ICI and neurosurgery separately.