A MODIFIED ROC ANALYSIS FOR THE SELECTION OF CUTOFF VALUES AND THE DEFINITION OF INTERMEDIATE RESULTS OF SERODIAGNOSTIC TESTS

A total number of 50 sera from clinically confirmed cases of canine Bo rrelia (B.) burgdorferi infection and 44 negative control sera were te sted with a B. burgdorferi specific antibody ELISA. The data were subm itted to the 'two-graph receiver operating characteristic' (TG-ROC) an alysis which is a plot of the test sensitivity (Se) and specificity (S p) against the threshold (cut-off) value assuming the latter to be an independent variable. Thus, in contrast to the conventional ROC analys is, valid pairs of Se and Sp can be read for pre-assigned threshold va lues directly from the TG-ROC plots. A cut-off that realises equal tes t parameters (Se = Sp = theta(0) (theta-zero)) can be obtained as the intersection point of the two graphs. Since the value for theta(0) is below a preselected accuracy level (95% or 90%), two cut-off values ar e selected that represent the bounds of an 'intermediate range' (IR). IR can be considered as a 'borderline' range for the clinical interpre tation of test results. The proportion of the measurement range (MR) t hat gives unambiguous test results can be expressed using IR as the 'v alid range proportion' (VRP = (MR - IR)/MR). VRP and theta(0) are usef ul parameters for test comparison since they do not depend upon the se lection of a single cut-off point. In addition, the selection of cut-o ff values is supported by graphical displays of efficiency, Youden's i ndex and likelihood ratios which can be considered as functions of the pre-assigned cut-off value. TG-ROC was derived as a user-defined temp late for a commercially available spreadsheet programme (MS-EXCEL, Mic rosoft).