Purpose: The GDx is a scanning laser polarimeter that assesses peripapillary nerve fiber layer thickness. In addition to the 14 existing outcome parameters, four new parameters have been described recently: the Ellipse Standard Deviation (ESD), the Normalized Superior Area (NSA), the Normalized Inferior Area (NIA) and the Discriminant Analysis (DA). The aim of this study was to investigate the sensitivity and specificity of these four new parameters.
Methods: Only one randomly selected eye of 263 healthy volunteers and 241 glaucoma patients was considered. The healthy group was randomly divided into a reference set (n = 132) to calculate the tenth percentile of the normal distribution and a test set (n = 131) to calculate the specificity against these newly established cut-off points. Sensitivity was calculated for all glaucoma patients (n = 241) and again for three separate subgroups: early glaucoma (n = 90), moderate glaucoma (n = 93), and advanced glaucoma (n = 58).
Results: When the tenth percentile of the normal distribution was used as a cut-off point, the sensitivity and specificity pairs of the new parameters were 61.8% and 87.6%, 61.8% and 89.1%, 50.2% and 92.2% and 72.6% and 95.3% for the ESD, NSA, NIA, and the DA, respectively. The Area under the ROC curve was 0.86, 0.86, 0.87, and 0.90, respectively. Among the existing parameters, the Number discriminated best (sensitivity and specificity: 76.8% and 89.1%, respectively; area under the ROC curve: 0.90). When compared with The Number, the DA was equally good, whereas the other three new parameters performed statistically significantly worse. In general, the area under the ROC curve increased from early to moderate to advanced glaucoma.
Conclusions: The new GDx parameters discriminated well between normal subjects and glaucoma patients. None of the new parameters discriminated better than The Number.