To evaluate the ability of scanning laser polarimetry (SLP) with a fixed corneal polarization compensator (GDx-FCC Nerve Fiber Analyzer) compared to one with a variable one (GDx-VCC) in the discrimination between healthy and early glaucomatous eyes.
Methods
Forty patients with early glaucomatous visual field defects, having a mean deviation of 3.1 ± 1.6 dB and a pattern standard deviation of 3.1 ± 0.9 dB, and 40 controls underwent both GDx-FCC and GDx-VCC. One eye per patient was considered. The cut-off point, taken as the value dividing healthy from glaucomatous eyes with highest probability, was determined for each GDx parameter. Linear discriminant functions (LDFs) were separately developed for GDx-FCC and GDx-VCC parameters. Sensitivity, specificity, and area under the receiver operating characteristic curve (AROC) for discriminating between healthy and glaucomatous eyes were calculated for each GDx parameter, both according to the GDx normative database and after the selection of new cut-off points, and for the LDFs.
Results
All software-provided parameters showed low sensitivity and high specificity. The selection of new cut-off points improved the performance of all GDx parameters: VCC parameters performed better than FCC parameters; the largest AROCs were associated with the superior/nasal ratio for the GDx-FCC (0.86) and with the Number for the GDx-VCC (0.87). The LDFs provided an AROC of 0.89 with both the GDx-FCC and the GDx-VCC parameters.
Conclusions
The GDx-VCC showed a higher ability in the early diagnosis of glaucoma when compared with the GDx-FCC. The individuation of the right cut-off point of selected parameters with both GDx settings performed better than the software-provided parameters, and comparably to the GDx parameters-based LDFs.
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