Objective: A study was conducted to determine the sensitivity and specificity of masked experts in classifying results obtained from normal eyes, glaucomatous eyes, and eyes suspicious for glaucoma using the Nerve Fiber Analyzer GDx (Laser Diagnostic Technologies, San Diego, CA), a scanning polarimeter.
Design: Retrospective consecutive case series.
Methods: Consecutive subjects seen in a glaucoma clinic were imaged with the GDX: The masked printouts were distributed to nine experienced users of the machine, who were asked to classify the scan from each eye as normal, suspicious for glaucoma, or consistent with glaucoma. No specific guidelines for interpretation were used. The interpretations were compared with the known diagnoses obtained from review of the subjects' medical records. The number of correct diagnoses, false-positive answers (specificity), and false-negative answers (sensitivity) were noted for each observer and averaged for all nine. Interobserver agreement was calculated by use of the kappa statistic.
Main outcome measures: False-positive and false-negative responses from reviewers' assessments compared with known diagnoses from chart review.
Results: A total of 104 eyes was included: 29 normal eyes, 33 eyes from glaucoma suspects, and 42 eyes with glaucoma. Interobserver agreement was moderate for normals (kappa = 0.42, P = 0.00) and glaucoma (kappa = 0.48, P = 0.00), but slight for glaucoma suspects (kappa = 0.09, P = 0.00). Overall, the average sensitivity was 74% and the specificity was 74%. If only considering whether or not the scan represented glaucoma (i.e., reviewer assessment of "suspect" meant "not glaucoma"), the average sensitivity was 86% and the specificity 83%. If only considering whether a scan was normal or not (i.e., reviewer assessment of "suspect" meant "abnormal"), the sensitivity was 83% and the specificity 86%. Eliminating glaucoma suspects (but allowing the assessment of "suspect") increased overall sensitivity to 80% and specificity to 79%. With suspects eliminated, sensitivity was 80% and specificity 91% for determination of glaucoma, and sensitivity was 93% and specificity 79% for determination of abnormality.
Conclusions: Scanning laser polarimetry may have good sensitivity and specificity for separating normal from abnormal eyes, but it is not as good for classifying unknown subjects when glaucoma suspects are included. Evaluation of new parameters and continued clinical experience may help develop guidelines for interpretation and/or specific indicators for glaucomatous damage to increase the sensitivity and specificity.