The sensitivity and specificity of scanning laser polarimetry in the detection of glaucoma in a clinical setting

Objective: A study was conducted to determine the sensitivity and specifici ty of masked experts in classifying results obtained from normal eyes, glau comatous eyes, and eyes suspicious for glaucoma using the Nerve Fiber Analy zer GDx (Laser Diagnostic Technologies, San Diego, CA), a scanning polarime ter. Design: Retrospective consecutive case series. Methods: Consecutive subjects seen in a glaucoma clinic were imaged with th e GDx. The masked printouts were distributed to nine experienced users of t he machine, who were asked to classify the scan from each eye as normal, su spicious for glaucoma, or consistent with glaucoma. No specific guidelines for interpretation were used. The interpretations were compared with the kn own diagnoses obtained from review of the subjects' medical records. The nu mber of correct diagnoses, false-positive answers (specificity), and false- negative answers (sensitivity) were noted for each observer and averaged fo r all nine. Interobserver agreement was calculated by use of the kappa stat istic. Main Outcome Measures: False-positive and false-negative responses from rev iewers' assessments compared with known diagnoses from chart review. Results: A total of 104 eyes was included: 29 normal eyes, 33 eyes from gla ucoma suspects, and 42 eyes with glaucoma. Interobserver agreement was mode rate 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, t he average sensitivity was 74% and the specificity was 74%, If only conside ring whether or not the scan represented glaucoma (i.e., reviewer assessmen t of "suspect" meant "not glaucoma"), the average sensitivity was 86% and t he specificity 83%. If only considering whether a scan was normal or not (i .e., reviewer assessment of "suspect" meant "abnormal"), the sensitivity wa s 83% and the specificity 86%, Eliminating glaucoma suspects (but allowing the assessment of "suspect") increased overall sensitivity to 80% and speci ficity to 79%, With suspects eliminated, sensitivity was 80% and specificit y 91% for determination of glaucoma, and sensitivity was 93% and specificit y 79% for determination of abnormality. Conclusions: Scanning laser polarimetry may have good sensitivity and speci ficity for separating normal from abnormal eyes, but it is not as good for classifying unknown subjects when glaucoma suspects are included. Evaluatio n of new parameters and continued clinical experience may help develop guid elines for interpretation and/or specific indicators for glaucomatous damag e to increase the sensitivity and specificity. Ophthalmology 2001;108:899-9 04 (C) 2001 by the American Academy of Ophthalmology.