Reproducibility of measurements with the nerve fiber analyzer (NFA/GDx)

Purpose: To determine the reproducibility of measurements with the Nerve Fi ber Analyzer, a scanning laser polarimeter designed for quantifying glaucom a in healthy patients and patients with glaucoma. The authors also assessed the variance of measurements between instruments. Methods: Measurements were made with the third generation Nerve Fiber Analy zer, the GDx. The study consisted of three parts. In the first part, the au thors measured the right eyes of 10 healthy volunteers on 5 consecutive day s. In the second part, 45 patients with glaucoma underwent Nerve Fiber Anal yzer measurements of one randomly selected eye on two separate days in a 5- week period. For all 14 available parameters, reproducibility of measuremen ts was expressed in terms of 95% limits of agreement and as the intraclass correlation coefficient. The Nerve Fiber Analyzer software has an option of creating a mean image from a selection of single images; for both parts of the study, the reproducibility of measurements was calculated for a "singl e image," and a "mean-of-three" image. In the third part of the study, 17 v olunteers underwent repeated Nerve Fiber Analyzer measurement sessions on e ach of three different instruments. Using multivariate analysis of variance , the authors determined the variance of measurements between instruments. Results: The reproducibility of measurements varied considerably across par ameters. Limits of agreement in mean images for superior maximum and inferi or maxi mum were 7.2 mu and 7.7 mu, respectively in the healthy volunteers, and 8.7 mu and 7.9 mu respectively in the patients with glaucoma. For heal thy patients, the intraclass correlation coefficient was greater than 90% i n 10 of 14 parameters. In patients with glaucoma, the intraclass correlatio n coefficient was greater than 90% in 13 of 14 parameters, Some parameters reproduced better in a mean than in a single image; these differences, howe ver, were small and generally not statistically significant. The between-in struments component also varied across parameters and was highest in ratio- based parameters. Conclusions: The reproducibility of measurements varied across parameters. In general, the reproducibility of measurements with the Nerve Fiber Analyz er was high. The reproducibility of measurements was similar between health y patients and patients with glaucoma. Any measured change in nerve fiber l ayer thickness would be statistically significant if it exceeded approximat ely 7 or 8 mu in the superior maximum or inferior maximum parameter in heal thy patients. Reproducibility of measurements hardly differed between singl e images and mean images. The reproducibility of measurements among the thr ee instruments we used was highest for straight parameters.