Improved reproducibility of measurements with the nerve fiber analyzer

This study was undertaken to assess and improve the repeatability and reproducibility of nerve fiber layer (NFL) measurements using the Nerve Fiber Analyzer (NFA I), a scanning laser polarimeter.

The repeatability of two subsequent measurements was determined in 100 healthy eyes. Four methods of image evaluation were performed. Polarimetric measurements of retinal NFL were obtained along a peripapillary band at the superior and inferior poles of the optic disk and analyzed in a one-pixel band of absolute retardation values (method 1) and relative to the nasal segment (method 2). Method 3 used measurements obtained in a 10-pixel band along the same peripapillary band. Method 4 comprised measurements in selected areas outside the peripapillary band, which did not contain blood vessels. The repeatability (r) and relative reliability coefficient (Rel) were assessed as indices of agreement of repeated measurements. In addition, the reproducibility (R) from day to day for methods 3 and 4 was assessed by taking 10 images on 10 consecutive days from each of 10 subjects.

The 95% critical value for r in the superior and inferior segments improved from about 30% for methods 1 and 2, to 23% for method 3, and to 11% for method 4. For the superior and inferior segments, the fourth method of image evaluation yielded a Rel of 98%, as compared with 54% in method 1, 80% in method 2, and 87% in method 3. The 95% critical value for R was 23% for method 3 but only 15% for method 4.

The improved reproducibility of measurements has made the NFA I a promising device. An improved reproducibility (smaller limit for detecting change) does not necessarily indicate good sensitivity and specificity for any instrument. The role of the scanning laser polarimeter in clinical practice requires further study.