Signal strength is an important determinant of accuracy of nerve fiber layer thickness measurement by optical coherence tomography.

PURPOSE: To investigate the effect of signal strength on the measurement of the retinal nerve fiber layer (RNFL) using optical coherence tomography (OCT).
METHODS: Eyes with known or suspected glaucoma or nonglaucomatous optic atrophy were scanned twice within the same visit using Stratus OCT's Fast Nerve Fiber Layer Thickness protocol. Only those eyes with 2 high-quality scans (signal strengths of at least 5 and different from each other, no error messages, and no obvious segmentation errors) were included in the study. The RNFL thickness measurements from the initial and the repeat scans were compared and then correlated with the differences in signal strength. Subgroup analyses were performed similarly among patients with average RNFL thickness less than 90 microm and those with at least 90 microm.
RESULTS: Scans with higher signal strengths are associated with greater RNFL thickness measurements if the signal strength is less than 7. Scans with signal strength of at least 7 have higher reproducibility. This is true among all patients and subgroups divided on the basis of average RNFL thickness. Additionally, we found that the greater the variability between the initial and repeat scans, the greater the variability in the RNFL thickness measurements. Scans with higher signal strengths have less variability, especially when the optic nerve is relatively healthy.
CONCLUSIONS: When measuring the RNFL thickness with the Stratus OCT, it is important to aim for a signal strength of at least 7. Visual field testing may be more reliable in some patients, especially when the optic nerve is significantly compromised.