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Q. Ghadiali, J. Manns, A. Llinas, A. Lin, C. Lee, V. C. Greenstein, J. M. Liebmann, J. G. Odel, R. Ritch, D. C. Hood; An Analysis of Normal Variability in OCT Retinal Nerve Fiber Layer Thickness Provides a Better Understanding of False Positives. Invest. Ophthalmol. Vis. Sci. 2007;48(13):524.
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To better understand the conditions under which optical coherence tomography (OCT) results in a false positive (i.e. an eye with normal vision being classified as abnormal), normal variations in retinal nerve fiber layer (RNFL) thickness were examined.
Fifty individuals with normal vision and refractive errors between ±6D were tested with the fast RNFL scan protocol of the OCT3 (Zeiss Meditech) and their 256 point RNFL profiles were exported for analysis. To assess the consequences of "true" intra-subject variability, steps were taken to minimize measurement error, including repeat testing on a second day. Care was taken to center the optic disc within the scanning circle. The positions and peak amplitudes of the temporal and nasal-most peaks of the RNFL profile were measured. The positions of the peaks were measured on the 256-point scale used by the OCT3 and then converted to degrees. 0 degrees represents the temporal-most location [9 o’clock (OD)] and 180 degrees the nasal-most location [3 o’clock (OD)]. A hemifield was defined as abnormal on the OCT if one of the 5 sectors (sectors at 3 and 9 o’clock were excluded) was at < 1% (red) or two sectors were at < 5% (yellow). This criterion (2Y1R) has high sensitivity and specificity.1
On average, the amplitude of the superior (inferior) RNFL peak was 153.5 ± 18.5 (160.3 ± 21.0) µm. They were located at 36.9 ± 5.6° (145.3 ± 5.7°). However, there was a wide variation in the shape of the individual RNFL profiles. For example, the position of the superior (inferior) peaks ranged over 27.7° (36.3°) and the range of the amplitudes spanned a factor of 1.8 (1.9). Not surprisingly, the 4 abnormal hemifields, on average, had smaller peak amplitudes. However, only 1 of the 4 amplitudes fell outside the 95% confidence intervals. More remarkably, the positions of the peaks for all 4 fell on, or outside, the 95% confidence limit. In general, the RNFL profiles of the two eyes were extremely similar.
RNFL thickness profiles with peaks located at the extremes of the normal range may be classified as abnormal. If a patient’s RNFL profile has peaks located near the extremes of the normal range, and if the profiles of the two eyes are the same, a false positive should be suspected. 1. Hood et al ARVO 2006
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