The Frontloading Fields Study: The Impact of False Positives and Seeding Point Errors on Visual Field Reliability When Using SITA-Faster

Abstract

Purpose: The purpose of this study was to evaluate the impact of two conventional reliability criteria (false positives [FPs] and seeding point errors [SPEs]) and the concurrent effect of low sensitivity points (≤19 dB) on intrasession SITA-Faster visual field (VF) result correlations.

Methods: There were 2320 intrasession SITA-Faster VF results from 1160 eyes of healthy, glaucoma suspects, and subjects with glaucoma that were separated into "both reliable" or "reliable-unreliable" pairs. VF results (mean deviation and pointwise sensitivity) were analyzed against the spectrum of FP rates and SPE, with and without censorship of sensitivity results ≤19 dB. Segmental linear regression was used to identify critical points where visual field results were significantly different between tests due to FP levels.

Results: There was a significant, but small (0.09 dB per 1% exceeding 12%) increase in mean deviation, and an increase in the number of points showing a >3 dB sensitivity increase (0.25-0.28 locations per 1% exceeding 12%). SPEs were almost exclusively related to a decrease in sensitivity at the primary seeding points but did not result in significant differences in other indices. Censoring sensitivity results ≤19 dB significantly improved the correlation between reliable and unreliable results.

Conclusions: Current criteria for judging an unreliable VF result (FP rate >15% and SPE) can lead to data being erroneously excluded, as many results do not show significant differences compared to those deemed "reliable." Censoring of sensitivity results ≤19 dB improves intrasession correlations in VF results.

Translational relevance: We provide guidelines for assessing the impact of FP, SPE, and low sensitivity results on VF interpretation.

Figure 1.

Examples of visual field results not meeting “reliability” criteria examined in the present study. Sensitivity maps (dB), pattern deviation maps, and select global indices are shown. (A) Seeding point error, where three of the four primary seeding locations are markedly reduced in isolation (blue circles). (B) False positive rate 45%, with most locations showing a sensitivity increase of >3 dB above the age-expected value (red bordered area). (C) False positive rate 18%, with no locations showing a sensitivity increase of >3 dB. (D) False positive 31% with a glaucomatous arcuate defect with sensitivity results less than or equal to 19 dB. See Methods for additional detail.

Figure 2.

Difference in mean deviation (dB) between “passed” and “failed” visual field results by criteria. A positive y-axis value indicates that the mean deviation was better (more positive) on the “passed” result, and a negative value indicates that the mean deviation was better on the “failed” result. (A) Difference in mean deviation as a function of difference in false positive rate (“failed” – “passed” result). (B) Difference in mean deviation as a function of the higher false positive rate within the pair of results. For A and B, a segmental linear regression was performed, indicated by the black solid line, with the point of inflection (X₀) and second slope shown in the inset. The point of inflection is also identified by the red arrow. (C) Distribution of difference in mean deviation found in the seeding point error (SPE) and groups with both results “passed.” The box and whiskers indicate the median, interquartile range, and full range. Each datum point indicates the result from one eye. The black dashed line indicates y = 0 (no difference in mean deviation).

Figure 3.

Heat maps showing the proportion of instances with a difference exceeding 3 dB between “passed” and “failed” results (left column, green color code), difference greater than 3 dB (lower sensitivity on the “failed” result; middle column, blue color code), and difference less than −3 dB (higher sensitivity on the “failed” result; right column, red color code). Numerical proportions are shown within each cell, indicating the position within the 24-2 test grid. The crosses indicate the two locations next to the physiological blind spot, which were excluded from analysis. The cells with dark borders, bolded text, and asterisks (* P < 0.05; ** P < 0.01; *** P < 0.001; **** P < 0.0001) in seeding point errors (middle row) and false positive rates greater than 15% (bottom row) indicate locations where the proportion was significantly different to the distribution of differences seen when both results were reliable (top row). The key to the color code is shown below each column. Note that at some locations the sum of proportions with greater than 3 dB difference (blue) and less than −3 dB difference (red) did not exactly equal the total proportion (green) due to decimal rounding.

Figure 4.

The number of points where sensitivity values were more than 3 dB greater found as a function of false positive rate. With “passed” – “failed” pairs, this indicated the number of occasions where the “failed” result was more than 3 dB compared to the “passed” result. When both results were “passed,” we compared the number found on the result with the relatively higher false positive rate, or if both were the same, in random order. A higher value on the y-axis indicates more points showing elevated sensitivity. Each datum point indicates the result from one eye. The blue solid line indicates the average number of points where there was a 3 dB increase in sensitivity when both results were “passed.” The red solid line indicates the segmental linear regression with the point of inflection (X₀) and second slope shown in the inset. The left column indicates the results when all test locations were included, and the right column indicates the results when points reaching an alternate measurement floor (19 dB) were excluded. The top row indicates results as a function of absolute false positive rate and the bottom row indicates results as a function of difference between the higher and lower false positive rates. For each regression analysis, a vertical black dashed line indicates the point of inflection.

Figure 5.

Correlation between “failed” result mean sensitivity (dB) and “passed” result mean sensitivity (dB) pairs for false positive rates >15% (top row) and seeding point error (bottom row) groups. The results from Figures 3 and 4 and Supplementary Figure S1 were used to create a model used to correct the “failed” visual fields, excluding test locations that were statistically likely to be unreliably elevated or depressed (orange for false positive rates >15% and purple for seeding point errors). The corrected mean sensitivity was compared with the uncorrected visual field result (black). Linear regression analysis is shown by the solid lines (R² values and root mean squared error [RSME] for corrected and uncorrected data are shown in the inset), and the dotted lines indicate the 95% prediction intervals (the width of the interval is shown by the brackets). The left column panels indicate the results when all data points were included, and the right column panels indicate the results when points reaching the alternate measurement floor (19 dB) were excluded. The 95% prediction intervals were notably narrower when using comparing all points and the condition where points reaching the measurement floor were excluded.