purpose. To compare intersubject variability and normal limits of threshold values between the new Swedish interactive test algorithm short wavelength automated perimetry (SITA SWAP) and the older Full Threshold SWAP programs (Carl Zeiss Meditec, Dublin, CA).

methods. Normal reduction of differential light sensitivity with age, age-corrected thresholds, intersubject variability, and normal limits of sensitivity were calculated from SITA SWAP and Full Threshold SWAP fields obtained in 53 normal subjects between 20 and 72 years of age.

results. Age influence on threshold sensitivity was the same with the two SWAP programs. On average, sensitivity decreased by 0.13 dB per year of age. Age-corrected normal threshold sensitivity was significantly higher (*P* < 0.0001) for SITA SWAP than for Full Threshold SWAP. The means for a subject 45.4 years of age were 28.8 dB with SITA SWAP and 24.4 dB with Full Threshold SWAP. Intersubject variance was 22% smaller with SITA SWAP than with Full Threshold SWAP. Normal limits at the *P* < 5% significance level were, on average, 14% narrower with SITA SWAP than with Full Threshold SWAP using Total Deviations from age-corrected normal thresholds and 11% narrower when applying Pattern Deviation, which is intended to adjust for general depression or elevation of the field.

conclusions. SITA SWAP test results from normal eyes showed higher sensitivities than results from the older Full Threshold SWAP. This represents an increase of the dynamic range, which implies that more patients can be tested with SWAP. The smaller intersubject variability with SITA SWAP means narrower normal limits and may be associated with more sensitive probability maps.

^{ 1 }

^{ 2 }

^{ 3 }

^{ 4 }

^{ 5 }However, limitations in the technology make SWAP difficult and impractical in everyday practice. One problem with SWAP has been the length of time it takes to administer the test.

^{ 6 }By modifying the SITA program originally developed for WWP

^{ 7 }

^{ 8 }for SWAP, we obtained radically reduced test times and maintained the same precision of threshold estimates, defined as threshold test–retest variability, compared with Full Threshold SWAP and Fastpac SWAP (all terms relate to the Humphrey Field Analyzer; Carl Zeiss Meditec, Dublin, CA).

^{ 9 }

^{ 6 }

^{ 10 }Small intersubject variability is desirable, because it results in narrow normal limits, which means that shallow defects also will be recognized and flagged as significantly depressed in probability maps. Probability maps, which are essential for interpretation of visual field test results

^{ 11 }are entirely based on such intersubject variability in normal populations.

^{ 12 }In Figure 1 , two fields of the same eye are displayed, one Full Threshold SWAP and one Swedish interactive test algorithm (SITA) Fast WWP. The difference in gray-tone maps of raw threshold data is mainly a scaling problem. The gray-tones originally designed for the Full Threshold WWP test have been transferred to SWAP without adjusting for differences in stimulus contrast between WWP and SWAP. The probability maps, which display significant deviations from age-corrected thresholds, appear more similar. Ocular media absorption of the SWAP stimulus, particularly in older subjects, partly explains the larger SWAP threshold intersubject variability in normal eyes compared with WWP.

^{ 13 }

^{ 14 }The Pattern Deviation concept

^{ 11 }included in the Statpac interpretation program

^{ 12 }was designed to reduce effects of cataract, which gives a general and similar reduction of sensitivity across the field. Thus, interindividual variability caused by cataract is reduced in Pattern Deviation data. Despite transformation of raw thresholds to Pattern Deviations, the normal limits applied in the probability maps available for the Full Threshold SWAP test on the HFA are wider than those for WWP. This indicates that SWAP variability is larger than for WWP and that this is not caused by lens status only. This is in agreement with data from Wild et al.,

^{ 6 }who found that intersubject variability was 1.9 times larger with SWAP than with WWP after correction for ocular media absorption.

^{ 9 }The current article addresses the other SWAP aspects that we have mentioned: large intersubject variability and reduced differential light sensitivity. The purpose of this study was to calculate preliminary normal limits for the new SITA SWAP program and to compare the results with those obtained with the older Full Threshold SWAP program using the same normal database.

^{ 15 }and color fundus photographs were obtained with a fundus camera (TRC-NW3; Topcon, Tokyo, Japan) through a dilated pupil. Color vision was tested using Standard Pseudoisochromatic Plates part 3 (SPP3; Igaku-Shoin, Tokyo, Japan). A congenital color vision deficit was noted but was not considered to be an exclusion criterion. It has been reported that color contrast thresholds in tritan color axes are not influenced by the presence of congenital red-green defects.

^{ 16 }

*IOVS*2000;41:ARVOAbstract 2539). Subjects were excluded if Fixation Losses exceeded 20% in both the SWAP and in the WWP test. SWAP alone could not be used for determining the patients’ fixation ability because the large SWAP stimulus, Goldmann size V, often tends to overestimate Fixation Losses with the blind spot method. Such a large stimulus is often seen when exposed in the blind spot, also when fixation seems to be perfect. Each patient’s first visit was considered to be a training session,

^{ 17 }

^{ 18 }and only test results from the second visit were used for calculation of results.

_{ijk}

^{2}, where i is individual, j is test program, and k is test point number.

_{i1.}

^{2}is the sum of squares of all points (.) for individual i with program 1 (SITA SWAP), and S

_{i2.}

^{2}is the sum of squares of all points (.) for individual i with program 2 (Full Threshold SWAP).

*t*-test of ratios.

^{ 12 }(i.e., by using the 85th percentile of the Total Deviation in each single test to adjust for general depression or elevation of the field). Distributions of Total Deviations and Pattern Deviations were plotted for each test point and SWAP program. The 5th percentile of all deviation distributions was calculated using the method of linear interpolation. In this way, the empiric normal limits at the

*P*< 5% level for both SITA SWAP and Full Threshold SWAP were determined.

*P*< 0.0001; Student’s

*t*-test) and of similar magnitude at all locations across the field. Age slopes of sensitivities were linear and the same for the two SWAP programs (Fig. 3) . The mean square error of residuals around the regression slope was 8.44 for Full Threshold and 5.18 for SITA SWAP, indicating smaller intersubject variability with SITA.

^{ 12 }Average skewness of all points was −0.51 dB for Total Deviation and −0.72 dB for Pattern Deviation. Distributions of both Total and Pattern Deviations were narrower with SITA SWAP than with Full Threshold SWAP. The Total Deviation 5th percentile indicating the

*P*< 5% normal limit was, on average, 14% closer to 0 dB, the age-corrected normal deviation value, with SITA SWAP than with Full Threshold SWAP. The average numerical difference was 1.05 dB, indicating that defects have to be more than 1 dB deeper with Full Threshold to be flagged as significantly depressed in the Total Deviation probability map at the investigated

*P*< 5% level. The numerical difference was significant (

*P*< 0.0001 paired

*t*-test; distribution of differences of percentiles at all test points between SITA SWAP and Full Threshold SWAP was Gaussian (Kolmogorov-Smirnov

*P*= 0.87). Similarly, the 5th percentile of Pattern Deviation was 11% closer to the normal value with SITA SWAP, mean difference 0.84 dB (

*P*< 0.0001 paired

*t*-test; distribution of differences was Gaussian; Kolmogorov-Smirnov

*P*= 0.61).

^{ 7 }whereas Full Threshold defines the threshold as the intensity of the last perceived stimulus at the end of a staircase procedure in which intensities are altered in 4-dB steps until a first reversal and then in 2-dB steps.

^{ 19 }Expected threshold estimates would be more similar if Full Threshold had defined the threshold as the mean of the last seen and not seen stimulus intensity. Reduced visual fatigue

^{ 20 }of the shorter SITA test is also a likely contributor to the higher sensitivity. Visual fatigue reduces threshold sensitivity along with increasing test time.

^{ 21 }The results of the present study were based on a considerably smaller number of normal subjects from just one center, but our purpose was not to establish definite normal limits for SITA SWAP, merely to compare SITA SWAP limits with Full Threshold SWAP limits.

^{ 22 }Nevertheless, all types of cataract affect SWAP sensitivity.

^{ 23 }but limits based on normal subjects are applied when differentiating between normal and abnormal visual fields.

*P*< 5% normal limits with Full Threshold SWAP than did those applied in the commercially available Full Threshold SWAP Statpac of the Humphrey Field Analyzer. This comparison may suggest that our subjects performed better than an average population. A comparison with the standard limits for SITA WWP tests indicated, on the contrary, that the subjects used in the present study may be very representative of a normal population Thus, the established

*P*< 5% limits of the SITA Fast WWP program were almost identical with the SITA Fast WWP

*P*< 5% limits obtained in the present study. The established SITA Fast WWP limits were based on 333 normal subjects collected at 10 different centers all over the world.

^{ 21 }

**Figure 1.**

**Figure 1.**

**Figure 2.**

**Figure 2.**

Subjects (n) | |
---|---|

Sum of LOCS II gradings >2 or LOCS II posterior subcapsular >1 | 3 |

Fixation loss | 6^{*} |

False-positive answers | 4^{, †} |

False-negative answers | 1^{, ‡} |

Unexpected eye disease | 1 |

Total number | 13 |

**Figure 3.**

**Figure 3.**

**Figure 4.**

**Figure 4.**

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