purpose. To determine the within-visit between-algorithm and the within-algorithm between-visit differences in sensitivity for the SITA Standard, SITA Fast, FASTPAC, and Full Threshold algorithms in stable primary open angle glaucoma.

methods. One designated eye from each of 29 patients (age 67.3 ± 10.2 years; mean ± SD) experienced in automated perimetry was examined with the four algorithms on each of three visits, using the Humphrey Field Analyzer 750 and Program 30-2.

results. The group mean Mean Sensitivity was 1.0 dB greater for SITA Standard
than Full Threshold (P < 0.001), 0.7 dB greater for SITA
Standard than FASTPAC (*P* < 0.001), 1.6 dB greater
for SITA Fast than FASTPAC (*P* < 0.001), and 0.9 dB
greater for SITA Fast than SITA Standard (*P* <
0.001). The higher pointwise sensitivity for SITA Fast compared to Full
Threshold, FASTPAC, and SITA Standard increased with increase in defect
depth. The examination duration for SITA Standard was 53% of that for
Full Threshold and 50% shorter for SITA Fast compared to FASTPAC
(*P* < 0.001), regardless of age
(*P* = 0.932). The examination duration increased
with increase in severity of field loss (*P* <
0.001), and this increase was proportionately greater for both SITA
algorithms (*P* < 0.001), particularly SITA Fast.
The Total and Pattern Deviation probability analyses of both SITA
algorithms yielded a statistically greater defect than Full Threshold
or FASTPAC (*P* < 0.001). The within-algorithm
between-visit differences were similar between SITA Standard and Full
Threshold and between SITA Fast and FASTPAC.

conclusions. Both SITA algorithms produce a marginally higher differential light sensitivity compared to existing algorithms but with a statistically deeper defect and a marked reduction in examination duration.

^{ 1 }The accuracy of the staircase procedure at any given stimulus location primarily increases with the use of smaller steps when the staircase position is close to the threshold, with an increase in the number of crossings of threshold and with an increase in the number of staircases.

^{ 2 }However, the improvement in accuracy is at the expense of an increase in the examination duration.

^{ 3 }

^{ 4 }

^{ 5 }

^{ 6 }

^{ 7 }

^{ 8 }

^{ 9 }which represent an increase in statistical sophistication over previous algorithms. Two SITA algorithms are currently available, SITA Standard and SITA FAST, which are analogous to the Full Threshold and FASTPAC algorithms, respectively. Both SITA algorithms use two likelihood functions for each stimulus location, one for normal responses and one for glaucomatous responses. The likelihood functions are adjusted after the positive or negative response to each individual stimulus presentation, and the shape of the function alters with increase in the number of responses. At any given moment in the examination, the height of the function describes the most likely threshold value at the given location, and the width describes the accuracy of the threshold estimate. The SITA Standard algorithm uses a 4–2 dB step size and the SITA Fast algorithm a 4 dB step size. The thresholding procedure at any given location is halted when a predetermined level of accuracy, as specified by the Error Related Factor, is obtained.

^{ 10 }At the end of the examination, the sensitivity at each stimulus location is recalculated using all the responses obtained from the examination. The resultant threshold is assumed to represent the stimulus luminance corresponding to a 50% probability on the frequency-of-seeing curve.

^{ 11 }and 1.9 dB

^{ 9 }higher than the Full Threshold algorithm. The SITA Fast algorithm for Program 30-2 was 50% shorter than FASTPAC and yielded a Mean Sensitivity 1.5 dB higher.

^{ 11 }The between-algorithm differences did not alter with age.

^{ 11 }The higher Mean Sensitivities for SITA compared to the Full Threshold and FASTPAC algorithms are present despite correction for the systematic differences in the designation of threshold between the algorithms.

^{ 8 }

^{ 9 }Interestingly, the between-subject normal variability at each stimulus location and upon which the confidence limits for normality are related was found to be approximately 10% narrower for the SITA algorithms than for the Full Threshold and FASTPAC algorithms at those stimulus locations within the configuration of Program 24-2.

^{ 11 }

^{ 12 }together with a repeatable visual field defect consistent with POAG. The mean age of the sample was 67.3 years (SD ± 10.2, range 42–79 years). The inclusion criteria comprised a visual acuity of 6/9 or better in each eye; a distance refractive error less than or equal to 5 D mean sphere and less than 2.5 D cylinder; lenticular changes not greater than NCIII, NOIII, CI, or PI by the Lens Opacity Classification System III;

^{ 13 }no systemic medication known to affect the visual field; and no history or family history of diabetes mellitus. All patients manifested well-controlled intraocular pressures (17.0 ± 3.4 mm Hg; mean ± SD) and stable visual fields. Twenty-one patients were controlled on a single topical agent (either a selective or nonselective β blocker, a topical carbonic anhydrase inhibitor, a prostaglandin or an α agonist); 6 patients required more than one topical agent for intraocular pressure control. No patients were on systemic carbonic anhydrase inhibitors or topical cholinergics. Three patients had undergone previous trabeculectomy, two of whom were not receiving any topical agents at the time of the study. All patients were experienced in automated threshold static perimetry, having undergone a minimum of three previous examinations.

^{ 14 }This classification describes the severity of loss in terms of the Mean Deviation visual field index and in terms of the number, severity, and proximity to fixation of the Pattern Deviation probability symbols. The sample comprised 14 eyes with early loss, 7 with moderate loss, and 8 with severe loss.

^{ 15 }The results were obtained with software revision A9. The first visit was considered as a familiarization period and the results were discarded before commencement of the remaining two visits. The research followed the tenets of the Declaration of Helsinki, informed consent was obtained from the subjects after explanation of the nature and possible consequences of the study, and the study was approved by the Aston University Human Science Ethical Committee and the Birmingham Heartlands Research and Ethics Committee.

*P*= 0.734). The group mean MS varied as function of algorithm (

*P*< 0.001), regardless of visit (

*P*= 0.541); the SITA Standard group mean MS was 1.0 dB higher than the Full Threshold MS and 0.7 dB higher than the FASTPAC MS. The SITA Fast group mean MS was 1.6 dB higher than FASTPAC and 0.9 dB higher than that of the SITA Standard. Group mean MS declined as a function of the severity of field loss (

*P*< 0.001), irrespective of algorithm (

*P*= 0.052). The differences in the MSs between the four algorithms were similar between visits (

*P*= 0.956) and were also independent of age (

*P*= 0.800). Group mean MS varied as a function of order of test (

*P*< 0.001), and this order effect was different between tests (

*P*= 0.01). The subgroup mean MS for the Full Threshold algorithm and for the SITA Standard algorithm was lower when the algorithms were undertaken as the second test at any given session, whereas that for the FASTPAC algorithm was higher as the second test of any session.

*P*= 0.075). It was similar for all four algorithms (

*P*= 0.291) regardless of visit (

*P*= 0.961). The Group mean MD became more negative as a function of the severity of field loss (

*P*< 0.001), irrespective of algorithm (

*P*= 0.577). It varied as a function of order of test (

*P*< 0.001), and this order effect was different between tests (

*P*= 0.004). The subgroup mean MD for the Full Threshold algorithm and for the SITA Standard algorithm was more negative when the algorithms were undertaken as the second test at any given session, whereas that for the FASTPAC algorithm was less negative as the second test of any session.

*P*= 0.004). It varied as function of algorithm (

*P*< 0.001), regardless of visit (

*P*= 0.368); the SITA Standard group mean PSD was approximately 0.2 dB higher than the Full Threshold PSD and approximately 0.6 dB higher than the FASTPAC PSD. The difference in the PSDs between algorithms increased as a function of the severity of field loss (

*P*< 0.001); the SITA Standard subgroup mean PSD was 0.8 dB higher than the Full Threshold for the severe field loss category (

*P*= 0.002). The differences in the PSD between the four algorithms were similar between visits (

*P*= 0.067) and were also independent of age (

*P*= 0.763). Group mean PSD varied as a function of order of test (

*P*< 0.001), but this order effect was not noticeably different between tests (

*P*= 0.092).

*P*= 0.284). The group mean examination duration was approximately 53% shorter for the SITA Standard algorithm compared to the Full Threshold algorithm and approximately 50% shorter for the SITA Fast algorithm compared to the FASTPAC algorithm (

*P*< 0.001), regardless of visit (

*P*= 0.145) and of age (

*P*= 0.932). The between-algorithm differences in group mean examination duration were similar between visits (

*P*= 0.967). The duration increased as a function of the severity of field loss (

*P*< 0.001), and this increase in time was proportionately greater for the SITA algorithms than for the Full Threshold and FASTPAC algorithms (

*P*< 0.001), particularly that of SITA Fast (Table 2) . The group mean examination duration also varied as a function-of-order of test (

*P*< 0.001), and this order effect was different between tests (

*P*< 0.001). The subgroup mean examination time for the Full Threshold algorithm and for the SITA Standard algorithm was longer when the algorithms were undertaken as the second test at any given session, whereas that for the FASTPAC algorithm was shorter as the second test of any session.

*P*= 0.975 and

*P*= 0.067, respectively). A more significant Total Deviation probability value (i.e., a more statistically significant defect depth) was found for the SITA Standard algorithm compared to the Full Threshold and to the FASTPAC algorithms (both

*P*< 0.001) and for the SITA FAST compared to the Full Threshold and FASTPAC algorithms (both

*P*< 0.001). The differences in the Pattern Deviation probability values between the Full Threshold and FASTPAC algorithms and between the SITA Standard and SITA Fast algorithms also did not reach statistical significance. (

*P*= 0.857 and

*P*= 0.083 respectively). A more significant Pattern Deviation probability value was found for the SITA Standard algorithm compared to the Full Threshold and to the FASTPAC algorithms (both

*P*< 0.001) and for the SITA FAST compared to the Full Threshold and FASTPAC algorithms (both

*P*< 0.001).

*P*= 0.164; FASTPAC,

*P*= 0.859; SITA Standard,

*P*= 0.401; SITA Fast,

*P*= 0.106). The differences in the distributions of the Pattern Deviation probability values between the two visits were not statistically significant for three of the four algorithms (Full Threshold,

*P*= 0.427; SITA Standard,

*P*= 0.972; SITA Fast,

*P*= 0.286). However, FASTPAC exhibited a more statistically significant Pattern Deviation defect at the second visit than at the third visit (

*P*= 0.006). The proportion of stimulus locations exhibiting deviations within the 95% confidence limits at both visits for the given algorithm declined in rank order from the Full Threshold to FASTPAC, SITA Standard, and SITA Fast for both the Total Deviation and the Pattern Deviation probability values. The proportion of such locations was greater for the Pattern than for the Total Deviation probability values. When those locations exhibiting between-visit deviations within the 95% confidence limits were excluded from the Total and the Pattern Deviation analyses, the similarity of the between-visit variability between the Full Threshold and SITA Standard and between FASTPAC and SITA Fast, respectively, was more pronounced for both the Total and the Pattern Deviation analyses. However, in proportionate terms, the FASTPAC algorithm yielded the greatest test-retest variability for both the Total and the Pattern Deviation probability analyses.

^{ 4 }

^{ 5 }

^{ 6 }

^{ 7 }

^{ 11 }Although the differences in MSs are statistically significant, they become clinically insignificant in the context of the statistically identical MDs, which describe alterations in the height of the hill of vision relative to the respective age corrected normal values of each algorithm. The SITA Standard algorithm yielded a marginally higher group mean PSD than the Full Threshold and FASTPAC algorithms, indicating a slightly greater defect depth for SITA Standard. The suggestion of a greater defect with the SITA Standard algorithm became more apparent as defect depth increased. The differences in the PSDs between the algorithms, although reaching statistical significance, were relatively unimportant clinically. Furthermore, the indices themselves are of limited clinical value in the detection of progressive visual field loss as they are merely summary measures of the sensitivities at all stimulus locations.

^{ 16 }

^{ 4 }

^{ 11 }

^{ 17 }which is present at both examinations. The irregularity of the increased spread of these percentiles can be attributed, in part, to the lower number of data points within the midsensitivity levels. The greater divergence of the 10th percentile for sensitivities of less than 10 dB compared to the 90th percentile is the mathematical consequence of a high value at the second test being subtracted from, and referenced to, a low value with large variability at the first test, hence biasing the differences toward negative values.

^{ 11 }

^{ 18 }Thus, the maximum between-individual discrepancy at any given stimulus location due to age would be in the region of 2.1 dB, i.e., generally within one interval of the scale on the abscissa of Figure 2 . The magnitude of the normal gradient of sensitivity across the Program 30-2 field varies with region, has an upper limit of approximately 9 dB,

^{ 18 }and governs the maximum within- and between-individual discrepancy between a normal peripheral value and an abnormal central value. The impact of age on the pointwise between-individual differences in sensitivity could have been reduced by considering the between-algorithm difference in sensitivity at the given stimulus location as a function of the deviation of measured sensitivity of the reference algorithm from the age-corrected normal value. Such an approach was adopted by Heijl and colleagues

^{ 17 }but does not distinguish normal reductions in sensitivity due to eccentricity from identical but abnormal values due to a defect. Moreover, such a technique would reduce the impact of any between-algorithm comparison of absolute values of sensitivity since the generated deviation values would be derived from each individual normal database. A comparison of the distributions of the within-visit between-algorithm and of the within-algorithm between-visit differences in sensitivity as a function of central and peripheral stimulus location yielded similar distributions between the two zones indicating that any potential differences due to stimulus eccentricity were masked by the underlying field loss. However, the analysis of the pointwise Total and Pattern Deviation probability values overcomes any limitations in the comparison of the absolute values of sensitivity, because the respective confidence limits are corrected for both age and eccentricity.

^{ 19 }

^{ 20 }The height adjustment of the Pattern Deviation approach also served to reduce any experimental error that might have affected the overall height of any given single field of any given patient, e.g., that arising from an increased homogeneous long-term fluctuation. Conversely, the Total Deviation approach is unaffected by any inappropriate readjustment in height.

^{ 11 }It must be noted that such analysis is based on differences in probability values and not differences in deviation values. The results also confirm the suggestion that the narrower confidence limits for the SITA algorithms correspond to lighter levels of gray in the greyscale printout than those for the Full Threshold algorithm, particularly for the proportionately larger deviations from normality and that the widths of the confidence limits for SITA are such that multiple changes in probability level can occur within a given level of gray.

^{ 11 }

^{ 21 }

^{ 22 }

^{ 23 }

^{ 24 }A reduction in the fatigue effect has been suggested as a possible explanation for the reduced between-subject normal variability of the SITA algorithms compared to the Full Threshold and FASTPAC algorithms. However, the nature of any between-algorithm differences in the fatigue effect is unknown. It is also likely that the statistically deeper defect depth of the SITA algorithms will largely militate against any apparent improvement of the SITA field in these patients. Alternatively, some patients who do not manifest a fatigue effect with the Full Threshold algorithm may exhibit fields that are worse with SITA Standard as a result of the statistically deeper SITA defect depth. These potential problems could be resolved at the designated follow-up by undertaking a Full Threshold examination followed, within a short time frame, by two SITA examinations. In this way, the SITA baseline for successive examinations would have been established.

Index | Visit | Threshold Algorithm | ||||||
---|---|---|---|---|---|---|---|---|

Full Threshold | FASTPAC | SITA Standard | SITA Fast | |||||

Mean Sensitivity (dB) | 2 | 22.18 ± 4.32 | 22.38 ± 3.90 | 23.15 ± 4.77 | 24.05 ± 4.12 | |||

3 | 22.26 ± 4.43 | 22.54 ± 4.13 | 23.21 ± 4.71 | 24.04 ± 4.17 | ||||

Mean Deviation (dB) | 2 | −4.97 ± 4.10 | −4.98 ± 3.76 | −5.11 ± 4.43 | −4.88 ± 4.02 | |||

3 | −4.94 ± 4.08 | −5.09 ± 4.06 | −5.20 ± 4.6 | −4.77 ± 4.10 | ||||

Pattern Standard Deviation (dB) | 2 | 6.90 ± 3.82 | 6.68 ± 3.77 | 7.15 ± 4.45 | 6.79 ± 4.11 | |||

3 | 7.32 ± 3.88 | 6.77 ± 3.77 | 7.46 ± 4.31 | 6.42 ± 4.25 | ||||

Short-term Fluctuation (dB) | 2 | 1.85 ± 0.92 | 2.12 ± 0.77 | — | — | |||

3 | 1.88 ± 1.24 | 2.26 ± 1.03 | — | — | ||||

Corrected Pattern Standard Deviation (dB) | 2 | 6.16 ± 3.72 | 5.99 ± 4.19 | — | — | |||

3 | 6.76 ± 4.02 | 5.78 ± 4.07 | — | — | ||||

Examination Duration (min) | 2 | 15.04 ± 1.54 | 9.56 ± 1.40 | 8.03 ± 1.14 | 4.81 ± 0.93 | |||

3 | 14.89 ± 1.76 | 9.45 ± 1.37 | 7.78 ± 0.91 | 4.68 ± 0.85 |

**Figure 1.**

**Figure 1.**

**Figure 2.**

**Figure 2.**

*Top left:*Full Threshold compared to FASTPAC;

*middle left:*Full Threshold compared to SITA Standard;

*bottom left:*Full Threshold compared to SITA Fast;

*top right:*FASTPAC compared to SITA Standard;

*middle right:*FASTPAC compared to SITA Fast;

*bottom right:*SITA Standard compared to SITA Fast. The data are expressed as a percentage in the summary table at the bottom.

*Top left:*Full Threshold compared to FASTPAC;

*middle left:*Full Threshold compared to SITA Standard;

*bottom left:*Full Threshold compared to SITA Fast;

*top right:*FASTPAC compared to SITA Standard;

*middle right:*FASTPAC compared to SITA Fast;

*bottom right:*SITA Standard compared to SITA Fast. The data are expressed as a percentage in the summary table at the bottom.

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